Rising atmospheric carbon dioxide concentration and the future of C4 crops for food and fuel

Crops with the C₄ photosynthetic pathway are vital to global food supply, particularly in the tropical regions where human well-being and agricultural productivity are most closely linked. While rising atmospheric [CO₂] is the driving force behind the greater temperatures and water stress, which threaten to reduce future crop yields, it also has the potential to directly benefit crop physiology. The nature of C₄ plant responses to elevated [CO₂] has been controversial. Recent evidence from free-air CO₂ enrichment (FACE) experiments suggests that elevated [CO₂] does not directly stimulate C₄ photosynthesis. Nonetheless, drought stress can be ameliorated at elevated [CO₂] as a result of lower stomatal conductance and greater intercellular [CO₂]. Therefore, unlike C₃ crops for which there is a direct enhancement of photosynthesis by elevated [CO₂], C₄ crops will only benefit from elevated [CO₂] in times and places of drought stress. Current projections of future crop yields have assumed that rising [CO₂] will directly enhance photosynthesis in all situations and, therefore, are likely to be overly optimistic. Additional experiments are needed to evaluate the extent to which amelioration of drought stress by elevated [CO₂] will improve C₄ crop yields for food and fuel over the range of C₄ crop growing conditions and genotypes.     

Leaf and canopy scale drivers of genotypic variation in soybean response to elevated carbon dioxide concentration

The atmospheric [CO₂] in which crops grow today is greater than at any point in their domestication history and represents an opportunity for positive effects on seed yield that can counteract the negative effects of greater heat and drought this century. In order to maximize yields under future atmospheric [CO₂], we need to identify and study crop cultivars that respond most favorably to elevated [CO₂] and understand the mechanisms contributing to their responsiveness. Soybean (Glycine max Merr.) is a widely grown oilseed crop and shows genetic variation in response to elevated [CO₂]. However, few studies have studied the physiological basis for this variation. Here, we examined canopy light interception, photosynthesis, respiration and radiation use efficiency along with yield and yield parameters in two cultivars of soybean (Loda and HS93‐4118) previously reported to have similar seed yield at ambient [CO₂], but contrasting responses to elevated [CO₂]. Seed yield increased by 26% at elevated [CO₂] (600 μmol/mol) in the responsive cultivar Loda, but only by 11% in HS93‐4118. Canopy light interception and leaf area index were greater in HS93‐4118 in ambient [CO₂], but increased more in response to elevated [CO₂] in Loda. Radiation use efficiency and harvest index were also greater in Loda than HS93‐4118 at both ambient and elevated [CO₂]. Daily C assimilation was greater at elevated [CO₂] in both cultivars, while stomatal conductance was lower. Electron transport capacity was also greater in Loda than HS93‐4118, but there was no difference in the response of photosynthetic traits to elevated [CO₂] in the two cultivars. Overall, this greater understanding of leaf‐ and canopy‐level photosynthetic traits provides a strong conceptual basis for modeling genotypic variation in response to elevated [CO₂].     

Productivity and water use of wheat under free-air CO2 enrichment

A free-air CO₂ enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE apparatus maintained the CO₂ concentration, [CO₂], at 550 μmol mol^?1 across four replicate 25-m-diameter circular plots under natural conditions in an open field. Four matching Control plots at ambient [CO₂] (about 370 μmol mol^?1) were also installed in the field. In addition to the two levels of [CO₂], there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system in a strip, split-plot design. Measurements were made of net radiation, R_n; soil heat flux, G_o; soil temperature; foliage or surface temperature; air dry and wet bulb temperatures; and wind speed. Sensible heat flux, H, was calculated from the wind and temperature measurements. Latent heat flux, λET, and evapotranspiration, ET, were determined as the residual in the energy balance. The FACE treatment reduced daily total R_n by an average 4%. Daily FACE sensible heat flux, H, was higher in the FACE plots. Daily latent heat flux, λET, and evapotranspiration, ET, were consistently lower in the FACE plots than in the Control plots for most of the growing season, about 8% on the average. Net canopy photosynthesis was stimulated by an average 19 and 44% in the Wet and Dry plots, respectively, by elevated [CO₂] for most of the growing season. No significant acclimation or down regulation was observed. There was little above-ground growth response to elevated [CO₂] early in the season when temperatures were cool. Then, as temperatures warmed into spring, the FACE plants grew about 20% more than the Control plants at ambient [CO₂], as shown by above-ground biomass accumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 °C warmer than the Controls from February through April in the well-watered plots, and we speculate that this temperature rise contributed to the earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, there was a narrowing of the final biomass and yield differences. The 20% mid-season growth advantage of FACE shrunk to about an 8% yield advantage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.     

Influence of certain environmental factors on photosynthesis and photorespiration in Simmondsia chinensis

The response of net photosynthesis and apparent light respiration to changes in [O₂], light intensity, and drought stress was determined by analysis of net photosynthetic CO₂ response curves. Low [O₂] treatment resulted in a large reduction in the rate of photorespiratory CO₂ evolution. Lightintensity levels influenced the maximum net photosynthetic rate at saturating [CO₂]. These results indicate that [CO₂], [O₂] and light intensity affect the levels of substrates involved in the enzymatic reactions of photosynthesis and photorespiration. Intracellular resistance to CO₂ uptake decreased in low [O₂] and increased at low leaf water potentials. This response reflects changes in the efficiency with which photosynthetic and photorespiratory substrates are formed and utilized. Water stress had no effect on the CO₂ compensation point or the [CO₂] at which net photosynthesis began to saturate at high light intensity. The relationship between these data and recently published in-vitro kinetic measurements with ribulose-diphosphate carboxylase is discussed.Abbreviations C _w intracellular CO₂ concentration - F _gross gross photosynthesis - F _net net photosynthesis - I light intensity - R _L light respiration rate - r _c carboxylation resistance - r ₈ leaf gas-phase resistance - r _i intracellular resistance; to CO₂ uptake - r _t resistance to CO₂ flux between the intercellular spaces and the carboxylation sites - T _L leaf temperature - _t leaf water potential - CO₂ compensation point     

Effects of fully open-air [CO2] elevation on leaf ultrastructure, photosynthesis, and yield of two soybean cultivars

The objective of this study was to investigate the effect of elevated (550 ± 17 ??mol mol^?1) CO₂ concentration ([CO₂]) on leaf ultrastructure, leaf photosynthesis and seed yield of two soybean cultivars [Glycine max (L.) Merr. cv. Zhonghuang 13 and cv. Zhonghuang 35] at the Free-Air Carbon dioxide Enrichment (FACE) experimental facility in North China. Photosynthetic acclimation occurred in soybean plants exposed to long-term elevated [CO₂] and varied with cultivars and developmental stages. Photosynthetic acclimation occurred at the beginning bloom (R1) stage for both cultivars, but at the beginning seed (R5) stage only for Zhonghuang 13. No photosynthetic acclimation occurred at the beginning pod (R3) stage for either cultivar. Elevated [CO₂] increased the number and size of starch grains in chloroplasts of the two cultivars. Soybean leaf senescence was accelerated under elevated [CO₂], determined by unclear chloroplast membrane and blurred grana layer at the beginning bloom (R1) stage. The different photosynthesis response to elevated [CO₂] between cultivars at the beginning seed (R5) contributed to the yield difference under elevated [CO₂]. Elevated [CO₂] significantly increased the yield of Zhonghuang 35 by 26% with the increased pod number of 31%, but not for Zhonghuang 13 without changes of pod number. We conclude that the occurrence of photosynthetic acclimation at the beginning seed (R5) stage for Zhonghuang 13 restricted the development of extra C sink under elevated [CO₂], thereby limiting the response to elevated [CO₂] for the seed yield of this cultivar.     

FACE facts hold for multiple generations; Evidence from natural CO2 springs

Rising atmospheric CO₂ concentration is a key driver of enhanced global greening, thought to account for up to 70% of increased global vegetation in recent decades. CO₂ fertilization effects have further profound implications for ecosystems, food security and biosphere‐atmosphere feedbacks. However, it is also possible that current trends will not continue, due to ecosystem level constraints and as plants acclimate to future CO₂ concentrations. Future predictions of plant response to rising [CO₂] are often validated using single‐generation short‐term FACE (Free Air CO₂ Enrichment) experiments but whether this accurately represents vegetation response over decades is unclear. The role of transgenerational plasticity and adaptation in the multigenerational response has yet to be elucidated. Here, we propose that naturally occurring high CO₂ springs provide a proxy to quantify the multigenerational and long‐term impacts of rising [CO₂] in herbaceous and woody species respectively, such that plasticity, transgenerational effects and genetic adaptation can be quantified together in these systems. In this first meta‐analysis of responses to elevated [CO₂] at natural CO₂ springs, we show that the magnitude and direction of change in eight of nine functional plant traits are consistent between spring and FACE experiments. We found increased photosynthesis (49.8% in spring experiments, comparable to 32.1% in FACE experiments) and leaf starch (58.6% spring, 84.3% FACE), decreased stomatal conductance (g_s, 27.2% spring, 21.1% FACE), leaf nitrogen content (6.3% spring, 13.3% FACE) and Specific Leaf Area (SLA, 9.7% spring, 6.0% FACE). These findings not only validate the use of these sites for studying multigenerational plant response to elevated [CO₂], but additionally suggest that long‐term positive photosynthetic response to rising [CO₂] are likely to continue as predicted by single‐generation exposure FACE experiments.     

Improving ecophysiological simulation models to predict the impact of elevated atmospheric CO2 concentration on crop productivity

Background

Process-based ecophysiological crop models are pivotal in assessing responses of crop productivity and designing strategies of adaptation to climate change. Most existing crop models generally over-estimate the effect of elevated atmospheric [CO₂], despite decades of experimental research on crop growth response to [CO₂].

Analysis

A review of the literature indicates that the quantitative relationships for a number of traits, once expressed as a function of internal plant nitrogen status, are altered little by the elevated [CO₂]. A model incorporating these nitrogen-based functional relationships and mechanisms simulated photosynthetic acclimation to elevated [CO₂], thereby reducing the chance of over-estimating crop response to [CO₂]. Robust crop models to have small parameterization requirements and yet generate phenotypic plasticity under changing environmental conditions need to capture the carbon–nitrogen interactions during crop growth.

Conclusions

The performance of the improved models depends little on the type of the experimental facilities used to obtain data for parameterization, and allows accurate projections of the impact of elevated [CO₂] and other climatic variables on crop productivity.     

Soybean leaf hydraulic conductance does not acclimate to growth at elevated [CO2] or temperature in growth chambers or in the field

Background and Aims

Leaf hydraulic properties are strongly linked with transpiration and photosynthesis in many species. However, it is not known if gas exchange and hydraulics will have co-ordinated responses to climate change. The objective of this study was to investigate the responses of leaf hydraulic conductance (K_leaf) in Glycine max (soybean) to growth at elevated [CO₂] and increased temperature compared with the responses of leaf gas exchange and leaf water status.

Methods

Two controlled-environment growth chamber experiments were conducted with soybean to measure K_leaf, stomatal conductance (g_s) and photosynthesis (A) during growth at elevated [CO₂] and temperature relative to ambient levels. These results were validated with field experiments on soybean grown under free-air elevated [CO₂] (FACE) and canopy warming.

Key results

In chamber studies, K_leaf did not acclimate to growth at elevated [CO₂], even though stomatal conductance decreased and photosynthesis increased. Growth at elevated temperature also did not affect K_leaf, although g_s and A showed significant but inconsistent decreases. The lack of response of K_leaf to growth at increased [CO₂] and temperature in chamber-grown plants was confirmed with field-grown soybean at a FACE facility.

Conclusions

Leaf hydraulic and leaf gas exchange responses to these two climate change factors were not strongly linked in soybean, although g_s responded to [CO₂] and increased temperature as previously reported. This differential behaviour could lead to an imbalance between hydraulic supply and transpiration demand under extreme environmental conditions likely to become more common as global climate continues to change.     

30 years of free‐air carbon dioxide enrichment (FACE): What have we learned about future crop productivity and its potential for adaptation?

Free‐air CO₂ enrichment (FACE) allows open‐air elevation of [CO₂] without altering the microclimate. Its scale uniquely supports simultaneous study from physiology and yield to soil processes and disease. In 2005 we summarized results of then 28 published observations by meta‐analysis. Subsequent studies have combined FACE with temperature, drought, ozone, and nitrogen treatments. Here, we summarize the results of now almost 250 observations, spanning 14 sites and five continents. Across 186 independent studies of 18 C₃ crops, elevation of [CO₂] by ca. 200 ppm caused a ca. 18% increase in yield under non‐stress conditions. Legumes and root crops showed a greater increase and cereals less. Nitrogen deficiency reduced the average increase to 10%, as did warming by ca. 2°C. Two conclusions of the 2005 analysis were that C₄ crops would not be more productive in elevated [CO₂], except under drought, and that yield responses of C₃ crops were diminished by nitrogen deficiency and wet conditions. Both stand the test of time. Further studies of maize and sorghum showed no yield increase, except in drought, while soybean productivity was negatively affected by early growing season wet conditions. Subsequent study showed reduced levels of nutrients, notably Zn and Fe in most crops, and lower nitrogen and protein in the seeds of non‐leguminous crops. Testing across crop germplasm revealed sufficient variation to maintain nutrient content under rising [CO₂]. A strong correlation of yield response under elevated [CO₂] to genetic yield potential in both rice and soybean was observed. Rice cultivars with the highest yield potential showed a 35% yield increase in elevated [CO₂] compared to an average of 14%. Future FACE experiments have the potential to develop cultivars and management strategies for co‐promoting sustainability and productivity under future elevated [CO₂].     

Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free‐air concentration enrichment (FACE)

The C₄ grass Zea mays (maize or corn) is the third most important food crop globally in terms of production and demand is predicted to increase 45% from 1997 to 2020. However, the effects of rising [CO₂] upon C₄ plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free‐air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated [CO₂] on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient [CO₂] (354 μ mol mol^?1) and the elevated [CO₂] (549 μmol mol^?1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C₄ photosynthesis is not typically enhanced by elevated [CO₂]. However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO₂]. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated [CO₂] significantly increased leaf photosynthetic CO₂ uptake rate (A) by up to 41%, and 10% on average. Greater A was associated with greater intercellular [CO₂], lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50‐year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C₄ photosynthesis is insensitive to elevated [CO₂] under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in [CO₂].     

Smaller than predicted increase in aboveground net primary production and yield of field-grown soybean under fully open-air [CO2] elevation

The Intergovernmental Panel on Climate Change projects that atmospheric [CO₂] will reach 550 ppm by 2050. Numerous assessments of plant response to elevated [CO₂] have been conducted in chambers and enclosures, with only a few studies reporting responses in fully open‐air, field conditions. Reported yields for the world's two major grain crops, wheat and rice, are substantially lower in free‐air CO₂ enrichment (FACE) than predicted from similar elevated [CO₂] experiments within chambers. This discrepancy has major implications for forecasting future global food supply. Globally, the leguminous‐crop soybean (Glycine max (L.) Merr.) is planted on more land than any other dicotyledonous crop. Previous studies have shown that total dry mass production increased on average 37% in response to increasing [CO₂] to approximately 700 ppm, but harvestable yield will increase only 24%. Is this representative of soybean responses under open‐air field conditions? The effects of elevation of [CO₂] to 550 ppm on total production, partitioning and yield of soybean over 3 years are reported. This is the first FACE study of soybean ( http://www.soyface.uiuc.edu ) and the first on crops in the Midwest of North America, one of the major food production regions of the globe. Although increases in both aboveground net primary production (17–18%) and yield (15%) were consistent across three growing seasons and two cultivars, the relative stimulation was less than projected from previous chamber experiments. As in previous studies, partitioning to seed dry mass decreased; however, net production during vegetative growth did not increase and crop maturation was delayed, not accelerated as previously reported. These results suggest that chamber studies may have over‐estimated the stimulatory effect of rising [CO₂], with important implications on global food supply forecasts.     

Do all leaf photosynthesis parameters of rice acclimate to elevated CO2, elevated temperature,and their combination,in FACE environments?

Leaf photosynthesis of crops acclimates to elevated CO₂ and temperature, but studies quantifying responses of leaf photosynthetic parameters to combined CO₂ and temperature increases under field conditions are scarce. We measured leaf photosynthesis of rice cultivars Changyou 5 and Nanjing 9108 grown in two free‐air CO₂ enrichment (FACE) systems, respectively, installed in paddy fields. Each FACE system had four combinations of two levels of CO₂ (ambient and enriched) and two levels of canopy temperature (no warming and warmed by 1.0–2.0°C). Parameters of the C₃ photosynthesis model of Farquhar, von Caemmerer and Berry (the FvCB model), and of a stomatal conductance (g_s) model were estimated for the four conditions. Most photosynthetic parameters acclimated to elevated CO₂, elevated temperature, and their combination. The combination of elevated CO₂ and temperature changed the functional relationships between biochemical parameters and leaf nitrogen content for Changyou 5. The g_s model significantly underestimated g_s under the combination of elevated CO₂ and temperature by 19% for Changyou 5 and by 10% for Nanjing 9108 if no acclimation was assumed. However, our further analysis applying the coupled g_s–FvCB model to an independent, previously published FACE experiment showed that including such an acclimation response of g_s hardly improved prediction of leaf photosynthesis under the four combinations of CO₂ and temperature. Therefore, the typical procedure that crop models using the FvCB and g_s models are parameterized from plants grown under current ambient conditions may not result in critical errors in projecting productivity of paddy rice under future global change.     

Will intra‐specific differences in transpiration efficiency in wheat be maintained in a high CO2 world? A FACE study

This study evaluates whether the target breeding trait of superior leaf level transpiration efficiency is still appropriate under increasing carbon dioxide levels of a future climate using a semi‐arid cropping system as a model. Specifically, we investigated whether physiological traits governing leaf level transpiration efficiency, such as net assimilation rates (A_net), stomatal conductance (g_s) or stomatal sensitivity were affected differently between two Triticum aestivum L. cultivars differing in transpiration efficiency (cv. Drysdale, superior; cv. Hartog, low). Plants were grown under Free Air Carbon dioxide Enrichment (FACE, approximately 550 µmol mol^?1 or ambient CO₂ concentrations (approximately 390 µmol mol^?1). Mean A_net (approximately 15% increase) and g_s (approximately 25% decrease) were less affected by elevated [CO₂] than previously found in FACE‐grown wheat (approximately 25% increase and approximately 32% decrease, respectively), potentially reflecting growth in a dry‐land cropping system. In contrast to previous FACE studies, analyses of the Ball et al. model revealed an elevated [CO₂] effect on the slope of the linear regression by 12% indicating a decrease in stomatal sensitivity to the combination of [CO₂], photosynthesis rate and humidity. Differences between cultivars indicated greater transpiration efficiency for Drysdale with growth under elevated [CO₂] potentially increasing the response of this trait. This knowledge adds valuable information for crop germplasm improvement for future climates.     

Seasonal patterns of photosynthetic response and acclimation to elevated carbon dioxide in field-grown strawberry

Strawberry (Fragaria × ananassa) plants were grown in field plots at the current ambient [CO₂], and at ambient + 300 and ambient + 600 μmol mol⁻¹ [CO₂]. Approximately weekly measurements were made of single leaf gas exchange of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosynthesis by elevated [CO₂], whether growth at elevated [CO₂] resulted in acclimation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosynthesis. Stimulation of photosynthetic CO₂ assimilation by short-term increases in [CO₂] increased strongly with measurement temperature. The stimulation exceeded that predicted from the kinetic characteristics of ribulose-1,5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthesis to growth at elevated [CO₂] was evident from early spring through summer, including the fruiting period in early summer, with lower rates under standard measurement conditions in plants grown at elevated [CO₂]. The degree of acclimation increased with growth [CO₂]. However, there were no significant differences between [CO₂] treatments in total nitrogen per leaf area, and photosynthetic acclimation was reversed one day after switching the [CO₂] treatments. Tests showed that acclimation did not result from a limitation of photosynthesis by triose phosphate utilization rate at elevated [CO₂]. Photosynthetic acclimation was not evident during dry periods in midsummer, when the elevated [CO₂] treatments conserved soil water and photosynthesis declined more at ambient than at elevated [CO₂]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditions and continued higher leaf starch content at elevated [CO₂]. Stomatal conductance responded little to short-term changes in [CO₂] except during drought, and changed in parallel with photosynthetic acclimation through the seasons in response to the long-term [CO₂] treatments. The data do not support the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO₂] in this species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Canopy warming caused photosynthetic acclimation and reduced seed yield in maize grown at ambient and elevated [CO2]

Rising atmospheric CO₂ concentration ([CO₂]) and attendant increases in growing season temperature are expected to be the most important global change factors impacting production agriculture. Although maize is the most highly produced crop worldwide, few studies have evaluated the interactive effects of elevated [CO₂] and temperature on its photosynthetic physiology, agronomic traits or biomass, and seed yield under open field conditions. This study investigates the effects of rising [CO₂] and warmer temperature, independently and in combination, on maize grown in the field throughout a full growing season. Free‐air CO₂ enrichment (FACE) technology was used to target atmospheric [CO₂] to 200 μmol mol^?1 above ambient [CO₂] and infrared heaters to target a plant canopy increase of 3.5 °C, with actual season mean heating of ~2.7 °C, mimicking conditions predicted by the second half of this century. Photosynthetic gas‐exchange parameters, leaf nitrogen and carbon content, leaf water potential components, and developmental measurements were collected throughout the season, and biomass and yield were measured at the end of the growing season. As predicted for a C₄ plant, elevated [CO₂] did not stimulate photosynthesis, biomass, or yield. Canopy warming caused a large shift in aboveground allocation by stimulating season‐long vegetative biomass and decreasing reproductive biomass accumulation at both CO₂ concentrations, resulting in decreased harvest index. Warming caused a reduction in photosynthesis due to down‐regulation of photosynthetic biochemical parameters and the decrease in the electron transport rate. The reduction in seed yield with warming was driven by reduced photosynthetic capacity and by a shift in aboveground carbon allocation away from reproduction. This field study portends that future warming will reduce yield in maize, and this will not be mitigated by higher atmospheric [CO₂] unless appropriate adaptation traits can be introduced into future cultivars.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Can improvement in photosynthesis increase crop yields?

The yield potential (Y_p) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture (?_i), the efficiency of conversion of the intercepted light into biomass (?_c) and the proportion of biomass partitioned into grain (η). Plant breeding brings η and ?_i close to their theoretical maxima, leaving ?_c, primarily determined by photosynthesis, as the only remaining major prospect for improving Y_p. Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y_p. By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO₂] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing ?_c by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO₂. Collectively, these changes could improve ?_c and, therefore, Y_p by c. 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10–15 years.     

Photosynthetic characteristics and nitrogen allocation in the black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) grown in a FACE system

Key message

The black locust is adapted to elevated [CO ₂ ] through changes in nitrogen allocation characteristics in leaves.

Abstract

The black locust (Robinia pseudoacacia L.) is an invasive woody legume within Japan. This prolific species has a high photosynthetic rate and growth rate, and undergoes symbiosis with N₂-fixing micro-organisms. To determine the effect of elevated CO₂ concentration [CO₂] on its photosynthetic characteristics, we studied the chlorophyll (Chl) and leaf nitrogen (N) content, and the leaf structure and N allocation patterns in the leaves and acetylene reduction activity after four growing seasons, in R. pseudoacacia. Our specimens were grown at ambient [CO₂] (370 μmol mol^?1) and at elevated [CO₂] (500 μmol mol^?1), using a free air CO₂ enrichment (FACE) system. Net photosynthetic rate at growth [CO₂] (A _growth) and acetylene reduction activity were significantly higher, but maximum carboxylation rate of RuBisCo (V _cmax), maximum rate of electron transport driving RUBP regeneration (J _max), net photosynthetic rate under enhanced CO₂ concentration and light saturation (A _max), the N concentration in leaf, and in leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) content were significantly lower grown at elevated [CO₂] than at ambient [CO₂]. We also found that RuBisCo/N were less at elevated [CO₂], whereas Chl/N increased significantly. Allocation characteristics from N in leaves to photosynthetic proteins, N_L (Light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and other proteins also changed. When R. pseudoacacia was grown at elevated [CO₂], the N allocation to RuBisCo (N_R) decreased to a greater extent but N_L and N remaining increased relative to specimens grown at ambient [CO₂]. We suggest that N remobilization from RuBisCo is more efficient than from proteins of electron transport (N_E), and from N_L. These physiological responses of the black locust are significant as being an adaptation strategy to global environmental changes.

     

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO₂ indicated an average photosynthetic enhancement of 44% when measured at the growth [CO₂]. When photosynthesis was measured at a common ambient [CO₂], photosynthesis of plants grown at elevated [CO₂] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO₂ enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO₂] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.Abbreviations A/C_i photosynthesis as a function of internal [CO₂] - J_max maximum rate of electron transport - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Vc_max maximum rate of carboxylation The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.