INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type o f computer printer.
The quality o f this reproduction is dependent upon the quality o f the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction.
In the unlikely event that the author did not send UMI a complete
manuscript and there are missing pages, these will be noted. Also, if
unauthorized copyright material had to be removed, a note will indicate the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back o f the book.
Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6” x 9” black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UM I directly to order.
UMI
A Bell & Howell Information Company 300 North Zeeb Road, Ann Arixjr MI 48106-1346 USA
Stom atal Control o f W hole-PIant Photosynthesis and Transpiration in C onifer Seedlings by Steeve Pepin B.Sc., University Laval, 1989 M .Sc., University Laval, 1991
A D issertation Submitted in Partial Fulfillm ent o f the Requirem ents for the Degree of
D OCTOR OF PHILOSOPHY in the Departm ent o f Biology We accept this dissertation as conform ing
to the required standard
Dr. N.J. Livingston, S upervisor (Departm ent o f Biology)
ins, D epartm enfâl M em ber (Department o f Biology)
Dr.-juA. HobgOBC^epartmental M em ber (Department of Biology)
Dr. X .O . m em an n . Outside M em ber (Departm ent o f Geography)
D& R .D rG uy, Ejltârnal M g ^ e r (Faculty o f Forestry, University o f B ritish Columbia)
Dr.Vs,. Sperry, External ExarnArëT(bepartment o f Botany, Duke University)
© Steeve Pepin, 1998 University of Victoria
All rights reserved. This dissertation may not be reproduced in w hole or in part, by photocopying or other means, without the permission o f the author.
Supervisor: Dr. N igel J. Livingston
ABSTRA CT
B ecause the exchange of carbon dioxide and w ater vapor betw een plants and the atmos phere is regulated by changes in stom atal conductance (gs), the responses o f stomata to fluctuations in environm ental variables have m ajor effects on leaf physiological processes such as carbon assimilation. This dissertation focuses on the stom atal control o f whole- plant photosynthesis and transpiration in conifer seedlings. Experim ents were conducted on well w atered one-year-old D ouglas-fir {Pseudotsuga m enziesii (Mirb.) Franco), west ern hem lock {Tsuga heterophylla (Raf.) Sarg.) and western redcedar {Thuja plicata Donn) seedlings to determ ine the effects of tem perature on w hole-plant photosynthetic and stom atal responses to short-term fluctuations in irradiance (Q). Follow ing a step change in (2, tim e constants ( t , the period over which 63% of the total change occurs) for g^ and
assim ilation rate (A) decreased linearly with increasing air tem perature (Ta,r). For exam ple, in w estern redcedar Ta decreased from 30 + 4 m inutes at 5 °C to 1 0 + 1 minutes at 25 °C. In all cases, Ta was within 10-15% o f Tg^. There was considerable variation in T am ong individuals within a given species. Differences between species becam e more pronounced w ith decreasing temperature. M ultiplicative m odels that included functions for T accounted for 99% o f the diurnal variability in A and g, for seedlings exposed to varying Taw, Q and vapor pressure deficit. Estim ates o f daily A were within 2% o f those m easured. Interm ittent cloud cover and understory shading were approxim ated by expos ing seedlings to 3 - 4 episodes (> 1 h) of shade (200 or 500 pm ol m"^ s“ ' ) or complete darkness during the day. In such cases, daily A was overestim ated by up to 4 and 21%, respectively, if a function for Twas excluded from the models. The results suggest that there is scope for selecting seedling stock for increased carbon assim ilation on the basis o f reduced time constants. For example, in western redcedar, a 40% reduction in T could
Ill
lead to increases in daily carbon gains o f alm ost 5% depending on the frequency and degree o f shading. If these daily gains w ere translated into increased dry m atter produc tion and com pounded, seasonal gains w ould be even larger.
E xperim ents were also conducted on one-year-old w estern redcedar seedlings to determ ine the response o f illum inated foliage to transient and reversible changes in total photosynthesizing foliage area (La). Reductions in La w ere brought about by either shad ing a portion o f the foliage or by reducing the am bient C O2 concentration (ca) o f the air surrounding the low er part o f the seedling. In the latter case, the vapor pressure was also changed so that transpiration rates (£) could be m anipulated independently of photosyn thesis rates. It w as hypothesized that following such treatm ents, there w ould be rapid short-term com pensatory changes in and A o f the rem aining foliage. These would be in response to hydraulic signals generated by changes in the w ater potential gradient rather than changes in the distribution o f sources and sinks o f carbon within the seedling. W hen a portion o f the foliage w as shaded, there was an im m ediate reduction in w hole-seedling
E and a concom itant increase in A and E in the rem aining illum inated foliage. How
ever, the intercellular C O2 concentration did not change. These com pensatory effects w ere fully reversed after the shade was removed. W hen the low er foliage A was reduced to < 0 pm ol m~2 s - ', by shading or lowering Cg, but E was either unchanged or increased, there was not a significant increase in gs and A in the rem aining foliage. I conclude that short-term com pensatory responses in illum inated foliage occur only w hen reductions in La are accom panied by a reduction in w hole-plant E. The relation betw een the reduction in w hole-seedling E and the increase in g, or A is highly linear (R^ = 0.69) and confirms the hypothesis o f the strong regulation o f gs by hydraulic signals generated within the seedling. I suggest that the m echanism o f the com pensatory effects is a com bination o f both increased C O2 supply, resulting from increased g, and a response o f the rate of carboxylation, possibly related to the activity of Rubisco.
IV
Exam iners:
■ ---Dr. N.J. Lu/ingston, Supervisor (Departm ent o f Biology)
Dr^B.J. J^ w k in s, D e p a r t m e n t M em ber (Departm ent o f Biology)
D r Æ ^ . H o b ^ n /D b p a rm ^ n ta l M em ber (Departm ent o f Biology)
Dr. ^ O . J^iemann, Outside M em ber (Departm ent o f Geography)
Guy, External M em beL fraculty o f Forestry, University o f B ritish Columbia)
Ta b l e o f c o n t e n t s
A b stra ct... ii
Table o f co n ten ts... v
List of ta b le s ... ix
List of fig u r e s ... x
List of sym bols and abbreviations... xiv
A cknow ledgm ents... xvi
G e n e ra l in tr o d u c tio n ... 1
C h a p te r 1 R ates o f stom atal opening in conifer seedlings in relation to air temperature an d daily carbon gain In tro d u c tio n ... 6
M aterials and m eth o d s... 8
Plant m aterial and growing co n d itio n s... 8
M easurem ents... 8
Gas exchange... 8
Photon flu x d e n s ity ... 9
Soil w ater c o n te n t... 10
Experim ental p ro to c o l... 10
Effects o f air temperature on stom atal o p e n in g ... 10
Effects o f length o f dark periods on stom atal o p en in g ... 11
Phenom enological m o d e l... 11
Estimation o f the param eters fo r the m o d e l... 12
Comparison between m odel and measurements f o r dynam ic changes in Q ... 12
R e s u lts ... 13
Response o f stomatal conductance and assimilation rate to changes in Q 13 Phenom enological model and daily carbon g a in ... 18
D iscussion... 22
VI
Chapter 2
Short-term responses o f transpiration and photosynthesis to transient and reversible changes in photosynthesising fo lia g e area in western redcedar (Thuja plica ta D onn.) seedlings
In tro d u c tio n ... 36
M aterials and m eth o d s... 38
Plant m aterial and grow ing co n d itio n s... 38
M ea su rem en ts... 39
G as exch a n g e... 39
P hoton flu x d e n s ity ... 40
Soil w ater c o n te n t... 42
E xperim ental p ro to c o l... 42
W hole-seedling responses to partial shading using the single cuvette 42 Response in upper foliage to shading the lower foliage using the dual-cuvette ... 43
(a) D in both cuvettes held co n sta n t... 43
(b) D in inner cuvette in crea se d ... 44
R esponse o f upper fo lia g e to exposure o f low er fo lia g e to low Ca... 44
(a) D held constant in both cu v e tte s... 44
(b) E held constant in both cuvettes... 45
R e s u lts ... 45
W hole-seedling responses to partial shading ... 45
R esponse o f upper foliage to shading the low er fo liag e... 48
R esponse o f upper foliage to changes in D and shading o f the low er foliage 50 R esponse o f upper foliage to changes in Ca around the low er foliage w ith no shade a p p lie d ... 54
D iscussion... 54
vu
Appendix A
Soil w ater retention curve, and photosynthetic and stom atal responses to environm ental fa cto rs
Soil w ater retention c u rv e ... 6 6 Photosynthetic and stom atal responses to photon flux d e n s ity ... 6 6 P hotosynthetic and stom atal responses to air tem p eratu re... 6 6 Photosynthetic and stom atal responses to vapor pressure d e fic it... 6 6
Appendix B
Tem perature-dependent m easurem ent errors in time dom ain reflectom etry determ inations o f soil w ater
A b stra ct... 72
In tro d u c tio n ... 73
M aterials and m eth o d s... 75
T D R m easurem ent sy ste m ... 75
Dielectric m o d e l... 76
M easurem ents in w a te r... 77
M easurem ents in s o il... 78
Precise tem perature control... 78
H igh-resolution TDR m e a su re m en ts... 80
Gravim etric determ ination o f soil w ater co n ten t... 81
D ata an a ly sis... 81
Results and d iscu ssio n ... 82
D istilled w a t e r ... 82
S o il... 82
P redicted vs. m easured relation between and tem perature... 82
A bsolute errors in soil w ater content... 8 8 S u m m ary... 90
v ia
Appendix C
A w hole-plant cuvette system to measure short-term responses o f conifer
seedlings to environm ental ch a n g e... 93
Appendix D
R esponse o f transpiration and photosynthesis to a transient change in
IX
Li s t o f t a b l e s
Ch a p t e r 1
Table 1.1 Tim e constants for increases in assim ilation rates o f western redcedar seedlings following a step change in photon flux density at three different air tem peratures follow ing (i) overnight darkness
and (ii) 1 - 2 h dark periods imposed during the d a y ... 17
Table 1.2 Values o f the coefficients in Eqns 7 -9 used to predict assimilation
rate and stom atal conductance o f western redcedar se e d lin g s... 19
Table 1.3 Values o f coefficients in the linear regression: A model = a + b Ameas for western redcedar seedlings exposed to periods of high photon flux density ( g ) interrupted by periods of darkness or shade, or
exposed to diurnal changes in Q w ithout interruption... 24
Ap p e n d i x B
Table B .l f/^air measured at 0y = 0, bulk density, probe length and soil water content determ ined gravim etrically and by time domain reflecto
Li s t o f f i g u r e s
Ch a p t e r 1
Figure 1.1 Tim e course o f (a) whole-seedling assim ilation rate and (b) stomatal conductance to water vapor in w estern hem lock in response to a step change in photon flux d en sity ... 15
Figure 1.2 Relation between time constant for increases in assimilation rate, follow ing a step change in photon flux density, and air temperature for (a) three conifer species, and (b) three western redcedar
s e e d lin g s ... 16
Figure 1.3 Tim e course o f (a) air tem perature and photon flux density, (b) vapor pressure and vapor pressure deficit. Typical daily course of measured and m odelled (c) assim ilation rate, (d) transpiration rate, (e) stomatal conductance and (f) w ater use efficiency o f a western redcedar
se e d lin g ...2 0 -2 1
Figure 1.4 Daily course o f m easured and m odelled assim ilation rate of a west ern redcedar seedling subjected to step changes in photon flux den sity and held at an air tem perature o f (a) 10 °C and (b) 20 ° C 23
Figure 1.5 Daily course o f m easured and m odelled assim ilation rate of a west ern redcedar seedling subjected to alternating periods of high photon flux density and shade and held at an air tem perature o f (a) 15 °C
and (b) 25 ° C ... 25
Figure 1.6 Assim ilation rate as a function o f intercellular C O2 concentration,
follow ing a step change in photon flux d e n sity ... 28
Figure 1.7 (a) Sim ulated time course o f air tem perature and photon flux density
(Q) for conditions when sinusoidal changes in Q are interrupted by
two 1 h episodes of total darkness and two 1 h episodes o f shade; (b) Increases in daily carbon gain predicted by Eqn 3 vs. the ratio o f the tim e constant for assim ilation rate to the m axim um tim e co n sta n t... 3 1
XI
Ch a p t e r 2
Figure 2.1 Schem atic diagram o f the dual-cuvette gas exchange system (not to scale) where M and S denote mass flow controllers and norm ally
closed solenoid valves, respectively... 41 Figure 2.2 Tim e course o f whole-plant net photosynthesis rate and stomatal
conductance to w ater vapor o f seedlings in response to partial shad ing im posed by inteiposing opaque screens betw een the light source and the c u v e tte ... 46 Figure 2.3 The ratio of the whole-plant net photosynthesis rate o f a partially
shaded seedling to that of a fully illuminated seedling vs. the ratio of shaded to total foliage a r e a ... 47 Figure 2.4 Tim e course o f (a) net photosynthesis rate, (b) transpiration rate,
(c) stomatal conductance to w ater vapor, and (d) intercellular CO2 concentration w hen the low er 65% o f a seedling’s foliage was
shaded for 6 h ... 49
Figure 2.5 C hanges in net photosynthesis rate and intercellular C O2 concentra tion o f three seedlings when a portion o f the low er foliage was
shaded for 3 - 4 consecutive d a y s ... 51
Figure 2.6 Tim e course o f (a) net photosynthesis rate, (b) transpiration rate, (c) stom atal conductance to w ater vapor, and (d) intercellular C O2 concentration when the low er 65% o f a seedling’s foliage was illu
m inated after having been shaded for 24 h ... 52
Figure 2.7 Tim e course o f (a) net photosynthesis rate, (b) transpiration rate, (c) stom atal conductance to w ater vapor, and (d) intercellular CO2 concentration when the low er 55% o f a seedling’s foliage was first subjected to an increase in vapour pressure deficit from 1 to 2 kPa
XII
Figure 2.8 Tim e course o f (a) net photosynthesis rate, (b) transpiration rate, (c) stom atal conductance to w ater vapor, and (d) intercellular CO2 concentration when the low er 63% o f a seedling’s foliage was ex posed to a reduction in am bient C O2 concentration from 350 to
50 /im ol mol“’ ... 55
Figure 2.9 Tim e course o f (a) net photosynthesis rate, (b) transpiration rate, (c) stom atal conductance to w ater vapor, and (d) intercellular C O2 concentration when the low er 65% o f a .seedling’s foliage was ex
posed to changes in am bient CO2 concentration... 56
Figure 2.10 C hanges in (a) net photosynthesis rate and (b) stomatal conductance to w ater vapor o f photosynthesizing foliage vs. the ratio o f non- photosynthesizing to total foliage area. Reductions in photosynthe sizing foliage area were brought about by either shading or lowering the am bient C O2 concentration below the com pensation p o in t 58
Figure 2.11 Changes in net photosynthesis rate in the upper (untreated) foliage vs. changes in w hole-seedling transpiration rate brought about by either shading the low er foliage or exposing the low er foliage to C O2 concentrations below the com pensation p o in t... 60
Appendix A
Figure A. 1 R elationship betw een soil water content and soil w ater potential for
the sand used in the present s tu d y ... 67
Figure A.2 Photon flux density vs. whole-seedling (a) net photosynthesis rate and (b) stomatal conductance norm alized to the m aximum measured over the d a y ... 6 8
Figure A.3 A ir tem perature vs. whole-seedling (a) net photosynthesis rate and (b) stom atal conductance norm alized to the m axim um m easured over the d a y ... 69
Xlll
Figure A .4 Vapour pressure deficit vs. whole-seedling (a) net photosynthesis rate and (b) stomatal conductance norm alized to the m axim um
m easured over the d a y ... 70
Appendix B
Figure B .l Predicted and m easured f/fair and A'a vs. tem perature for distilled
w ater using a 0.58 m long pro b e... 83
Figure B.2 Predicted and m easured t/fair and vs. tem perature for a wet coarse sand using (a) 0.33 m (w ater content, = 0.293 m^ m"^) and (b) 0.58 m (0v = 0.294 m^ m“^) long p r o b e s ... 84
Figure B.3 Predicted and m easured f/tair and vs. tem perature for a dry coarse sand using (a) 0.33 m (w ater content, = 0.090 m^ m~^) and (b) 0.30 m (0v = 0.119 m^ m"^) long p ro b e s... 85
Figure B.4 Predicted and m easured t/fair and vs. tem perature for loam (water content, = 0.487 m^ m“^) using a 0.58 m p ro b e ... 86
Figure B.5 Predicted and measured f/fair and vs. tem perature for saturated
peat (w ater content, dy = 0.810 m^ m“^) using a 0.30 m probe 87
Figure B.6 Predicted and measured absolute error in w ater content vs. soil water c o n te n t... 89
XIV Li s t o f s y m b o l s a n d a b b r e v ia t io n s A A r A t '^max C Ca Ca low er Ci D Dq.5 ^lo w er D C . Ca Cs E Et g s g s m ax ID K ' K ” K , E-dtr Kc ^ s o lid s ^water / La 4 4
net photosynthesis rate; net assimilation rate (/imol C O2 m ”^ s“')
ratio o f w hole-plant net photosynthesis rate o f a partially shaded seedling to that o f a fully illum inated seedling (dimensionless)
w hole-plant net photosynthesis rate (/imol CO2 m“^ s~')
steady-state assim ilation rate measured over one hour, 2 -3 h after the lights w ere sw itched on (/imol CO2 m“^ s“ ')
speed o f light (m s“ ' )
am bient C O2 concentration (/imol m ol"' )
am bient C O2 concentration in the lower cuvette (/imol m o l"') intercellular C O2 concentration (/imol m ol"')
vapor pressure deficit (kPa)
vapor pressure deficit when A and gs are half the m axim um (kPa) vapor pressure deficit in the lower cuvette (kPa)
direct current (V)
am bient w ater vapor pressure (kPa) w ater vapor pressure at saturation (kPa) transpiration rate (mmol H2O m"^ s"')
w hole-plant transpiration rate (mmol H2O m"^ s"') stom atal conductance to water vapor (mmol H2O m"^ s"')
steady-state stom atal conductance measured over one hour, 2 -3 h after the lights were sw itched on (mmol H2O m"^ s"')
internal diam eter (m)
real part o f the dielectric constant (dimensionless)
im aginary part o f the dielectric constant; dielectric loss (dim ensionless) apparent dielectric constant (dimensionless)
dielectric constant o f air (dimensionless) com posite dielectric constant (dimensionless) dielectric constant o f solids (dimensionless) dielectric constant o f water (dimensionless) length o f a probe or transm ission line (m) photosynthesizing foliage area (m^)
ratio o f shaded to total foliage area (dimensionless) w hole-plant one-sided foliage area (m^)
XV
ns nanosecond (1 0“^ s) OD outer diam eter (m)
P probability o f a statistical test ps picosecond ( 10“ ' ^ s)
PVC polyvinyl chloride
Q photon flux density for photosynthetically active radiation (/xmol photons m "^s“ ')
Q ' light com pensation point (/imol photons m““ s“ ' ) coefficient of determ ination
R ubisco ribulose 1,5-bisphosphate carboxylase/oxygenase
t time (s or h)
t propagation tim e o f an electrom agnetic pulse; time delay (s) fair time delay in air (s)
fdry soil time delay in a dry soil (s)
fsolids time delay due to the solid fraction o f a soil (s) fwater time delay in w ater (s)
T tem perature (°C) fair air tem perature (°C) Tjeaf leaf tem perature (°C)
fmax tem perature at w hich A and gs are at a maxim um (°C) fwater tem perature of w ater (°C)
TDR time dom ain reflectom etry V velocity (m s" ')
W UE w ater use efficiency (|im ol C O i m“^ s“ ’ mmol H2O m“^ s“ ’)
a alpha, the level of significance o f a test; probability of a Type 1 error 0g gravim etric w ater content o f a soil (Mg m“^)
0v volum etric w ater content o f a soil (m^ m“^)
(p total porosity o f a soil (dimensionless) Pi, bulk density o f a soil (M g m“^)
density of w ater (M g m “^) T time constant (m inute) Tgg time constant for gs (minute) Ta tim e constant for A (minute)
XVI
Ac k n o w l e d g m e n t s
I am truly indebted to Dr. Nigel J. Livingston. He is a great supervisor and a good friend. His generosity, understanding, patience and his numerous advices have allowed me to persue and com plete this program. I w ould also like to thank him for providing me with financial support through his operating grant from Natural Sciences and Engineering R esearch Council of Canada. I am very grateful to the m embers o f my graduate com m it tee, Drs. B arbara J. Hawkins, Louis A. Hobson, K. O laf Niemann, and R obert D. Guy for offering advice during this project, for reviewing this dissertation and for providing helpful criticism and com ments. I wish to thank Greg Filek and Hugh Hinskins for intro ducing me to the w orld o f electronics and instrumentation, W illiam R. H ook for sharing w ith me his interest and knowledge o f time domain reflectom etry and Brad M. Binges for his assistance in the m aintenance o f plant growth facilities. Thanks are also due to my lab m ates: Erin, Robin, Wendy, Edgar, Sun, Sonu, Dale, Rob, Gilbert and Tracy, and to colleagues o f the Centre for Forest Biology for creating such a dynam ic and friendly research environm ent. And m ost importantly, special thanks to Suzanne for her support and understanding.
G eneral introduction
A pproxim ately one m illion hectares o f forest are harvested annually in Canada (A nony m ous 1996). A significant proportion o f the harvested areas is left to regenerate naturally and the rem aining areas are restocked by planting (=45% ), direct seeding (=3%) or scari fication (=1.5% ). T he reforestation of ‘understocked’ forest areas and recently harvested sites requires m ore than 600 million seedlings each year.
R eforestation success has increased significantly over the past decades, mainly because o f im provem ents in stock quality, planting techniques and site preparation (Anonym ous 1991). Nevertheless, seedling establishm ent and stand productivity are often lim ited by a range o f environm ental stresses. A better understanding o f the physi ological processes that determ ine seedling responses to such stresses is central to the successful developm ent o f breeding program s for the production o f high performance planting stock. Such inform ation is also crucial for the developm ent of mechanistic m odels o f plant perform ance.
Stom atal regulation provides the m echanism by which plants control fluxes of carbon and w ater betw een their leaves and the atm osphere. A good knowledge o f sto matal behavior and its influence on leaf gas-exchange is needed to predict carbon assim i lation and w ater use by whole trees, or to model transfer processes in forest canopies (Herbst 1995). It is, therefore, hardly surprising that considerable research has been focused on characterizing the behavior and functioning of stomata. Studies on the dynam ics o f stom atal responses to individual environm ental factors have explored several interesting approaches, ranging from experim ents conducted in controlled laboratory conditions, to m easurem ents perform ed in natural systems. Em pirical models, rather than m echanistic m odels, have also been used to exam ine the interactions between seedlings and the aerial environm ent (Jones 1992). These m odels are generally successful in de scribing and predicting the overall variation in stom atal conductance (Jarvis 1976;
Ge n e r a li n t r o d u c t i o n 2
Livingston & B lack 1987; Jones 1992). However, they have a lim ited scope for general ized application, as they lack a good understanding of the underlying response m echa nisms. In m any cases, em pirical models eventually becom e ‘sem i-m echanistic’ as more physiological know ledge is incorporated into them. M odels that couple stom atal conduct ance to the rate o f photosynthesis at the leaf scale provide a good exam ple o f this (Ball, W oodrow & B erry 1987; Lloyd 1991; Leuning 1995).
T his dissertation investigates some of the factors that control whole-plant gas- exchange in conifer seedlings. A quantitative approach is proposed to further improve our understanding o f the processes regulating stomatal conductance (gs) and photosynthesis rate (A). In particular, the photosynthetic and stomatal responses o f whole seedlings to fluctuations in photon flux density are quantified in terms o f tim e constant (a measure of response tim e). A dditionally, the short-term responses o f w hole-plant transpiration and photosynthesis to transient and reversible changes in illum inated foliage area are exam ined in detail. T he results of these studies are presented in the tw o chapters o f this disser tation, which has been w ritten in ajo u rn ai publication format.
In C hapter 1, the effects o f tem perature and varying the length o f dark periods on the time constant for net photosynthesis rate and stomatal conductance (t^ and Tg^, the period over w hich 63% o f the total change in A and gs, respectively, occurs) of Douglas fir, w estern hem lock and w estern redcedar seedlings are described. A phenom enological gas-exchange m odel, based on m easured photosynthetic and stom atal responses o f west ern redcedar to three environm ental variables (photon flux density, air tem perature and vapor pressure deficit), is developed. The model is then used to: (i) determ ine if the inclusion o f functions that account for photosynthetic and stom atal dynam ics would im prove our estim ates o f daily carbon gain; and (ii) determ ine w hether the selection of individuals w ith low er tim e constant would result in increased carbon assim ilation in a range o f environm ents.
Ge n e r a lin t r o d u c t i o n 3
In C hapter 2, the short-term responses o f illum inated foliage to transient and revers ible reductions in photosynthesizing foliage area, brought about by either shading a portion o f a seedling or by reducing the am bient C O2 concentration o f the air surrounding part o f a seedling, are described. Responses to changes in vapor pressure to manipulate w hole-seedling transpiration rate (£), independently o f photosynthesis, are reported. The changes in w hole-seedling E and in the distribution of sources and sinks o f carbon within a seedling associated with a reduction in photosynthesizing foliage area are discussed in relation to hom eostatic adjustm ent in plant water potential and photosynthesis. Finally, the m echanism s underlying these com pensatory responses are discussed.
In A ppendix A, the w ater retention curve for the sand used in the present study, and the responses o f net photosynthesis rate and stomatal conductance to changes in photon flux density, air tem perature and vapor pressure deficit for western redcedar seedlings are presented.
A ppendix B describes the time dom ain reflectom etry (TDR) technique used in this study to m easure soil w ater content. In particular, the effects o f tem perature on the soil apparent dielectric constant {K^) and the m easurem ent errors in associated with varia tions in soil tem perature are reported. The m easured changes in with tem perature are com pared w ith those predicted by a com posite dielectric mixing model. Finally, the tem perature dependence o f the dielectric constant o f water is discussed in relation to the effects o f soil m atrix on K^.
A ppendix C is a previously published paper that describes the w hole-plant cuvette system (originally developed by Dr. Livingston) used in this study. The effects of in creased tem perature and vapor pressure deficit on net photosynthesis rate and stomatal conductance o f western redcedar and white spruce [Picea glauca (M oench) Voss] seed lings are presented.
Ge n e r a li n t r o d u c t i o n 4
A ppendix D is a recently published paper that describes experim ents conducted in N ew Zealand, in collaboration with Drs. David W hitehead, Frank Kelliher and Kevin H ogan, to determ ine w hether the com pensatory mechanisms that had been observed in our laboratory experim ents w ould also occur in trees under natural conditions. Changes in transpiration flux density, tree conductance, stomatal conductance and photosynthesis of a Pinus radiata D. Don tree in response to transient reductions in illuminated foliage area (by shading) are presented. The physiological processes involved in these com pensa tory responses are also discussed.
Re f e r e n c e s
A nonym ous (1991 ) Reforestation: it’s a law in British Colum bia and our com m itm ent to the future. M inistry o f Forest, Victoria, British Colum bia, Canada.
A nonym ous (1996) National forestry database: Summ ary 1996. Canadian Council o f Forest M inisters, Ottawa, Ontario, Canada.
Ball J.T., W oodrow I.E. & Berry J.A. (1987) A model predicting stom atal conductance and its contribution to the control o f photosynthesis under different environm ental conditions. In Progress in Photosynthesis Research, Vol IV. (ed. J. Biggins). M artinus N ijhoff Publishers, Dordrecht, pp. 221-224.
H erbst M. (1995) Stom atal behaviour in a beech canopy: an analysis o f Bowen ratio m easurem ents com pared with porom eter data. Plant, Cell and Environment 18,
1010- 1020.
Jarvis P.O. (1976) The interpretation o f the variations in leaf w ater potential and stomatal conductance found in canopies in field. Philosophical Transcripts o f the Royal Society
o f London, Series B. 273, 593-610.
Jones H.G. (1992) Plants and microclimate: a quantitative approach to environm ental plant physiology. (2"^ ed.) Cam bridge Univ. Press, Cambridge. 428 pp.
Leuning R. (1995) A critical appraisal of a com bined stom atal-photosynthesis m odel for C3 plants. Plant, Cell and Environm ent 18, 339-357.
Ge n e r a li n t r o d u c t i o n 5
L ivingston N .J. & B lack T.A. (1987) Stomatal characteristics and transpiration o f conifer seedlings planted on a south-faced clear-cut. Canadian Journal o f Forest Research 17,
1273-1282.
Lloyd J. (1991) M odelling stom atal response to environm ent in M acadam ia integrifolia.
Chapter 1
Rates o f stom atal op en in g in conifer seed lin g s in relation to air
temperature and daily carbon gain
In t r o d u c t i o n
Assim ilation rates (A) o f photo-induced leaves adjust alm ost instantaneously to fluctua tions in photon flux density (Q) whereas changes in stom atal conductance (gs) occur m ore slowly. Stom atal responsiveness to varying Q differs am ong species (Knapp & Sm ith 1989) and is influenced by a num ber o f factors that include plant water status (Davies & Kozlow ski 1975; Knapp & Smith 1989), plant nutrition (Davies & Kozlowski 1974), air tem perature (Ng & Jarvis 1980; W hitehead & Teskey 1995) and light condi tions (W oods & T urner 1971; Tinoco-O janguren & Pearcy 1993a,b).
In broad-leafed tree species, the response time o f stom ata (that is, the period over w hich stom ata are in transition from one steady-state level to another) to rapid changes in irradiance typically ranges from 3 -4 5 min (Woods & Turner 1971; Davies & Kozlowski
1975). In conifers, stom atal responses to fluctuating irradiance are generally slower. Knapp & Sm ith (1989) reported that in two alpine conifers (Pinus fle x ilis Jam es and
Pinus conforta D ougl.) gs was relatively unresponsive to alternating periods of full sun
light (Q > 1800 /im ol m~^ s~' for 8 min) and shade (Q = 4 0 0 -4 5 0 /tm ol m“^ s“ ' for 5 min). Livingston (1994) reported that in western redcedar (Thuja plicata Donn.) seed lings, once stom ata w ere fully open, there were no significant changes in whole-plant transpiration rate (£ ) and gs in response to rapid and continuous fluctuations in Q (200 to
1100 yumol m““ s“ ' over 3 -5 min).
In som e circum stances, com plete stomatal adjustm ent to illum ination (after a period in the dark) can take over 4 h (Watts & Neilson 1978; N g & Jarvis 1980). However, responses are usually faster. F or example, the stomatal time constant (Tg^, the period over
C h a p t e r 1 - Stom atal opening in relation to temperature 1
which 63% o f the total change in occurs), for foliage in a Pinus radiata D. Don tree that had been shaded for 36 h, was only 30-35 min (W hitehead et al. 1996). Sitka spruce
{Picea sitchensis (Bong.) Carr.) seedlings achieved two-thirds o f their stom atal response
to illumination in 40 m in (Watts & N eilson 1978).
Generally, stom ata take longer to open than to close in response to a change in illum ination (Watts & N eilson 1978; W hitehead & Teskey 1995). Further, the rate of opening is slower after overnight darkness than after a shorter period o f darkness im posed during the day (W hitehead & Teskey 1995).
It has been suggested that the distribution o f species among contrasting habitats m ight be related to the optim ization o f stom atal behavior in order to lim it w ater expendi ture for a given carbon gain (DeLucia 1987; Fites & Teskey 1988; Tinoco-O janguren & Pearcy 1993a,b). For example, it m ight be advantageous for trees growing in dry environ ments (i.e. interior provenances) to open their stom ata rapidly in the morning to m axi mize their productivity and w ater use efficiency (WC/£) by confining the bulk of their carbon assim ilation to periods of relatively low and vapor pressure deficit (D). It is not uncom m on for seedlings grow ing in an understory to experience long periods o f very low Q interceded by episodes (typically ranging from a few minutes to m ore than 1 h) of intense sunlight (Young & Smith 1979; Knapp & Smith 1988; Pfitsch & Pearcy 1989; Chen & K linka 1997). Seedlings that rapidly open their stomata, in response to increased <2, would likely m inim ize stom atal lim itation to photosynthesis. Additionally, in the early morning, they could take full advantage o f high CO2 concentrations before turbulent eddies introduce air from above the canopy at low er C O2 concentrations (Jarvis, Jam es & Landsberg 1976). In contrast, rapid stom atal opening should be less im portant for carbon gain in wet (i.e. coastal provenances) and unshaded environm ents.
The purpose o f this study was to quantify, in terms o f time constants, the stomatal and photosynthetic dynam ics o f conifer seedlings represented by three species
[Douglas-C h a p t e r 1 - Stom atal opening in relation to temperature 8
fir {Pseudotsuga m enziesii (M irb.) Franco), w estern hem lock (Tsuga heterophylla (Raf.) Sarg.) and w estern redcedar {Thuja plicata Donn.)] that have a broad distribution within B ritish Colum bia. Specific objectives were: (i) to establish the relation between air tem perature and T for both A and (ii) to determ ine the effects o f varying the length of dark periods on t ; (iii) to determ ine w hether the incorporation, within phenom enological gas exchange m odels, o f functions that account for the photosynthetic and stomatal responses to changing irradiance would im prove our ability to predict daily carbon gain; and (iv) to use such m odels to determ ine w hether the selection of individuals with lower T would result in increased carbon uptake in a range o f environm ents.
M a t e r i a l s a n d m e t h o d s
Plant material and growing conditions
O ne-year-old D ouglas-fir o f interior and coastal provenances, western redcedar from dry and w et habitats, and western hem lock seedlings were used in this study. Nursery raised seedlings w ere transplanted to 7 dm^ plastic containers filled with fine sand and kept outdoors from m id-A pril to m id-Novem ber in field facilities at the University of Victoria. Seedlings w ere watered once or twice a week. A com m ercial (20:20:20) N:P:K fertilizer was applied bi-weekly.
Measurements
Gas exchange
W hole-seedling transpiration and assimilation rates w ere m onitored continuously with a closed gas exchange system as described by Livingston et al. (1994). In this system, m easurem ents o f cham ber CO2 concentration ( c j and w ater vapor pressure (e^) are made using a dual detector infrared gas analyzer (LI-6262, L i-C or Inc., Lincoln, NE, USA). N et assim ilation rates are determ ined by integrating the output (recorded as 1 min
run-C h a p t e r 1 - Stom atal opening in relation to temperature 9
ning averages) o f a mass flow controller (ly ia n Corp., Carson, CA, USA) used to inject CO] into the cham ber to balance that taken up by the seedling. Vapor pressure is control led by circulating cham ber air through a column o f C aS0 4 w hen Ca exceeds a given set point. The desiccant colum n is supported on a balance with 1 mg resolution. Transpira tion rate is determ ined as the change in desiccant mass over time (typically 60 s). Instan taneous W U E is calculated as A/E. C onductance to w ater vapor is calculated as E/{L x D) where L is the seedling projected leaf area measured using a leaf area m eter (LI-3100, Li- Cor Inc.). Intercellular C O ] concentration (c,) is calculated as described by Field, Ball & Berry (1991). B ecause values o f E, gs and q are derived from m easurem ents made at fixed intervals, they are plotted as discrete points. The flow rate in the cham ber is ap proxim ately 0.025 m^ s“', giving rise to very high boundary layer conductances (> 2 mol m“^ s“ ’ for seedlings w ith L < 0.06 m^; Livingston et al. 1994) relative to stomatal con ductances (typically 0 - 0 .2 mol m~^ s“ '). In separate experim ents, it was determ ined that leaf tem peratures (Tieaf) w ere within 0.1 °C o f air tem peratures (both m easured with fine wire therm ocouples) and fo r the present study, it was assum ed that Tieaf =
Fair-A ir tem perature in the cham ber is controlled to better than ± 0.1 °C over 0 to 35 °C. There are slight (< 0.25 °C) tem perature excursions from the set point when the light source is sw itched on or o ff to provide an abrupt change in Q but these are corrected within 20 min o f the change (Livingston et al. 1994).
Photon flu x density
The light control system described by Livingston (1994) was used to provide constant Q (typically 1000 ± 5 /tm ol m~^ s“') at seedling height over a 1 0 -1 2 h photoperiod. Rapid (=10 //m ol nT ^ s~^) or slow er changes in Q are brought about by varying the amount of dyed liquid in a tank placed between the light source and the cuvette. This system was also used to accurately sim ulate diurnal changes in Q.
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 0
S oil w ater content
Soil w ater content (0) w as m easured using tim e dom ain reflectom etry and probes with rem otely shorted diodes as described by Hook et al. (1992). Transpired w ater was re placed every 2 or 3 days and soil water, unless stated otherwise, was generally m ain tained betw een 0 .0 6 -0 .0 7 m^ m ”^ (see A ppendix A, Fig. A .l).
Experimental protocol
E ffects o f air temperature on stom atal opening
P rior to treatm ent, seedlings were acclim atized in the cuvette system for three to four days, and A and E m easured continuously. During this period, Fair, D and Ca were main tained at 20 ± 0.05 °C, 1 ± 0.02 kPa and 350 ± 2 /im ol mol”', respectively.
R esponses of whole-seedling A and to a step change in Q (from overnight dark- o 1 0 0 0 fxmo\ m”^
exponential equations:
ness to 1 0 0 0 /nmol m ^ s ') w ere described as a function of time (t) using first-order
A = A m a x [ l - e ( - ^ ^ 4 ( 1 1 )
g s = g s m a x [ l - e ( ” ^ % ) ] ( 1 . 2 )
w here A,nax and g; max are the steady-state assim ilation rate and stomatal conductance, respectively, m easured over one hour, 2 -3 h after the lights were sw itched on; and Tg^ are the tim e constant for photosynthetic and stomatal responses, respectively. Estimates o f T were obtained w ith a non-linear least squares routine using the Levenberg-M arquardt algorithm (K aleidaG raph, Abelbeck Software, Reading, PA, USA). The time taken for a 90% change is calculated as 2.3 T.
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 1
Experim ents were carried out on three western redcedar seedlings to determ ine m easurem ent repeatability. Each seedling was held at a given air tem perature for at least 20 h fo r the determ ination o f Ta and Tg^. Tim e constants were determ ined over a 5 -2 5 °C range im posed at random. Any changes in Tair were imposed at a m axim um rate o f 0.33 °C m in” ' and w ere com pleted at least 8 -1 2 h before lights were switched on. At each tem perature, m easurem ents were repeated (once per day) three to nine times per seedling. F or Fair ^ 10 °C, D was held at 1.0 kPa and for Tgir < 10 °C, D was 0.6 kPa.
A second set o f experim ents was carried out to determ ine intra- and inter-species variation in 1% and Tg^. M easurem ents were m ade on at least three seedlings per species and provenance. Time constants were m easured at Fair o f 5, 15 and 25 °C selected at random . In all cases, D w as 0.6 kPa.
E jfects o f length o f dark periods on stom atal opening
Experim ents w ere conducted on western redcedar seedlings to establish the relation betw een the length o f the dark period and Ta and Tg^ in response to illum ination. Time constants w ere determ ined after overnight darkness (1 2 -1 4 h) and after 1-2 h periods of darkness im posed at different times during the day. M easurem ents were carried out at Tgir o f 10, 15, and 20 °C (im posed at random) w ith D at 1.0 kPa. At each temperature, meas urem ents w ere repeated at least three times per seedling.
P henom enological m odel
Sim ple m ultiplicative models (Jarvis 1976; Livingston & Black 1987; Jones 1992) were used to predict A and g^ of western redcedar seedlings as a function o f Q, Fair, D and T ,
whereby:
Ch a p t e r I - Stomatal opening in relation to temperature 1 2
8 s ~ 8s max f ( Q ) ^ ( ^ a i r ) h{D) ( 1 - 4 )
w h e re /, g and h are functions that describe the relation between steady-state i4 and and
Q, Tair and D , respectively; and j describes the relation betw een A and g^ and time during
the transition dynam ics (Eqns 1.1 and 1.2). For simplicity, all variables were assum ed to act independently but multiplicatively. W hen determ ining the individual functions/ g and h in Eqns 1.3 and 1.4, attempts w ere made to use general form s o f relationships already established in the literature. Daily carbon assim ilation and transpiration were calculated by integrating A and E over the corresponding photoperiod.
E stim ation o f the param eters f o r the m odel
F u n c tio n s/, g and h used in Eqns 1.3 and 1.4 were determ ined for three seedlings. For each seedling, the photosynthetic and stomatal responses to the following sequence of treatm ents w ere determ ined over three successive days: (i) Q was progressively de creased from 1200 to 0 /tm ol m“^ s~' over 10 h while Fair was held at 25 °C and D = 0.75 kPa; (ii) Tgjr w as reduced from 25 to 5 °C by 5 °C every 2 h, while Q and D were held at
1200 /tm ol m“^ s“ ' and 0.75 kPa, respectively; (iii) D was increased by 0.5 kPa every 2 h from 0.5 to 3.0 kPa with Q = 1200 /imol m“^ s“' and Tgir = 25 °C. All o f the above treat m ents w ere im posed 1-2 h after the lights were sw itched on, when A and gs had reached steady-state. In further experim ents, seedlings were subjected to overnight changes in air tem perature to determ ine the relation between Fair and respiration in the dark.
C om parison between m odel and measurements f o r dynam ic changes in Q
The seedlings used to derive the coefficients in Eqns 1.3 and 1.4 w ere subjected to diur nal fluctuations of Q (/tmol m~^ s“ ') and Fgjr described by the following:
C h a p t e r I - Stomatal opening in relation to temperature 1 3
7’air= 15 + [10 X sin 0 .2618 ( r -10)] (1.6)
where t is the time o f day in h. These conditions approxim ate those found in coastal southern B ritish C olum bia in mid-spring. In separate experim ents, the vapor pressure deficit was eith er held at 0.75 kPa throughout the day or was varied between 0.4 and 2.7 kPa as a function of tem perature. In both cases, the 12 h days were assum ed to be cloud less. In additional experim ents, seedlings were exposed to variations in Q that simulated cloudy conditions and under-canopy shading. Interm ittent cloud cover and understory shading w ere approxim ated by exposing seedlings to 3 - 4 episodes (> 1 h) o f shade (200 o r 500 n m o l m"^ s“’) or com plete darkness during the day.
M easurem ents o f /I and gs were com pared to estim ates obtained using Eqns 1.3 and 1.4, respectively, with and w ithout (Model 1 and M odel 2, respectively) time constant functions.
R e s u l t s
Response of stomatal conductance and assimilation rate to changes in Q
To facilitate com parison o f the photosynthetic and stom atal responses of different conifer seedlings to a step change in Q at given air tem peratures, A and gs w ere normalized with respect to A^ax and gs max. and tim e constants (Ta and %^) were determ ined using Eqns
1.1 and 1.2, respectively. Coefficients o f determ ination (R^) o f the fits to Eqns 1.1 and 1.2 w ere typically > 0 .9 0 and never less than 0.80. Standard errors o f the estimates (SEE) o f Ta and Tg^ w ere generally < 5%.
B etw een 5 and 25 °C, A and gs generally increased with increasing temperature (Fig. 1.1). Typically, differences in A and gs among species were largest at the lowest Fair- Tim e constants for A and gs, following a step change in Q, increased linearly with de creasing air tem perature (Fig. 1.2). In western redcedar seedlings, Ta increased three fold
C h a f p e r 1 - Stomatal opening in relation to temperature 1 4
(from 10 min) w hen was decreased from 25 to 5 °C. In general, values for Tg^ were slightly larger (1 0 -1 5 %) than those for W ithin a given species, there was consider able variability in T betw een individual seedlings, particularly at low tem peratures. For example at Fair o f 5 °C, 1% in w estern hem lock seedlings (n = 4) ranged from 35 to 61 min. A sim ilar variation w as found in loblolly pine [Tg^ ranged from 36 to 73 min {n = 6); D. W hitehead (Landcare Research, Lincoln, NZ), personal com m unication]. Coefficients of variation for all species varied betw een 6 and 28%. However, for a given individual, variation in T was generally small. F or exam ple, repeated m easurem ents (one per day
over 7 d) m ade on a w estern redcedar seedling at 10 °C yielded a m ean of 28.7 min with a standard deviation o f 2 . 0 min.
Differences in Ta am ong species w ere least pronounced at 25 °C but increased significantly with decreasing Fair (Fig. 1.2). Below 25 °C, western hem lock had the largest time constants follow ed by western redcedar and Douglas-fir. T here was not a statistically significant difference in x between coastal and interior provenances o f Doug las-fir. W hile there were som e individuals from wet habitats (e.g. redcedar from Jervis inlet; Fig. 1.2b) that had larger Tthan those from dry habitats (Duncan), in general, for a given species, there was not a consistent relationship between habitat type and time constant. For exam ple, for w estern redcedar seedlings from the Sunshine Coast (Fig.
1.2a), a relatively wet habitat, was sim ilar to that o f seedlings from the relatively dry Duncan habitat (Fig. 1.2b).
Tim e constants for increases in A, after periods o f darkness that ranged from 1-2 h and were im posed at different tim es during a 12 h photoperiod, were generally lower than those m easured after overnight darkness (Table 1.1). Even though these differences were not statistically significant (F-test: P > 0.44), the low er time constants were used to model photosynthetic and stom atal responses to short periods o f darkness im posed during the day.
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 5 25 °C 15 °C I
s
I
=1 5 °C<
- 2 300 180 240 -6 0 0 60 120Time after illumination (min)
200 25 °C
: (b)
160 CP 15 °C O— o. o o 5 °C 40 180 240 300 0 60 120 -6 0Time after illumination (min)
Figure 1.1 Tim e course o f (a) whole-seedling assim ilation rate (A) and (b) stomatal
conductance to w ater vapor (gs) in western hem lock in response to a step change (at time = 0) in photon flux density from 0 to 1000 /tm ol m“^ s"*. M easurem ents were conducted over three successive days at 25, 5 and 15 °C.
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 6 0
■ (a )
A Western hemlock □ Western redcedar o Douglas fir c < 40I
100 1 ---o Jervis inlet - □ Duncan■ (b )
80 40 10 15 20 25Air temperature (°C)
30Figure 1.2 R elation betw een tim e constant (t^) for increases in assim ilation rate (/t),
follow ing a step change in photon flux density from 0 to 1 0 0 0 Jim ol m~^ s” ', and air tem perature (Tair, °C) for (a) three conifer species: each data point represents the average value (±SD) o f at least three seedlings. Douglas-fir (n = 6), Ta = 2 1 .4 -0 .4 5 T air,
= 0.99; W estern redcedar {n = 3), Ta = 34 .2 - 0.967air. = 0.99; W estern hem lock (n = 4), Ta = 52.5 - 1.697’air, = 0.97; (b) three redcedar seedlings: one from Jervis inlet (relatively wet habitat) and two from Duncan (relatively dry habitat). Each data point represents the m ean value (± SD ) of three to nine m easurem ents at each Ta,r for each seedling. Soil w ater contents during measurem ents were 0 .0 2 6 -0 .0 5 3 m^ m“^ for the Jervis inlet provenance and 0 .0 6 4 -0 .1 0 5 m^ m“^ for the Duncan provenance. Jervis inlet: Ta = 90.8 - 2.81 Tair, R^ = 0.81; Duncan: Ta = 35.8 - 1.01 Tair, R^ = 0.96.
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 7
Table 1.1 Tim e constants (m ean ± SD) for increases in assim ilation rates {A) o f western
redcedar seedlings follow ing a step change in photon flux density (0 to 1 0 0 0 /tmol m“^ s“') at three different air tem peratures following (i) ovem ight darkness and (ii) 1 - 2 h dark periods im posed during the day. N um bers o f measurem ents are shown in parentheses.
Length of the dark period (h)
Tim e constant (min)
10 °C 15 °C 20 °C 1 - 2 12-14 20.3 ± 4 .3 ( 1 7 ) 24.5 ± 4 .5 ( 1 3 ) 14.9 ± 1.5 (7 ) 18.7 ± 3 .2 (6) 10.1 ± 1.7 ( 1 6 ) 1 3 . 4 ± 2 . 0 ( l l )
C h a p t e r 1 - Stomatal opening in relation to temperature 1 8
Phenomenological model and daily carbon gain
F ollow ing Livingston & B lack (1987) and Jones (1992), photosynthetic and stomatal responses to changes in Q, and D were best described by the follow ing functions:
yC6 ) = 1 - e -= (G -G 'l (1.7)
g ( W = 1 - 6 ( T a i r - T m a x ) ^ ( 1 . 8 )
h { D ) = m + { D / D o . 5 f ] (1.9)
w here Q ’ is the light com pensation point; T^ax is the tem perature at w hich A and gs are at a m axim um ; Dq.s is the vapor pressure deficit when A and g , are half the m axim um, and
a, b and c are constants (see Appendix A, Figs A 2-A 4). Values o f the coefficients a, b, c, Q \ Tmax and Dq.j used in Eqns 1.7-1.9 are given in Table 1.2. Coefficients of determ ina
tion for the relationship between m easured and modelled A or gs using a function o f any single variable ranged from 0.86 to 0.99 (Table 1.2).
T im e courses o f A, E, g, and WUE in response to diurnal changes (assuming a cloudless day) in Q, fair (Eqns 1.5 & 1.6, respectively), and D (Figs 1.3a-b) were very well described by the phenom enological m odels regardless o f whether or not a function that accounted for T was included (Figs 1.3c-f). For example, there was an overall differ ence o f 2.2% (R^ = 0.99, ^ =17, Table 1.3) and 1.4% (R^ = 0.99, =10) between m od elled and m easured daily A for M odel 1 ( t function included) and M odel 2 ( t function excluded), respectively (Fig. 1.3c). Generally, gsund E were slightly overestim ated for the first 2 -3 h after lights were sw itched on, and at the end o f the photoperiod (Figs 1 .3 d -e). Typically, there was about a 2% difference between m odelled (including t) and m easured daily E w hen D was held constant at 0.75 kPa (data not shown) and a 5% difference when D w as allowed to vary with fair (Fig. 1.3d).
Ch a p t e r 1 - Stomatal opening in relation to temperature 1 9
Table 1.2 Values o f the coefficients (± SEE) in Eqns 1.7-1.9 used to predict whole-plant
assim ilation rate (A, jUmol m”^ s“’) and stomatal conductance (gs, mmol s“ ') of w estern redcedar seedlings. Q is the photon flux density (/imol m“^ s“ '); 7air is the air tem perature (°C); D is the vapor pressure deficit (kPa); Q ’ is the light com pensation point (jUmol s“'), Ttnax is the tem perature (°C) at w hich A and gs are at a maximum; and
Do s is the vapor pressure deficit (kPa) when A and g, are half the m aximum. Also shown
are the coefficients o f determ ination (R'^) o f the fits to Eqns 1.7-1.9.
Variable Function Coefficients
/?’-A a = 0.0056 ( ± 0 . 0 0 0 3 ) Q ’ = 19.2 ( ± 3 . 1 ) 0.99 6 = 0.0023 ( ± 0 . 0 0 0 3 ) = 2 1 .2 ( ± 0 . 9 ) 0.99 A(D) c = 2.2 (± 0.4) Dgg = 3.6 (±0.3) 0.89 S s ycG) a = 0.0043 ( ± 0 . 0 0 0 3 ) Q '= 1 0 0 0.96 6 = 0.0012 (±0.0004) = 25.3 (±3.3) 0 .8 6 A(D) c = 2.4 (± 0.4) , = 1.5 (±0.1) 0.91
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 0 1 0 0 0 - 8 0 0 ^ 6 0 0 4 0 0 •a 200 2 4 Cu Time (h) (b )
I
2 . 5 I 1 . 5 CL, 5 1. 0 o S' > 0 . 5 20 Time (h) M o d el 2 M o d el 1 M easured M odel 2 - 2 Time (h) 0 - 1 ui
CLI
f t
J!
Figure 1.3 (a-c) Tim e course o f (a) air tem perature (Tair) and photon flux density (Q),
(b) vapor pressure (g j and vapor pressure deficit (£)). (c) Typical daily course of m easured and m odelled assim ilation rate (2\) of a western redcedar seedling. Model 1 (Eqn 1.3) includes a tim e constant function (T^; Eqn 1.1) that is om itted from M odel 2. The differences betw een m easured and m odelled values are shown in (c).
Ch a p t e r 1 - Stomatal opening in relation to temperature 21 M o d el 2 M easu red 1 2 15 Time (h) M o d el 2 M o d el I M easured M o d el 2 12 15 Time (h) E 1 0 6xr 0 1% 'e - 1 0 1 00 " o E M easu red
I
E "oi
u D M odel 2 - 5 Time (h)Figure 1.3 (d-f) Typical daily course o f measured and modelled (d) transpiration rate (£),
(e) stomatal conductance (gg) and (f) w ater use efficiency (W UE) o f a western redcedar seedling. M odel 1 (Eqn 1.3) includes a tim e constant function (tx; Eqn 1.1) that is omitted from M odel 2. The differences betw een measured and m odelled values are shown in (e).
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 2
W hen periods o f relatively high g (1000 ^m ol vcT^ s“ ') were interdispersed with episodes o f com plete darkness, and and D kept constant, Model 1 predicted A suc cessfully (Fig 1.4). There was a 0.4% difference between m odelled and m easured daily A w hen Tair = 10 °C = 0.99, ^ = 42, Table 1.3) and only a 0.2% difference when Tgir = 20 °C (R^ = 0.99, = 51). In contrast. M odel 2 overestim ated changes in A with Q, leading to a difference of 21.4% and 13.0% between m odelled and m easured daily A for Tair o f 10 °C and 20 °C, respectively (Table 1.3). In cases where periods of high g (1 0 0 0 - 1500 /im ol m ““ s“ ’) alternated w ith periods o f shade (2 0 0 -5 0 0 /im ol m “^ s“’), and Tair and D w ere kept constant, fluctuations in A w ith Q were well described by both models (Fig. 1.5). However, the inclusion o f a function for T led to slightly better estim ates of daily A (Table 1.3). The difference between m odelled (including t) and m easured daily A
was 2.3% when Tair = 15 °C (Fig. 1.5a) and 0.6% when Tair = 25 °C (Fig. 1.5b; Table 1.3). In all cases w here there was one or m ore periods o f shade or total darkness during the day, the inclusion o f a time constant function in Eqn 1.3 led to higher R^ and low er ^ values. This was particularly significant at low Tair.
Di s c u s s i o n
Step changes in Q, following prolonged periods of darkness, bring about increases in A that result from changes in both stom atal conductance and photosynthetic induction state (K irschbaum & Pearcy 1988a,b; Tinoco-O janguren & Pearcy 1993b; Pearcy et al. 1994). There is evidence that upon illum ination with relatively high Q, enzym e activation (in cluding that o f R ubisco) and replenishm ent o f m etabolite pools are usually com plete within 10 min (Pearcy et al. 1994). Further, studies o f the regulation o f Rubisco activity by light indicate that biochem ical limitations o f A during induction do not differ among species (Seem ann et al. 1988; Woodrow & M ott 1989; Tinoco-O janguren & Pearcy 1993b). In this study, I did not specifically exam ine the relative contributions of stomatal
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 3
I
eI
<
Time (h)
1 2(a):
M easured M odel 1 M odel 2 10 8 6 4 2 0 2 18 15 21 6 9 12 12 M easured M odel 1 M odel 2 10 8 6 4 2 0 2 15 18 21 6 9 12Time (h)
F ig u re 1.4 D aily course of m easured and modelled assim ilation rate (4) o f a western redcedar seedling subjected to step changes in photon flux density ( 0 between 0 and 1000 jtrmol m“^ s“ ' and held at an air temperature o f (a) 10 °C and (b) 20 °C. The vapor pressure deficit was 1.0 kPa. M odel 1 (Eqn 1.3) includes a tim e constant function (t^; Eqn 1.1) that is om itted from M odel 2. The solid bars indicate the periods during which darkness w as im posed. The arrows indicate w hen the lights were switched on in the m orning and o ff at night.
Ch a p t e r I - Stomatal opening in relation to temperature 2 4
Table 1.3 Values o f coefficients (± SEE) in the linear regression Eqn: Amodel = a + b Ameas for w estern redcedar seedlings exposed to periods o f high photon flux density (g ) interrupted by periods o f darkness or shade, or exposed to diurnal changes in Q without interruption. A ir tem peratures (fair) were held at 10, 15, 20, and 25 °C, or varied diur- nally. The vapor pressure deficit w as 1.0 kPa when Tair was held constant, or varied diurnally as a function o f T^\^. M odel 1 (Eqn 1.3) includes a time constant function (T*; Eqn 1.1) that is om itted from M odel 2. Also shown are the coefficients of determ i nation (/?2) and the chi-square (%^) values o f the regressions, and the differences (%) between daily Amodel and daily
Amcas-Conditions Model a b R} t ‘Diff(%)
Diurnal changes -0.12 ( ±0.04) 1.02 (±0.01) 0.99 17 -2.2 in 6 and T.^ 2 (no T^) -0.08 ( ±0.03) 1.02 (±0.01) 0.99 10 -1.4 Dark periods 1 0.03 (±0.02) 0.99 (±0.01) 0.99 42 +0.4
( r =io°C)
2 0.49 ( ±0.08) 1.03 (±0.02) 0.87 735 +21.4 Dark periods 1 -0.05 (±0.02) 1.02 (±0.01) 0.99 51 -0.2 ( r = 2 0 °C ) 2 0.19 ( ±0.06) 1.05 (±0.01) 0.93 462 +13.0 Shade periods 1 -0.01 (±0.02) 0.98 (±0.01) 0.99 45 -2.3( r = i5°c)
2 0.16 ( ±0.06) 1.00 (±0.01) 0.93 295 +3.6 Shade periods 1 0.16 ( ±0.03) 0.98 (±0.01) 0.99 37 +0.6 ( r = 2 5 = 0 2 0.25 ( ±0.04) 0.98 (±0.01) 0.98 111 +2.3^ approaches 0 w hen deviations from fit are small. * D iff (% ) = [(M odel - M easured)/M easured] x 100
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 5
I
12 10 1500 8 1000 1000 6 500 4 200 200 2 M easured M odel 1 M odel 2 0 - 2 15 18 21 6 9 12Time (h)
01
12(b):
10 1500 1500 1 000 , 1000 8 6 500 4 2 0 0 M easured M odel 1 M odel 2 W 200 2 0 2Time (h)
F ig u re 1.5 Daily course of m easured and m odelled assim ilation rate (A) o f a western redcedar seedling subjected to alternating periods of high photon flux density {Q = 1000 or 1500 /im ol m“^ s " ') and shade (200 or 500 /im ol m~^ s”’) and held at an air tem pera ture o f (a) 15 °C and (b ) 25 °C. The vapor pressure deficit was 1.0 kPa. M odel 1 (Eqn 1.3) includes a tim e constant function (t^; Eqn 1.1) that is om itted from Model 2. The solid and hatched bars indicate the periods during w hich darkness and shade, respec tively, w ere imposed. T he num bers represent Q for each period o f illum ination. The arrows indicate when the lights w ere switched on in the m orning and off at night.
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 6
and biochem ical lim itations during the various phases of induction. However, the results suggest that any differences in photosynthetic response am ong species and individuals that persisted for at least 10 min after a step change in Q, m ust have been related prim a rily to differences in their stom atal response.
T he linear increases in and Tg^, with decreasing Tair were alm ost certainly directly related to the effects o f tem perature on enzym e activity. Typically, the rates o f reactions catalyzed by enzym es increase exponentially with increasing tem perature until dénatura tion rapidly reduces the activity o f the enzym es (Taiz & Z eiger 1991). However, for the range o f Tair used in this study, increases in reaction rates are generally linear.
T here is som e evidence that in woody angiosperms, stom ata open m ore rapidly in shade-tolerant than in shade-intolerant species (Woods & T urner 1971; D avies & K ozlow ski 1974). However, Pereira & Kozlow ski ( 1977) did not find a consistent rela tion betw een r a n d shade tolerance. Western redcedar and w estern hem lock are both considered to be very tolerant to shade, w hereas D ouglas-fir is regarded as having inter m ediate shade tolerance (Kramer & Kozlowski 1979). B ased on this tolerance rating, the results suggest that shade-tolerant seedlings have relatively large T for m orning stomatal opening. However, the values of fg, for western hemlock seedlings are sim ilar to those reported for loblolly pine seedlings (W hitehead & Teskey 1995), a species considered to be shade intolerant (K ram er & Kozlowski 1979).
Tim e constants for 4 upon illum ination following dark periods o f 1-2 h were about 25% low er than those m easured after overnight darkness, at any given tem perature. This is consistent with results obtained for loblolly pine (W hitehead & Teskey 1995), that revealed that as the length of shade periods was increased from 5 to 60 min, there was a corresponding and linear increase in Tg, upon illum ination. However, the loblolly pine data show that beyond 60 min (tg^ = 46 + 3 m in), there were only small differences in Tg^ follow ing shade or overnight darkness (Tg^ = 50 ± 6 min ). These results suggest that in
Ch a p t e r 1 - Stomatal opening in relation to temperature 2 7
conifer seedlings, com plete stom atal adjustm ent and enzyme deactivation are generally attained w ithin 1 h o f shade or darkness.
In experim ents (data not shown) during w hich conifer seedlings w ere exposed to sinusoidal variations in tem perature but Q and D kept constant, late afternoon A was consistently higher than that m easured at the same tem perature in the early morning. These differences in A generally disappeared when sinusoidal changes in Tair were im posed 1 - 2 h after, rather than before illumination, that is after A and g, had both reached steady-state values. This suggests that follow ing illumination after prolonged darkness, stom ata can lim it C O2 assim ilation for extensive periods. However, the extent o f stomatal lim itation o f A does vary am ong coniferous species (Teskey et al. 1986; M einzer 1982a). These results are consistent w ith the observations that after a step change in Q, and Tg^ were very similar, giving rise to concom itant increases in A and
gs-In other experim ents when sudden shade was imposed, there was a corresponding decrease in C O2 assim ilation, followed by a slow er decrease as A w as lim ited by the slow er change in gs. This type o f response has also been reported by W hitehead & Teskey (1995).
F urther evidence o f stom atal lim itation o f A, after a rapid increase in Q, was pro vided, using the analysis o f K irschbaum & Pearcy (1988a) and Barradas & Jones (1996), by plots o f A as a function o f intercellular CO2 concentration (q ). Initially, dA/dq was negative and points w ere below the steady-state A vs. q curve (Fig. 1.6). However, over the next 15 to 20 m in, changes in Cj were small as dA/dcj approached the steady-state, and thereafter dA/dcj was positive and constant indicating that increases in A resulted from higher c, brought about by increases in g,.
G as exchange m odels that include functions that account for photosynthetic and stom atal dynam ics (e.g. K irschbaum , Gross & Pearcy 1988; Knapp 1992, 1993; Barradas & Jones 1996; Pearcy, Gross & He 1997) are often used to assess the carbon gain and
