Stomatal sensitivity to vapour pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grassland

In the present study the response of stomatal conductance (g_s) to increasing leaf‐to‐air vapour pressure difference (D) in early season C₃ (Bromus japonicus) and late season C₄ (Bothriochloa ischaemum) grasses grown in the field across a range of CO₂ (200–550 µmol mol^?1) was examined. Stomatal sensitivity to D was calculated as the slope of the response of g_s to the natural log of externally manipulated D (dg_s/dlnD). Increasing D and CO₂ significantly reduced g_s in both species. Increasing CO₂ caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO₂ for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which g_s varies to maintain constant transpiration and leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non‐hydraulic signals influenced guard cell behaviour. Photosynthesis (A) declined with increasing D in both species, but analyses of the ratio of intercellular to atmospheric CO₂ (C_i/C_a) suggested that stomatal limitation of A occurred only in Br. japonicus. Rising CO₂ had the greatest effect on g_s and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthetic responses to CO₂ were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by a seasonal transition from C₃ to C₄ photosynthesis. Interspecific variation in the response of g_s to D therefore has implications for predicting seasonal ecosystem responses to CO₂.     

Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Responses of stomatal conductance (g_s) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of g_s to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of g_s at low D ( ≤ 1 kPa). To test this relationship we used the function g_s = g_sref–m· lnD where m is the stomatal sensitivity and g_sref = g_s at D = 1 kPa. Regardless of species or methodology, m was highly correlated with g_sref (average r² = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of g_s in relation to leaf-surface D (D_s), or on sap flux-based stomatal conductance of whole trees (G_Si), or stand-level stomatal conductance (G_S) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.     

Leaf stomatal responses to vapour pressure deficit under current and CO2-enriched atmosphere explained by the economics of gas exchange

Using the economics of gas exchange, early studies derived an expression of stomatal conductance ( g ) assuming that water cost per unit carbon is constant as the daily loss of water in transpiration ( f _e) is minimized for a given gain in photosynthesis ( f _c). Other studies reached identical results, yet assumed different forms for the underlying functions and defined the daily cost parameter as carbon cost per unit water. We demonstrated that the solution can be recovered when optimization is formulated at time scales commensurate with the response time of g to environmental stimuli. The optimization theory produced three emergent gas exchange responses that are consistent with observed behaviour: (1) the sensitivity of g to vapour pressure deficit ( D ) is similar to that obtained from a previous synthesis of more than 40 species showing g to scale as 1 −  m  log( D ), where m   ∈  [0.5,0.6], (2) the theory is consistent with the onset of an apparent 'feed-forward' mechanism in g , and (3) the emergent non-linear relationship between the ratio of intercellular to atmospheric [CO₂] ( c _i/ c _a) and D agrees with the results available on this response. We extended the theory to diagnosing experimental results on the sensitivity of g to D under varying c _a.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments

Two Italian CO₂ springs allowed us to study the long-term effect of a 350–2600 μ mol mol^–1 increase in CO₂ concentrations on the surface structures of leaves of Quercus ilex L. Carbon dioxide increased the quantity of cuticular waxes, above an apparent threshold of 750 μ mol mol^–1 CO₂. Leaf wettability was not modified by CO₂ concentrations. Reduction in stomatal frequency was observable up to 750 μ mol mol^–1 CO₂, the slope being almost the same as that estimated for the increase in CO₂ concentration from pre-industrial times to the present. At higher concentrations, CO₂ seemed to exert no more impact on stomatal frequency.     

Stomatal behaviour, photosynthesis and transpiration under rising CO2

Definitions of the variables used and the units are given in Table 1

The literature reports enormous variation between species in the extent of stomatal responses to rising CO₂. This paper attempts to provide a framework within which some of this diversity can be explained. We describe the role of stomata in the short-term response of leaf gas exchange to increases in ambient CO₂ concentration by developing the recently proposed stomatal model of Jarvis & Davies (1998 ). In this model stomatal conductance is correlated with the functioning of the photosynthetic system so that the effects of increases in CO₂ on stomata are experienced through changes in the rate of photosynthesis in a simple and mechanistically transparent way. This model also allows us to consider the effects of evaporative demand and soil moisture availability on stomatal responses to photosynthesis and therefore provides a means of considering these additional sources of variation. We emphasize that the relationship between the rate of photosynthesis and the internal CO₂ concentration and also drought will have important effects on the relative gains to be achieved under rising CO₂.  

  Table 1 . Abbreviations     

8.

Elevated CO₂ concentrations and stomatal density: observations from 17 plant species growing in a CO₂ spring in central Italy     

Isabella Bettarini  Francesco P. Vaccari  Franco Miglietta 《Global Change Biology》1998,4(1):17-22

Stomatal density (SD) and stomatal conductance ( g _s) can be affected by an increase of atmospheric CO₂ concentration. This study was conducted on 17 species growing in a naturally enriched CO₂ spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO₂], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO₂ spring and this indicated that CO₂ effects on g _s are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO₂-enriched area. These results did not support the hypothesis that long-term exposure to elevated [CO₂] may cause adaptive modification in stomatal number and in their distribution.     

9.

Effects of elevated CO₂ on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech     

J. HEATH  G. KERSTIENS 《Plant, cell & environment》1997,20(1):57-67

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

10.

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO₂ and/or O₃   总被引：3，自引：2，他引：3  

A. Noormets  A. Sôber  E. J. Pell  R. E. Dickson  G. K. Podila  J. Sôber  J. G. Isebrands  & D. F. Karnosky 《Plant, cell & environment》2001,24(3):327-336

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

11.

Interactions of chronic exposure to elevated CO₂ and O₃ levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica)   总被引：2，自引：0，他引：2  

THORSTEN E. E. GRAMS  SABINE ANEGG  KARL-HEINZ HÄBERLE  CHRISTIAN LANGEBARTELS  & RAINER MATYSSEK 《The New phytologist》1999,144(1):95-107

     

12.

Limitation of net CO₂ assimilation rate by internal resistances to CO₂ transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)     

D. EPRON  D. GODARD  G. CORNIC  B. GENTY 《Plant, cell & environment》1995,18(1):43-51

Using a combination of gas-exchange and chlorophyll fluorescence measurements, low apparent CO₂/O₂ specificity factors (1300 mol mol_?1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO₂ mole fraction between the intercellular airspace (C_i) and the catalytic site of Rubisco (C_c) due to internal resistances to CO₂ transfer. C_c. was calculated assuming a specificity of Rubisco value of 2560 mol mol^?1. The drop between C_i and C_c was used to calculate the internal conductance for CO₂ (g_i). A good correlation between mean values of net CO₂ assimilation rate (A) and g_i was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO₂ transfer, was 24–30%. High internal resistances to CO₂ transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/C_i curves.     

13.

Elevated atmospheric CO₂ alters stomatal responses to variable sunlight in a C₄ grass   总被引：1，自引：1，他引：0  

A. K. KNAPP  J. T. FAHNESTOCK  C. E. OWENSBY 《Plant, cell & environment》1994,17(2):189-195

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

14.

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO₂ and water vapour in the air     

P. J. APHALO  P. G. JARVIS 《Plant, cell & environment》1993,16(7):771-783

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

15.

Stem respiration increases in CO₂-enriched sweetgum trees   总被引：1，自引：1，他引：0  

Nelson T. Edwards  Timothy J. Tschaplinski  Richard J. Norby 《The New phytologist》2002,155(2):239-248

     

16.

Modelling a spring wheat crop under elevated CO₂ and drought     

S. Grossman-Clarke  P. J. Pinter Jr    T. Kartschall  B. A. Kimball  D. J. Hunsaker  G. W. Wall  R. L. Garcia  R. L. LaMorte 《The New phytologist》2001,150(2):315-335

     

17.

The Effect of Elevated [CO₂] on Uptake and Allocation of ¹³C and ¹⁵N in Beech (Fagus sylvatica L.) during Leafing     

J. Dyckmans  H. Flessa  rea Polle  F. Beese 《Plant biology (Stuttgart, Germany)》2000,2(1):113-120

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

18.

Effects of water vapour pressure deficit and stomatal conductance on photosynthesis,internal pressurization and convective flow in three emergent wetland plants   总被引：5，自引：0，他引：5  

Sorrell  B.K.  Brix  H. 《Plant and Soil》2003,253(1):71-79

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (G _s) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and G _s under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australisand lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 mol m^–2 s^–1 photosynthetically active photon flux density (PPFD)) to < 200 and < 10 mol m^–2 s^–1PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and G _s. However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than G _s. In B. articulata, pressures increased at low G _s values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low.     

19.

Non-stomatal limitation of CO₂ assimilation in three tree species during natural drought conditions   总被引：1，自引：0，他引：1  

G. M. Briggs  T. W. Jurik  D. M. Gates 《Physiologia plantarum》1986,66(3):521-526

The effect of drought on CO₂ assimilation and leaf conductance was studied in three northern hardwood species: Quercus rubra L., Acer rubrum L. and Populus grandidentata Michx. Leaf gas exchange characteristics at two CO₂ levels (320 and 620 μl I⁻¹) and temperatures from 20 to 35°C were measured at the end of a dry period and shortly after 10 cm of rainfall. The effects of drought varied with species, temperature and CO₂ level. Calculated values of internal CO₂ concentration showed little or no decline during drought. Differences in assimilation, before vs after the rains, were most apparent at the higher CO₂ level. These latter two observations indicate nonstomatal disruption of CO₂ assimilation during the dry period. In P. grandidentata there was a substantial interaction between drought and temperature, with a resultant shift in the temperature for maximum assimilation to lower temperatures during drought. During drought, internal CO₂ concentrations increased sharply in all three species under the combined conditions of high temperatures and the higher CO₂ level.     

20.

Shade‐tolerance as a predictor of responses to elevated CO₂ in trees     

Gerhard Kerstiens 《Physiologia plantarum》1998,102(3):472-480

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers of differing shade‐tolerance was analysed. The number of intraphyletic comparisons in which the more shade‐tolerant species showed the greater relative increase of biomass in elevated CO₂ was significantly higher than would be expected by chance alone. It is suggested that more shade‐tolerant species are inherently better disposed, in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO₂ and that these traits are responsible for the interaction between shade‐tolerance and CO₂ concentration. Compared with less shade‐tolerant angiosperm trees, more shade‐tolerant angiosperm species generally have a lower leaf area ratio in ambient CO₂ and show a smaller relative reduction in elevated CO₂. Furthermore, leaf nitrogen content is usually lower in more shade‐tolerant angiosperm species and tends to be more strongly reduced by elevated CO₂ in those species. Within angiosperm trees, more shade‐tolerant species showed a stronger stimulation of net leaf photosynthetic rate in most experiments, but this trend was not significant.     

Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy

Stomatal density (SD) and stomatal conductance ( g _s) can be affected by an increase of atmospheric CO₂ concentration. This study was conducted on 17 species growing in a naturally enriched CO₂ spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO₂], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO₂ spring and this indicated that CO₂ effects on g _s are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO₂-enriched area. These results did not support the hypothesis that long-term exposure to elevated [CO₂] may cause adaptive modification in stomatal number and in their distribution.     

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

Limitation of net CO2 assimilation rate by internal resistances to CO2 transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)

Using a combination of gas-exchange and chlorophyll fluorescence measurements, low apparent CO₂/O₂ specificity factors (1300 mol mol_?1) were estimated for the leaves of two deciduous tree species (Fagus sylvatica and Castanea sativa). These low values contrasted with those estimated for two herbaceous species and were ascribed to a drop in the CO₂ mole fraction between the intercellular airspace (C_i) and the catalytic site of Rubisco (C_c) due to internal resistances to CO₂ transfer. C_c. was calculated assuming a specificity of Rubisco value of 2560 mol mol^?1. The drop between C_i and C_c was used to calculate the internal conductance for CO₂ (g_i). A good correlation between mean values of net CO₂ assimilation rate (A) and g_i was observed within a set of data obtained using 13 woody plant species, including our own data. We report that the relative limitation of A, which can be ascribed to internal resistances to CO₂ transfer, was 24–30%. High internal resistances to CO₂ transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calculated from A/C_i curves.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

Effects of water vapour pressure deficit and stomatal conductance on photosynthesis,internal pressurization and convective flow in three emergent wetland plants

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (G _s) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and G _s under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australisand lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 mol m^–2 s^–1 photosynthetically active photon flux density (PPFD)) to < 200 and < 10 mol m^–2 s^–1PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and G _s. However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than G _s. In B. articulata, pressures increased at low G _s values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low.     

Non-stomatal limitation of CO2 assimilation in three tree species during natural drought conditions

The effect of drought on CO₂ assimilation and leaf conductance was studied in three northern hardwood species: Quercus rubra L., Acer rubrum L. and Populus grandidentata Michx. Leaf gas exchange characteristics at two CO₂ levels (320 and 620 μl I⁻¹) and temperatures from 20 to 35°C were measured at the end of a dry period and shortly after 10 cm of rainfall. The effects of drought varied with species, temperature and CO₂ level. Calculated values of internal CO₂ concentration showed little or no decline during drought. Differences in assimilation, before vs after the rains, were most apparent at the higher CO₂ level. These latter two observations indicate nonstomatal disruption of CO₂ assimilation during the dry period. In P. grandidentata there was a substantial interaction between drought and temperature, with a resultant shift in the temperature for maximum assimilation to lower temperatures during drought. During drought, internal CO₂ concentrations increased sharply in all three species under the combined conditions of high temperatures and the higher CO₂ level.     

Shade‐tolerance as a predictor of responses to elevated CO2 in trees

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers of differing shade‐tolerance was analysed. The number of intraphyletic comparisons in which the more shade‐tolerant species showed the greater relative increase of biomass in elevated CO₂ was significantly higher than would be expected by chance alone. It is suggested that more shade‐tolerant species are inherently better disposed, in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO₂ and that these traits are responsible for the interaction between shade‐tolerance and CO₂ concentration. Compared with less shade‐tolerant angiosperm trees, more shade‐tolerant angiosperm species generally have a lower leaf area ratio in ambient CO₂ and show a smaller relative reduction in elevated CO₂. Furthermore, leaf nitrogen content is usually lower in more shade‐tolerant angiosperm species and tends to be more strongly reduced by elevated CO₂ in those species. Within angiosperm trees, more shade‐tolerant species showed a stronger stimulation of net leaf photosynthetic rate in most experiments, but this trend was not significant.