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.     

Exploiting leaf starch synthesis as a transient sink to elevate photosynthesis, plant productivity and yields

Improvements in plant productivity (biomass) and yield have centered on increasing the efficiency of leaf CO₂ fixation and utilization of products by non-photosynthetic sink organs. We had previously demonstrated a correlation between photosynthetic capacity, plant growth, and the extent of leaf starch synthesis utilizing starch-deficient mutants. This finding suggested that leaf starch is used as a transient photosynthetic sink to recycle inorganic phosphate and, in turn, maximize photosynthesis. To test this hypothesis, Arabidopsis thaliana and rice (Oryza sativa L.) lines were generated with enhanced capacity to make leaf starch with minimal impact on carbon partitioning to sucrose. The Arabidopsis engineered plants exhibited enhanced photosynthetic capacity; this translated into increased growth and biomass. These enhanced phenotypes were displayed by similarly engineered rice lines. Manipulation of leaf starch is a viable alternative strategy to increase photosynthesis and, in turn, the growth and yields of crop and bioenergy plants.     

Genotypic variation in source and sink traits affects the response of photosynthesis and growth to elevated atmospheric CO2

This study aimed to understand the response of photosynthesis and growth to e-CO₂ conditions (800 vs. 400 μmol mol⁻¹) of rice genotypes differing in source–sink relationships. A proxy trait called local C source–sink ratio was defined as the ratio of flag leaf area to the number of spikelets on the corresponding panicle, and five genotypes differing in this ratio were grown in a controlled greenhouse. Differential CO₂ resources were applied either during the 2 weeks following heading (EXP1) or during the whole growth cycle (EXP2). Under e-CO₂, low source–sink ratio cultivars (LSS) had greater gains in photosynthesis, and they accumulated less nonstructural carbohydrate in the flag leaf than high source–sink ratio cultivars (HSS). In EXP2, grain yield and biomass gain was also greater in LSS probably caused by their strong sink. Photosynthetic capacity response to e-CO₂ was negatively correlated across genotypes with local C source–sink ratio, a trait highly conserved across environments. HSS were sink-limited under e-CO₂, probably associated with low triose phosphate utilization (TPU) capacity. We suggest that the local C source–sink ratio is a potential target for selecting more CO₂-responsive cultivars, pending validation for a broader genotypic spectrum and for field conditions.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Contrasting Effects of Carbon Dioxide and Irradiance on the Acclimation of Photosynthesis in Developing Soybean Leaves

Leaves developed at high irradiance (I) often have higher photosynthetic capacity than those developed at low I, while leaves developed at elevated CO₂ concentration [CO₂] often have reduced photosynthetic capacity compared with leaves developed at lower [CO₂]. Because both high I and elevated [CO₂] stimulate photosynthesis of developing leaves, their contrasting effects on photosynthetic capacity at maturity suggest that the extra photosynthate may be utilized differently depending on whether I or [CO₂] stimulates photosynthesis. These experiments were designed to test whether relationships between photosynthetic income and the net accumulation of soluble protein in developing leaves, or relationships between soluble protein and photosynthetic capacity at full expansion differed depending on whether I or [CO₂] was varied during leaf development. Soybean plants were grown initially with a photosynthetic photon flux density (PPFD) of 950 µmol m^–2 s^–1 and 350 µmol [CO₂] mol^–1, then exposed to [CO₂] ranging from 135 to 1400 µmol mol^–1 for the last 3 d of expansion of third trifoliolate leaves. These results were compared with experiments in which I was varied at a constant [CO₂] of 350 µmol mol^–1 over the same developmental period. Increases in area and dry mass over the 3 d were determined along with daily photosynthesis and respiration. Photosynthetic CO₂ exchange characteristics and soluble protein content of leaves were determined at the end of the treatment periods. The increase in leaflet mass was about 28 % of the dry mass income from photosynthesis minus respiration, regardless of whether [CO₂] or I was varied, except that very low I or [CO₂] increased this percentage. Leaflet soluble protein per unit of area at full expansion had the same positive linear relationship to photosynthetic income whether [CO₂] or I was varied. For variation in I, photosynthetic capacity varied directly with soluble protein per unit area. This was not the case for variation in [CO₂]. Increasing [CO₂] reduced photosynthetic capacity per unit of soluble protein by up to a factor of 2.5, and photosynthetic capacity exhibited an optimum with respect to growth [CO₂]. Thus CO₂ did not alter the relationship between photosynthetic income and the utilization of photosynthate in the net accumulation of soluble protein, but did alter the relationship between soluble protein content and photosynthetic characteristics in this species.     

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].     

Opposite effects of exogenous sucrose on growth, photosynthesis and carbon metabolism of in vitro plantlets of tomato (L. esculentum Mill.) grown under two levels of irradiances and CO2 concentration

The long-term effects of exogenous sucrose (3 percnt;) on growth, photosynthesis and carbon metabolism ofin vitro tomato plantlets were investigated under two sets of growth conditions that respectively favor source- or sink-limitations of photosynthesis: 1) low photosynthetic photon flux (PPF) (50 μmol m⁻² · s⁻¹) and low CO₂ concentration (400 μmol mol⁻¹) and 2) high PPF (500 μmol m⁻² · s⁻¹ and high CO₂ concentration (4000 μmol mol⁻¹). The supply of sucrose under source-limitation conditions increased the growth, the maximal photosynthetic rate, the chl content, the maximal quantum yield of Photosystem II estimated by the Fv/Fm chl fluorescence ratio as well as the soluble sugars (hexoses, sucrose) and starch contents in roots, young and mature leaves when compared to those of photo-autotrophic plantlets. Also, sucrose feeding under these conditions strongly increased the activity of sucrose synthase (SS) (EC 2.4.1.13) in roots and young leaves whereas the activities of sucrose phosphate synthase (SPS) (EC 2.4.1.14), acid invertase (INV) (EC 3.2.1.26) and ADP-glucose pyrophosphorylase (ADPGppase) (EC 2.7.7.27) were highly stimulated in roots and mature leaves. Contrary to these observations, the supply of sucrose to plantlets developed under high PPF and CO₂ concentration decreased growth and led to a somewhat lower maximal photosynthetic rate relative to photo-autotrophic plantlets. These negative responses to exogenous sucrose were accompanied by stronger accumulations of hexose and starch, larger stimulation of INV in mature leaves developed under conditions of sink limitation than those from source limitation conditions. Moreover, under high PPF and high CO₂ concentration, exogenous sucrose led to a marked repression of the SPS activity and caused much lower stimulations of ADPGppase in mature leaves than those observed at low PPF and low CO₂ concentration. We therefore conclude that under our experimental conditions, the interactive effects of exogenous sucrose and environmental conditions on growth and photosynthesis could be rationalized by the source-sink equilibrium of thein vitro tomato plantlets.     

Responses of Rice Cultivars to the Elevated CO2

The effect of CO₂ concentration elevated to 575 – 620 µmol mol^–1 on growth, tillering, grain yield, net photosynthetic rate, dark respiration rate, stomatal conductance, sugar content and protein profile of two rice (Oryza sativa L.) cultivars Pusa Basmati-1 and Pusa-677 at flowering stage was studied using open top chambers. The cultivar Pusa Basmati-1 responded more markedly for most of the growth and physiological parameters compared to Pusa-677. The increase in grain yield in Pusa Basmati-1 attributed largely to increased grain number. The increased net photosynthetic rate and greater accumulation of sugar contributed significantly to the accelerated development of leaves and tillers in both the cultivars. The reduction in the low molecular mass proteins including Rubisco and increase in high molecular mass photosystem 2 proteins was observed in both the cultivars. Additional sugars may possibly help in balancing the profile of photosynthetic proteins and sustain greater growth and productivity in rice cultivars.     

Similar photosynthetic response to elevated carbon dioxide concentration in species with different phloem loading strategies

Species have different strategies for loading sugars into the phloem, which vary in the route that sugars take to enter the phloem and the energetics of sugar accumulation. Species with passive phloem loading are hypothesized to have less flexibility in response to changes in some environmental conditions because sucrose export from mesophyll cells is dependent on fixed anatomical plasmodesmatal connections. Passive phloem loaders also have high mesophyll sugar content, and may be less likely to exhibit sugar-mediated down-regulation of photosynthetic capacity at elevated CO₂ concentrations. To date, the effect of phloem loading strategy on the response of plant carbon metabolism to rising atmospheric CO₂ concentrations is unclear, despite the widespread impacts of rising CO₂ on plants. Over three field seasons, five species with apoplastic loading, passive loading, or polymer-trapping were grown at ambient and elevated CO₂ concentration in free air concentration enrichment plots. Light-saturated rate of photosynthesis, photosynthetic capacity, leaf carbohydrate content, and anatomy were measured and compared among the species. All five species showed significant stimulation in midday photosynthetic CO₂ uptake by elevated CO₂ even though the two passive loading species showed significant down-regulation of maximum Rubisco carboxylation capacity at elevated CO₂. There was a trend toward greater starch accumulation at elevated CO₂ in all species, and was most pronounced in passive loaders. From this study, we cannot conclude that phloem loading strategy is a key determinant of plant response to elevated CO₂, but compelling differences in response counter to our hypothesis were observed. A phylogenetically controlled experiment with more species may be needed to fully test the hypothesis.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Elevated CO<Subscript>2</Subscript> reduces stomatal and metabolic limitations on photosynthesis caused by salinity in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

The future environment may be altered by high concentrations of salt in the soil and elevated [CO₂] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO₂] stimulates photosynthesis by increasing CO₂ availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO₂] (350 or 700 μmol mol⁻¹ CO₂, respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO₂, the rate of CO₂ diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO₂ availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO₂ partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.     

Hormone and sugar effects on rice sucrose transporter <Emphasis Type="Italic">OsSUT1</Emphasis> expression in germinating embryos

The rice (Oryza sativa L.) OsSUT1 gene encodes a sucrose transporter protein. OsSUT1 was suggested to contribute to phloem loading of sucrose. OsSUT1 expression is highly induced in embryos after seeds were imbibed in water and peaked at 2 days after imbibition, but mRNA levels decline gradually afterwards. In this study, we demonstrated that phytohormones and sugars regulate OsSUT1 expression. Antagonism of abscisic acid and gibberellic acid appeared to play an important role in regulating OsSUT1 expression during embryo germination. In addition, our data showed a glucose and sucrose effect on OsSUT1 expression that represented a bi-phase process. Initially, glucose and sucrose functioned as negative regulators of OsSUT1 expression in germinating embryos after a 1-day treatment; however, when the treatment duration was extended to 5 days, OsSUT1 expression was significantly enhanced. Therefore, we hypothesized that the glucose and sucrose effect might occur in combination with other side effects, such as changes in hormone content or catabolism. Based on the effects that sugar analogs have on OsSUT1 expression, we suggest that the signal transduction for regulating glucose-responsive OsSUT1 expression in embryos occurs via a hexokinase-mediated pathway.     

Effects of age and ontogeny on photosynthetic responses of a determinate annual plant to elevated CO2 concentrations

Plant responses to elevated CO₂ concentrations ([CO₂]) may be regulated by both accelerated ontogeny and allocational changes as plants grow. However, isolating ontogeny‐related effects from age‐related effects are difficult because these factors are often confounded. In this study, the roles of age and ontogeny in photosynthetic responses to elevated [CO₂] were examined on Xanthium strumarium L. grown at ambient (365 µmol mol^?1) and elevated (730 µmol mol^?1) [CO₂]. To examine age‐related effects, six cohorts were planted at 5‐day intervals. To examine ontogeny‐related effects, all plants were induced to flower at the same time; ontogeny in Xanthium is relatively unaffected by growth in elevated [CO₂]. Growth in elevated [CO₂] increased net photosynthetic rates by approximately 30% throughout vegetative growth (i.e. active carbohydrate sinks), approximately 10% during flowering (i.e. minimal sink activity), and approximately 20% during fruit production (i.e. active sinks). At the harvest, the ratio of source to sink tissue significantly decreased with increasing plant age and was correlated with leaf soluble sugar concentration. Leaf soluble sugar concentration was negatively correlated with the relative photosynthetic response to elevated [CO₂]. These results suggest that age and ontogeny independently affect photosynthetic responses to elevated [CO₂] and the effects are mediated by reversible changes in source : sink balance.     

The temporal and species dynamics of photosynthetic acclimation in flag leaves of rice (Oryza sativa) and wheat (Triticum aestivum) under elevated carbon dioxide

In this study, we tested for the temporal occurrence of photosynthetic acclimation to elevated [CO₂] in the flag leaf of two important cereal crops, rice and wheat. In order to characterize the temporal onset of acclimation and the basis for any observed decline in photosynthetic rate, we characterized net photosynthesis, g_s, g_m, C_i/C_a, C_i/C_c, V_cmax, J_max, cell wall thickness, content of Rubisco, cytochrome (Cyt) f, N, chlorophyll and carbohydrate, mRNA expression for rbcL and petA, activity for Rubisco, sucrose phosphate synthase (SPS) and sucrose synthase (SS) at full flag expansion, mid‐anthesis and the late grain‐filling stage. No acclimation was observed for either crop at full flag leaf expansion. However, at the mid‐anthesis stage, photosynthetic acclimation in rice was associated with RuBP carboxylation and regeneration limitations, while wheat only had the carboxylation limitation. By grain maturation, the decline of Rubisco content and activity had contributed to RuBP carboxylation limitation of photosynthesis in both crops at elevated [CO₂]; however, the sharp decrease of Rubisco enzyme activity played a more important role in wheat. Although an increase in non‐structural carbohydrates did occur during these later stages, it was not consistently associated with changes in SPS and SS or photosynthetic acclimation. Rather, over time elevated [CO₂] appeared to enhance the rate of N degradation and senescence so that by late‐grain fill, photosynthetic acclimation to elevated [CO₂] in the flag leaf of either species was complete. These data suggest that the basis for photosynthetic acclimation with elevated [CO₂] may be more closely associated with enhanced rates of senescence, and, as a consequence, may be temporally dynamic, with significant species variation.     

Reversibility of photosynthetic acclimation of swiss chard and sugarbeet grown at elevated concentrations of CO2

Although leaf photosynthesis and plant growth are initially stimulated by elevated CO₂ concentrations, increasing insensitivity to CO₂ (acclimation) is a frequent occurrence. In order to examine the acclimation process, we studied photosynthesis and whole plant development in swiss chard (Beta vulgaris L. Koch ssp. ciela) and sugarbeet (Beta vulgaris L. ssp. vulgaris) grown at either ambient or twice ambient concentrations of CO₂. In an initial controlled environment study, photosynthetic acclimation to elevated CO₂ levels was observed in both subspecies 24 days after sowing (DAS) but was not observed at 42 and 49 DAS for sugarbeet or at 49 DAS for swiss chard. Although sugarbeet and swiss chard differed in root size and morphology, this was not a factor in the onset of photosynthetic acclimation. The reversal of photosynthetic acclimation that was observed in older plants grown at elevated CO₂, concentrations was associated with a rapid increase in root development (i.e. increased root: shoot [R/S] ratio), increased sucrose levels in sinks (roots) and no differences in total soluble leaf protein of either subspecies relative to the ambient CO₂ condition. In a second set of experiments, swiss chard and sugarbeet were grown in outdoor Plexiglass chambers at different times of the year (i.e. summer and early fall). Average 24-h temperature was 30.7 and 19.4°C for the summer and fall plantings, respectively. In agreement with the controlled environment study, lack of photosynthetic acclimation, determined from the response of photosynthesic rate to internal CO₂ concentration, was correlated with increased root biomass and sucrose concentration relative to the ambient condition. However, photo-synthetic acclimation was observed depending on the season, i.e. summer (swiss chard) or fall (sugarbeet), suggesting that acclimation was affected by environmental factors, such as temperature. Data from both experiments suggest that continued long-term photosynthetic stimulation may be dependent upon the ability of increased CO₂ to stimulate new sink development which would allow full utilization of the additional carbon made available in a high CO₂ environment.     

Carbon source–sink limitations differ between two species with contrasting growth strategies

Understanding how carbon source and sink strengths limit plant growth is a critical knowledge gap that hinders efforts to maximize crop yield. We investigated how differences in growth rate arise from source–sink limitations, using a model system comparing a fast‐growing domesticated annual barley (Hordeum vulgare cv. NFC Tipple) with a slow‐growing wild perennial relative (Hordeum bulbosum). Source strength was manipulated by growing plants at sub‐ambient and elevated CO₂ concentrations ([CO₂]). Limitations on vegetative growth imposed by source and sink were diagnosed by measuring relative growth rate, developmental plasticity, photosynthesis and major carbon and nitrogen metabolite pools. Growth was sink limited in the annual but source limited in the perennial. RGR and carbon acquisition were higher in the annual, but photosynthesis responded weakly to elevated [CO₂] indicating that source strength was near maximal at current [CO₂]. In contrast, photosynthetic rate and sink development responded strongly to elevated [CO₂] in the perennial, indicating significant source limitation. Sink limitation was avoided in the perennial by high sink plasticity: a marked increase in tillering and root:shoot ratio at elevated [CO₂], and lower non‐structural carbohydrate accumulation. Alleviating sink limitation during vegetative development could be important for maximizing growth of elite cereals under future elevated [CO₂].     

Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory

We compared the CO₂- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 μl l^–1 CO₂. Naturally-established saplings were fumigated with a free-air CO₂ enrichment system. Light-saturated photosynthetic rates were 159–190% greater for Ce. canadensis saplings grown and measured under elevated CO₂. This species had the greatest CO₂ stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO₂ enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO₂ stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO₂. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO₂. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO₂ make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO₂ did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO₂. Received: 16 February 1999 / Accepted: 30 August 1999     

Exogenous sucrose can decrease <Emphasis Type="Italic">in vitro</Emphasis> photosynthesis but improve field survival and growth of coconut (<Emphasis Type="Italic">Cocos nucifera</Emphasis> L.) <Emphasis Type="Italic">in vitro</Emphasis> plantlets

Summary Coconut (Cocos nucifera L.) plantlets grown in vitro often grow slowly when transferred to the field possibly, due to a limited photosynthetic capacity of in vitro-cultured plantlets, apparently caused by the sucrose added to growth medium causing negative feedback for photosynthesis. In this paper, we tested the hypothesis that high exogenous sucrose will decrease ribulose 1,5-bisphosphate carboxylase (Rubisco) activity and photosynthesis resulting in limited ex vitro growth. Plantlets grown with high exogenous sucrose (90 gl⁻¹) had reduced photosynthetic activity that resulted in a poor photosynthetic response to high levels of light and CO₂. These plantlets also had low amounts of Rubisco protein, low Rubisco activity, and reduced growth despite showing high survival when transferred to the field. Decreasing the medium’s sucrose concentration from 90 to 22.5 gl⁻¹ or 0 gl⁻¹ resulted in increased photosynthetic response to light and CO₂ along with increased Rubisco and phosphoenolpyruvate carboxylase (PEPC) activities and proteins. However, plantlets grown in vitro without exogenous sucrose died when transferred ex vitro, whereas those grown with intermediate exogenous sucrose showed intermediate photosynthetic response, high survival, fast growth, and ex vitro photosynthesis. Thus, exogenous sucrose at moderate concentration decreased photosynthesis but increased survival, suggesting that both in vitro photosynthesis and exogenous sucrose reserves contribute to field establisment and growth of coconut plantlets cultured in vitro.     

Acclimation of photosynthesis to elevated CO2 and temperature in five British native species of contrasting functional type

Acclimation of photosynthesis to growth at elevated CO₂ concentration varies markedly between species. Species functionally classified as stress-tolerators (S) and ruderals (R), are thought to be incapable, or the least capable, of responding positively in terms of growth to elevated [CO₂]. Is this pattern of response also apparent in leaf photosynthesis of wild S- and R-strategists? Acclimatory loss of a photosynthetic and growth response to elevated [CO₂] is assumed to reflect limitation on capacity to utilize additional photosynthate. The doubling of pre-industrial global [CO₂] is expected to coincide with a 3 °C increase in mean temperature which could stimulate growth; will photosynthetic capacity at elevated [CO₂] be greater when the concurrent temperature increase is simulated? Five species from natural grassland of NW Europe and of contrasting ecological strategy were grown in hemispherical greenhouses, environmentally controlled to track the external microclimate. Within a replicated design, plants were grown at (i) current ambient [CO₂] and temperature, (ii) elevated [CO₂] (ambient + 340 μmol mol^–1) and ambient temperature, (iii) ambient [CO₂] and elevated temperature (ambient + 3 °C), or (iv) elevated [CO₂] and elevated temperature. After 75–104 days, the CO₂ response of light-saturated rates of photosynthesis (A_sat) was analysed in controlled-environment cuvettes in a field laboratory. There was no acclimatory loss of photosynthetic capacity with growth in elevated [CO₂] or elevated temperature over this period in Poa alpina (S), Bellis perennis (R) or Plantago lanceolata (mixed C-S-R strategist), and a significant (P ? ? bl 0.05) increase in capacity in Helianthemum nummularium (S) and Poa annua (R). Photosynthetic rates of leaves grown and measured in elevated [CO₂] were therefore significantly higher than rates for leaves grown and measured in ambient [CO₂], for all species. With the exception of Poa alpina, stomatal conductance and stomatal limitation on A_sat showed no acclimatory response to growth in elevated [CO₂]. Carboxylation efficiency, determined from the initial slope of the response of A_sat to intercellular CO₂ concentration was significantly increased by elevated [CO₂] and elevated temperature in H.nummularium, implying a possible increase in in vivo RubisCO activity. Increased carboxylation efficiency of this species was also reflected by an increase in the CO₂- and light-saturated rates of photosynthesis, indicating an increased capacity for regeneration of the primary CO₂ acceptor in photosynthesis. The results show that R-strategists and slow-growing S-strategists, are inherently capable of large increases in leaf photosynthetic capacity with growth in elevated [CO₂] in contrast to expectations from growth studies. With the exception of P.annua, where there was a significant negative interaction between CO₂ and temperature, concurrent increase in growth temperature had little effect on this pattern of response.