植被-大气相互作用中的气孔导度及其尺度转换

气孔导度是衡量植物和大气间水分、能量及CO2平衡和循环的重要指标,探讨气孔导度在叶片、冠层及区域尺度间的尺度转换及累积效应,对更好地认识植被与大气间的水热运移过程,合理评价植被在陆面过程中的地位和作用具有重要意义.本文着重从叶片尺度气孔导度模拟、气孔导度在冠层尺度的累积表现、冠层到区域尺度转换研究及气孔导度累积效应在陆面过程模型中的作用等4个层次总结了近期国内外研究状况,指出其中存在的异质性等问题,并就今后应加强多尺度间的同步观测提出了展望.     

The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions

This review summarizes current understanding of the mechanisms that underlie the response of photosynthesis and stomatal conductance to elevated carbon dioxide concentration ([CO2]), and examines how downstream processes and environmental constraints modulate these two fundamental responses. The results from free-air CO2 enrichment (FACE) experiments were summarized via meta-analysis to quantify the mean responses of stomatal and photosynthetic parameters to elevated [CO2]. Elevation of [CO2] in FACE experiments reduced stomatal conductance by 22%, yet, this reduction was not associated with a similar change in stomatal density. Elevated [CO2] stimulated light-saturated photosynthesis (Asat) in C3 plants grown in FACE by an average of 31%. However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis at elevated [CO2] had greater potential for increases in Asat than those where photosynthesis became ribulose-1,5-bisphosphate (RubP)-limited at elevated [CO2]. Both nitrogen supply and sink capacity modulated the response of photosynthesis to elevated [CO2] through their impact on the acclimation of carboxylation capacity. Increased understanding of the molecular and biochemical mechanisms by which plants respond to elevated [CO2], and the feedback of environmental factors upon them, will improve our ability to predict ecosystem responses to rising [CO2] and increase our potential to adapt crops and managed ecosystems to future atmospheric [CO2].     

Plant water relations at elevated CO2 -- implications for water-limited environments

Long-term exposure of plants to elevated [CO2] leads to a number of growth and physiological effects, many of which are interpreted in the context of ameliorating the negative impacts of drought. However, despite considerable study, a clear picture in terms of the influence of elevated [CO2] on plant water relations and the role that these effects play in determining the response of plants to elevated [CO2] under water-limited conditions has been slow to emerge. In this paper, four areas of research are examined that represent critical, yet uncertain, themes related to the response of plants to elevated [CO2] and drought. These include (1) fine-root proliferation and implications for whole-plant water uptake; (2) enhanced water-use efficiency and consequences for drought tolerance; (3) reductions in stomatal conductance and impacts on leaf water potential; and (4) solute accumulation, osmotic adjustment and dehydration tolerance of leaves. A survey of the literature indicates that the growth of plants at elevated [CO2] can lead to conditions whereby plants maintain higher (less negative) leaf water potentials. The mechanisms that contribute to this effect are not fully known, although CO2-induced reductions in stomatal conductance, increases in whole-plant hydraulic conductance and osmotic adjustment may be important. Less understood are the interactive effects of elevated [CO2] and drought on fine-root production and water-use efficiency, and the contribution of these processes to plant growth in water-limited environments. Increases in water-use efficiency and reductions in water use can contribute to enhanced soil water content under elevated [CO2]. Herbaceous crops and grasslands are most responsive in this regard. The conservation of soil water at elevated [CO2] in other systems has been less studied, but in terms of maintaining growth or carbon gain during drought, the benefits of CO2-induced improvements in soil water content appear relatively minor. Nonetheless, because even small effects of elevated [CO2] on plant and soil water relations can have important implications for ecosystems, we conclude that this area of research deserves continued investigation. Future studies that focus on cellular mechanisms of plant response to elevated [CO2] and drought are needed, as are whole-plant investigations that emphasize the integration of processes throughout the soil--plant--atmosphere continuum. We suggest that the hydraulic principles that govern water transport provide an integrating framework that would allow CO2-induced changes in stomatal conductance, leaf water potential, root growth and other processes to be uniquely evaluated within the context of whole-plant hydraulic conductance and water transport efficiency.     

Elevated CO2 effects on canopy and soil water flux parameters measured using a large chamber in crops grown with free-air CO2 enrichment

An arable crop rotation (winter barley-sugar beet-winter wheat) was exposed to elevated atmospheric CO(2) concentrations ([CO(2) ]) using a FACE facility (Free-Air CO(2) Enrichment) during two rotation periods. The atmospheric [CO(2) ] of the treatment plots was elevated to 550 ppm during daylight hours (T>5°C). Canopy transpiration (E(C) ) and conductance (G(C) ) were measured at selected intervals (>10% of total growing season) using a dynamic CO(2) /H(2) O chamber measuring system. Plant available soil water content (gravimetry and TDR probes) and canopy microclimate conditions were recorded in parallel. Averaged across both growing seasons, elevated [CO(2) ] reduced E(C) by 9%, 18% and 12%, and G(C) by 9%, 17% and 12% in barley, sugar beet and wheat, respectively. Both global radiation (Rg) and vapour pressure deficit (VPD) were the main driving forces of E(C) , whereas G(C) was mostly related to Rg. The responses of E(C) and especially G(C) to [CO(2) ] enrichment were insensitive to weather conditions and leaf area index. However, differences in LAI between plots counteracted the [CO(2) ] impact on E(C) and thus, at least in part, explained the variability of seasonal [CO(2) ] responses between crops and years. As a consequence of lower transpirational canopy water loss, [CO(2) ] enrichment increased plant available soil water content in the course of the season by ca. 15 mm. This was true for all crops and years. Lower transpirational cooling due to a [CO(2) ]-induced reduction of E(C) increased canopy surface and air temperature by up to 2 °C and 0.5 °C, respectively. This is the first study to address effects of FACE on both water fluxes at canopy scale and water status of a European crop rotation.     

Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].     

Decreases in stomatal conductance of soybean under open-air elevation of [CO2] are closely coupled with decreases in ecosystem evapotranspiration

Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO(2) ([CO(2)]) have been shown to decrease stomatal conductance (g(s)) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease in g(s) will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO(2)] on g(s) at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 micromol CO(2) mol(-1) air) and elevated [CO(2)] (approximately 550 micromol mol(-1)) using free air CO(2) enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO(2)] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with g(s) of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in g(s) results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO(2)]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased g(s) of upper canopy leaves at elevated [CO(2)] results in decreased transfer of water vapor to the atmosphere.     

Lower responsiveness of canopy evapotranspiration rate than of leaf stomatal conductance to open‐air CO2 elevation in rice

An elevated atmospheric CO₂ concentration ([CO₂]) can reduce stomatal conductance of leaves for most plant species, including rice (Oryza sativa L.). However, few studies have quantified seasonal changes in the effects of elevated [CO₂] on canopy evapotranspiration, which integrates the response of stomatal conductance of individual leaves with other responses, such as leaf area expansion, changes in leaf surface temperature, and changes in developmental stages, in field conditions. We conducted a field experiment to measure seasonal changes in stomatal conductance of the uppermost leaves and in the evapotranspiration, transpiration, and evaporation rates using a lysimeter method. The study was conducted for flooded rice under open‐air CO₂ elevation. Stomatal conductance decreased by 27% under elevated [CO₂], averaged throughout the growing season, and evapotranspiration decreased by an average of 5% during the same period. The decrease in daily evapotranspiration caused by elevated [CO₂] was more significantly correlated with air temperature and leaf area index (LAI) rather than with other parameters of solar radiation, days after transplanting, vapor‐pressure deficit and FAO reference evapotranspiration. This indicates that higher air temperatures, within the range from 16 to 27 °C, and a larger LAI, within the range from 0 to 4 m² m^?2, can increase the magnitude of the decrease in evapotranspiration rate caused by elevated [CO₂]. The crop coefficient (i.e. the evapotranspiration rate divided by the FAO reference evapotranspiration rate) was 1.24 at ambient [CO₂] and 1.17 at elevated [CO₂]. This study provides the first direct measurement of the effects of elevated [CO₂] on rice canopy evapotranspiration under open‐air conditions using the lysimeter method, and the results will improve future predictions of water use in rice fields.     

Water relations and gas exchange in poplar and willow under water stress and elevated atmospheric CO2

Predictions of shifts in rainfall patterns as atmospheric [CO2] increases could impact the growth of fast growing trees such as Populus spp. and Salix spp. and the interaction between elevated CO2 and water stress in these species is unknown. The objectives of this study were to characterize the responses to elevated CO2 and water stress in these two species, and to determine if elevated CO2 mitigated drought stress effects. Gas exchange, water potential components, whole plant transpiration and growth response to soil drying and recovery were assessed in hybrid poplar (clone 53-246) and willow (Salix sagitta) rooted cuttings growing in either ambient (350 &mgr;mol mol-1) or elevated (700 &mgr;mol mol-1) atmospheric CO2 concentration ([CO2]). Predawn water potential decreased with increasing water stress while midday water potentials remained unchanged (isohydric response). Turgor potentials at both predawn and midday increased in elevated [CO2], indicative of osmotic adjustment. Gas exchange was reduced by water stress while elevated [CO2] increased photosynthetic rates, reduced leaf conductance and nearly doubled instantaneous transpiration efficiency in both species. Dark respiration decreased in elevated [CO2] and water stress reduced Rd in the trees growing in ambient [CO2]. Willow had 56% lower whole plant hydraulic conductivity than poplar, and showed a 14% increase in elevated [CO2] while poplar was unresponsive. The physiological responses exhibited by poplar and willow to elevated [CO2] and water stress, singly, suggest that these species respond like other tree species. The interaction of [CO2] and water stress suggests that elevated [CO2] did mitigate the effects of water stress in willow, but not in poplar.     

Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 micromol mol(-1)) and elevated [CO2] (550 micromol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.     

Comparative field water relations of three Mediterranean shrub species co-occurring at a natural CO(2) vent

Annual variations in the water relations and stomatal response of Erica arborea, Myrtus communis and Juniperus communis occurring at a natural CO(2) vent were analysed under Mediterranean field conditions. A distinct gradient of CO(2)concentration ([CO(2)]) exists between two sites near a natural CO(2)-emitting vent, with higher [CO(2)] (700 micromol mol(-1)) in the proximity of the CO(2) spring. Plants at the CO(2) spring site have been growing for generations at elevated [CO(2)]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during the summer drought were severe. Leaf conductance and water potential recovered after major rainfalls in September to predrought values. Strong relationships between leaf conductance, predawn water potential, and leaf-specific hydraulic resistance are consistent with the role of stomata in regulating plant water status. Considerable between-species variation in sensitivity of water potentials and stomatal characters to elevated [CO(2)] were observed. Common to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response to elevated [CO(2)]. Elevated [CO(2)] decreased the sensitivity of leaf conductance to vapour pressure deficit. Morphological characters (including stomatal density and degree of sclerophylly) showed site-dependent variations, but degree and sign of such changes varied with the species and/or the season. Measurements of discrimination against (13)C provided evidence for long-term decreases of water use efficiency in CO(2) spring plants. Analysis of C isotope composition suggested that a downward adjustment of photosynthetic capacity may have occurred under elevated [CO(2)]. Elevated [CO(2)] effects on water relations and leaf morphology persisted in the long term, but the three shrubs growing in the same environment showed species-specific responses.     

Effects of elevated [CO2] and/or ozone on limitations to CO2 assimilation in soybean (Glycine max)

Soybean (Glycine max) was grown in open-top field chambers at ambient (360 mol mol^-1) or doubled [CO2] either in charcoal-filtered air (20 nmol mol^-1 [O3]) or in non-filtered air supplemented to 1,5 x ambient [O3] (70 nmol mol^-1) to determine the major limitations to assimilation under conditions of elevated [CO2] and/or [O3]. Through plant ontogeny, assimilation versus intercellular CO2 concentration (A/Ci) responses were measured to assess the limitations to assimilation imposed by the capacity for Rubisco carboxylation, RuBP regeneration, and stomatal diffusion.In the vegetative stages, no significant treatment effects of elevated [CO2] and/or [O3] were observed on Rubisco carboxylation efficiency (CE), light and CO2-saturated assimilation capacity (Amax), and chlorophyll content (Chl). However, for plants grown in elevated [CO2], the assimilation rate at growth [CO2] (A) was 60% higher than at ambient [CO2] up to the seed maturation stage, and the potential rate of assimilation by Rubisco capacity (Ap) was increased. Also in elevated [CO2]: A was 51% of Ap; the relative stomatal limitation (%Stomata) was 5%; and the relative RuBP regeneration limitation (%RuBP) was 44%. In ambient [CO2], O3 gradually decreased A per unit leaf area, but had little effect on Ap and the relative limitations to assimilation where A remained 51% of Ap, %Stomata was 27%, and %RuBP was 22%.During reproduction, CE declined for plants grown in elevated [CO2] and/or [O3]; Ap was unaffected by elevated [CO2], but was reduced by [O3] at ambient [CO2]; A increased to 72% of Ap in elevated [CO2] and/or [O3]-fumigated air; the %Stomata increased; and the %RuBP decreased, to become non significant in elevated [CO2] from the beginning of seed growth on, and in O3-fumigated air at ambient [CO2] at the seed maturation stage. The decrease in %RuBP occurred concomitantly with an increase in Amax and Chl. Significant [CO2] x [O3] interactions support the lack of an O3 effect on assimilation and its limitations at elevated [CO2] during seed maturation. These data suggest that elevated [CO2] alleviated some of the effects of O3 on photosynthesis.Keywords: CO2 by O3 interactions, elevated [CO2], O3 fumigation, Rubisco carboxylation efficiency, RuBP regeneration.      

Increasing canopy photosynthesis in rice can be achieved without a large increase in water use—A model based on free‐air CO2 enrichment

Achieving higher canopy photosynthesis rates is one of the keys to increasing future crop production; however, this typically requires additional water inputs because of increased water loss through the stomata. Lowland rice canopies presently consume a large amount of water, and any further increase in water usage may significantly impact local water resources. This situation is further complicated by changing the environmental conditions such as rising atmospheric CO₂ concentration ([CO₂]). Here, we modeled and compared evapotranspiration of fully developed rice canopies of a high‐yielding rice cultivar (Oryza sativa L. cv. Takanari) with a common cultivar (cv. Koshihikari) under ambient and elevated [CO₂] (A‐CO₂ and E‐CO₂, respectively) via leaf ecophysiological parameters derived from a free‐air CO₂ enrichment (FACE) experiment. Takanari had 4%–5% higher evapotranspiration than Koshihikari under both A‐CO₂ and E‐CO₂, and E‐CO₂ decreased evapotranspiration of both varieties by 4%–6%. Therefore, if Takanari was cultivated under future [CO₂] conditions, the cost for water could be maintained at the same level as for cultivating Koshihikari at current [CO₂] with an increase in canopy photosynthesis by 36%. Sensitivity analyses determined that stomatal conductance was a significant physiological factor responsible for the greater canopy photosynthesis in Takanari over Koshihikari. Takanari had 30%–40% higher stomatal conductance than Koshihikari; however, the presence of high aerodynamic resistance in the natural field and lower canopy temperature of Takanari than Koshihikari resulted in the small difference in evapotranspiration. Despite the small difference in evapotranspiration between varieties, the model simulations showed that Takanari clearly decreased canopy and air temperatures within the planetary boundary layer compared to Koshihikari. Our results indicate that lowland rice varieties characterized by high‐stomatal conductance can play a key role in enhancing productivity and moderating heat‐induced damage to grain quality in the coming decades, without significantly increasing crop water use.     

Long-term growth of soybean at elevated [CO2] does not cause acclimation of stomatal conductance under fully open-air conditions

Accurately predicting plant function and global biogeochemical cycles later in this century will be complicated if stomatal conductance (g(s)) acclimates to growth at elevated [CO(2)], in the sense of a long-term alteration of the response of g(s) to [CO(2)], humidity (h) and/or photosynthetic rate (A). If so, photosynthetic and stomatal models will require parameterization at each growth [CO(2)] of interest. Photosynthetic acclimation to long-term growth at elevated [CO(2)] occurs frequently. Acclimation of g(s) has rarely been examined, even though stomatal density commonly changes with growth [CO(2)]. Soybean was grown under field conditions at ambient [CO(2)] (378 micromol mol(-1)) and elevated [CO(2)] (552 micromol mol(-1)) using free-air [CO(2)] enrichment (FACE). This study tested for stomatal acclimation by parameterizing and validating the widely used Ball et al. model (1987, Progress in Photosynthesis Research, vol IV, 221-224) with measurements of leaf gas exchange. The dependence of g(s) on A, h and [CO(2)] at the leaf surface was unaltered by long-term growth at elevated [CO(2)]. This suggests that the commonly observed decrease in g(s) under elevated [CO(2)] is due entirely to the direct instantaneous effect of [CO(2)] on g(s) and that there is no longer-term acclimation of g(s) independent of photosynthetic acclimation. The model accurately predicted g(s) for soybean growing under ambient and elevated [CO(2)] in the field. Model parameters under ambient and elevated [CO(2)] were indistinguishable, demonstrating that stomatal function under ambient and elevated [CO(2)] could be modelled without the need for parameterization at each growth [CO(2)].     

Effects of Elevated Atmospheric CO2 Concentration on Leaf Anatomy and Morphology in Panicum Species Representing Different Photosynthetic Modes

Panicum tricanthum Nees, Panicum antidotale Retz., and Panicum decipiens Nees ex Trin. were selected to represent C3, C4, and C3/C4 intermediate perennial species of Panicum, respectively. Plants grown from seed with 900 ppm [CO2] under natural sunlight and controlled temperatures (30 degrees /22 degrees C) were compared with plants grown with ambient [CO2]. The anatomy of the last fully expanded leaf of the main tiller was studied by light microscopy with computerized graphic image analysis and by transmission electron microscopy. Leaf anatomy did not change qualitatively in response to elevated [CO2], but there were changes in leaf thickness and in the proportions of total transsectional area occupied by mesophyll, bundle sheath cells, vascular elements, and sclerenchyma, according to species. The abaxial stomatal frequency decreased by 22% for P. tricanthum but increased by ca. 30% for the other two species. With 900 ppm CO2, all three species showed a considerable increase in leaf starch content (to >30% of dry matter). Starch granules accumulated in chloroplasts of the mesophyll and bundle sheath cells. Increased leaf glaucousness in response to elevated [CO2] was the result of increased or modified deposition of epicuticular wax on both leaf surfaces, a response to elevated [CO2] that is unusual and one that has not been previously recorded for monocotyledons. The wax patterns were studied by scanning electron microscopy. Panicum decipiens did not respond to elevated [CO2] in a truly intermediate fashion; its responses resembled those of either the C3 or the C4 species. C3/C4 intermediates may thus be interpreted as developmental chimeras and not as species in transition between C3 and C4 modes in an evolutionary sense.     

Photosynthetic stimulation under long-term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE)

The long-term response of leaf photosynthesis to rising CO2 concentrations [CO2] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO2] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand. After 6 yr of exposure to free-air CO2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar (Populus) short-rotation coppice. Net photosynthetic rate (A(growth)) was 49% higher in poplars grown in elevated [CO2], independently of the canopy position. Jmax significantly increased (15%), whereas leaf carboxylation capacity (Vcmax), leaf nitrogen (N(a)) and chlorophyll (Chl(a)) were unaffected in elevated [CO2]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown. These results suggest that the photosynthetic stimulation by elevated [CO2] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.     

Effects of water deficit and its interaction with CO(2) supply on the biochemistry and physiology of photosynthesis in sunflower

Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO(2) (A=350 or E=700 micromol mol(-1), respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Psi(w)) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g(s)) and thereby transpiration, but increased net CO(2) assimilation rate (P(n), short-term measurements); therefore, water-use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P(n) to intercellular CO(2) concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in energy metabolism associated with E were small, despite the decreased Rubisco content. Water deficits decreased g(s) and P(n): metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO(2). The decrease in P(n) with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO(2) during growth and water deficit with respect to photosynthetic metabolism. Elevated CO(2 )will benefit sunflower growing under water deficit by marginally increasing P(n), and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.     

Leaf and canopy photosynthetic characteristics of cotton (Gossypium hirsutum) under elevated CO2 concentration and UV-B radiation

Increases in both atmospheric CO2 concentration ([CO2]) and ultraviolet-B (UV-B) radiation on the Earth's surface are features of current climate change patterns. An experiment was conducted in sunlit, controlled environment chambers known as Soil-Plant-Atmosphere-Research (SPAR) units to determine interactive effects of elevated [CO2] and UV-B radiation on leaf and canopy photosynthetic characteristics of cotton. Six treatments were comprised of two CO2 levels of 360 (ambient) and 720 (elevated) microL L(-1) and three levels of 0 (control), 8, and 16 kJ m(-2) d(-1) biologically effective UV-B radiation. Treatments were imposed for 66 days from crop emergence through three weeks after the first flower stage. Plants grown in elevated [CO2] had significantly greater leaf area, higher leaf and canopy net photosynthetic rates (PN), lower dark respiration rate (Rd), and lower light compensation point (LCP) than plants grown in ambient [CO2]. There was no difference in CO2 compensation point (gamma), maximum rate of Rubisco activity (Vcmax), or light-saturated rate of electron transport (Jmax) between ambient and elevated CO2 treatments. When plants were grown in 8 kJ m(-2) d(-1) UV-B radiation, most of the measured photosynthetic parameters did not differ from control plants. High UV-B (16 kJ) radiation, however, caused 47-50% smaller leaf area, 38-44% lower leaf PN, 72-74% lower Vcmax, and 61-66% lower Jmax compared to the control. There were no interactive effects of [CO2] and UV-B radiation on most of the photosynthetic parameters measured. From the results, it is concluded that decreased canopy photosynthesis due to enhanced UV-B radiation in cotton is associated with both smaller leaf area and lower leaf PN, and loss of Rubisco activity and electron transport are two major factors in UV-B inhibition of leaf PN.     

How do leaf hydraulics limit stomatal conductance at high water vapour pressure deficits?

A reduction in leaf stomatal conductance (g) with increasing leaf-to-air difference in water vapour pressure (D) is nearly ubiquitous. Ecological comparisons of sensitivity have led to the hypothesis that the reduction in g with increasing D serves to maintain leaf water potentials above those that would cause loss of hydraulic conductance. A reduction in leaf water potential is commonly hypothesized to cause stomatal closure at high D. The importance of these particular hydraulic factors was tested by exposing Abutilon theophrasti, Glycine max, Gossypium hirsutum and Xanthium strumarium to D high enough to reduce g and then decreasing ambient carbon dioxide concentration ([CO2]), and observing the resulting changes in g, transpiration rate and leaf water potential, and their reversibility. Reducing the [CO2] at high D increased g and transpiration rate and lowered leaf water potential. The abnormally high transpiration rates did not result in reductions in hydraulic conductance. Results indicate that low water potential effects on g at high D could be overcome by low [CO2], and that even lower leaf water potentials did not cause a reduction in hydraulic conductance in these well-watered plants. Reduced g at high D in these species resulted primarily from increased stomatal sensitivity to [CO2] at high D, and this increased sensitivity may mediate stomatal responses to leaf hydraulics at high D.     

气孔导度对CO_2浓度变化的模拟及其生理机制

基于气孔运动的生理生化机制重点进行了气孔导度(gs)对CO2浓度变化的响应机制分析,并推导得到气孔导度(gs)对CO2浓度变化响应模型,并以9种植物进行了模型验证。结果表明:随着CO2浓度的升高,气孔导度会逐渐降低,且下降的幅度会随着CO2浓度的升高而逐渐减弱。气孔导度对CO2浓度(Cs)变化的响应模型可以表达为gs=gmax/(1+Cs/Cs0),其中式中gmax是最大气孔导度和Cs0是实验常数。该模型较好地模拟了气孔导度随CO2浓度变化的规律,模型参数具有明确的生理意义,与Jarvis模型和Ball-Berry模型相比,该模型如何实现多种环境因子的耦合有待进一步突破。另外,模型是在短期改变叶片CO2浓度的条件下得出的,在CO2浓度长期胁迫下的适用性也有待进一步确认。     

Morphology and Stomatal Function of Douglas Fir Needles Exposed to Climate Change: Elevated CO2 and Temperature

Climate change may have an impact on the productivity of conifer trees by influencing the morphology (size and surface characteristics) and function (capacity for gas exchange) of conifer needles. In order to test the responses of needles to climatic variables, Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), saplings were grown in sunlit controlled environment chambers at ambient or elevated (+200 parts per million above ambient) CO2 and at ambient or elevated temperature (+4 degrees C above ambient). Needle characteristics, including length, width, area, stomatal density (stomata per mm2), percentage of stomatal occlusion, and the morphology of epicuticular wax, were evaluated. Needle function was evaluated as stomatal conductance to water vapor and transpiration. Needle length increased significantly with elevated temperature but not with elevated CO2. Neither elevated CO2 nor elevated temperature affected stomatal density or stomatal number in these hypostomatous needles. Epicuticular wax was less finely granular at elevated than at ambient temperature and was similar in appearance at elevated and ambient CO2. Stomatal conductance and transpiration increased with elevated temperature and associated increased vapor pressure deficit; however, neither conductance nor transpiration was affected by elevated CO2. These results indicate that simulated climate change influences Douglas fir needle structure and function.