Effect of pod removal on leaf senescence in soybeans

Depodding soybean (Glycine max [L] Merr. cv Wye) plants results in an apparent inhibition of senescence as indicated by leaf chlorophyll and soluble protein retention. However, leaf photosynthesis and ribulose bisphosphate carboxylase (Rubisco) levels begin to decline earlier in depodded than in control, podded plants. The initial decline in photosynthesis is correlated with a decrease in leaf transpiration, while the latter decline is associated with the loss of Rubisco. Total soluble protein remains high in depodded plants because several polypeptides, three in particular, increase in amounts sufficient to offset the loss of Rubisco. Thus, depodding appears to change the function of the leaf rather than simply delaying or preventing the decline in leaf function. Changes in specific leaf weight and starch content following depodding suggest that the leaf may be changing to a storage organ.     

Regulation of leaf photosynthetic rate correlating with leaf carbohydrate status and activation state of Rubisco under a variety of photosynthetic source/sink balances

There is evidence suggesting that in plants changes in the photosynthetic source/sink balance are an important factor that regulates leaf photosynthetic rate through affects on the leaf carbohydrate status. However, to resolve the regulatory mechanism of leaf photosynthetic rate associated with photosynthetic source/sink balance, information, particularly on mutual relationships of experimental data that are linked with a variety of photosynthetic source/sink balances, seems to be still limited. Thus, a variety of manipulations altering the plant source/sink ratio were carried out with soybean plants, and the mutual relationships of various characteristics such as leaf photosynthetic rate, carbohydrate content and the source/sink ratio were analyzed in manipulated and non-manipulated control plants. The manipulations were removal of one-half or all pods, removal of one-third or two-third leaves, and shading of one-third or one-half leaves with soybean plants grown for 8 weeks under 10 h light (24 degrees C) and 14 h darkness (17 degrees C). It was shown that there were significant negative correlations between source/sink ratio (dry weight ratio of attached leaves to other all organs) and leaf photosynthetic rate; source/sink ratio and activation ratio (percentage of initial activity to total activity) of Rubisco in leaf extract; leaf carbohydrate (sucrose or starch) content and photosynthetic rate; carbohydrate (sucrose or starch) content and activation ratio of Rubisco; amount of protein-bound ribulose-1,5-bisphosphate (RuBP) in leaf extract and leaf photosynthetic rate; and the amount of protein-bound RuBP and activation ratio of Rubisco. In addition, there were significant positive correlations between source/sink ratio and leaf carbohydrate (sucrose or starch) content; source/sink ratio and the amount of protein-bound RuBP; carbohydrate (sucrose or starch) content and amount of protein-bound RuBP and the activation ratio of Rubisco and leaf photosynthetic rate. The plant water content, leaf chlorophyll and Rubisco contents were not affected significantly by the manipulations. There is a previous report in Arabidopsis thaliana that the amount of protein-bound RuBP in leaf extract correlates negatively with the activation ratio of Rubisco in the leaf extract. Therefore, the results obtained from the manipulation experiments indicate that there is a regulatory mechanism for the leaf photosynthetic rate that correlates negatively with leaf carbohydrate (sucrose and starch) status and positively with the activation state of Rubisco under a variety of photosynthetic source/sink balances.     

Influence of nitrogen supply and sink strength on changes in leaf nitrogen compounds during senescence in two wheat cultivars

Changes in various nitrogen compounds during senescence of the fourth leaf were studied in two cultivars of spring wheat (Triticum aestivum L.). One of the cultivars (Yecora) was supplied with two N levels; the other (Tauro) was grown with the high N level and pruned above the fourth leaf, whereas the control was left intact. In both cultivars grown with high N supply, net nitrogen export from the fourth leaf did not occur until 35 days after sowing (DAS). Loss of leaf soluble proteins started earlier than that of chlorophylis, and coincided initially with an increase in insoluble protein. In N deficient plants the level of total N, soluble protein, and the activity of nitrate reductase (NRA. EC 1.6.6.1) started to decrease about 5 days earlier, and along with chlorophyll, continued to decrease at a faster rate, than in high N plants. Also, with low N supply, the large subunit (LSU, 58 kDa) of ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) decreased in greater proportion than other soluble proteins, while with high N supply the decrease in Rubisco LSU was similar to that of other soluble proteins. Nitrogen deficiency caused a greater decrease in soluble proteins than in insoluble proteins, and NRA relative to soluble proteins. The faster senescing Tauro cultivar had lower levels of most parameters, especially NRA, soluble protein and, after 35 DAS. Rubisco LSU as a proportion of soluble protein. The decrease in sink strength due to shoot pruning did generally not affect the level of the various nitrogenous compounds until 35 DAS; thereafter the levels of most parameters, especially soluble protein, Rubisco LSU and, at late stages of senescence, insoluble protein, were higher in pruned than in control shoots. Thus, shoot pruning slows down senescence. The 56- and 78-kDa polypeptides increased, rather than decreased, with leaf age; the level of these two polypeptides showed a negative relationship with Rubisco LSU (r = -0.933 and r = -0.758, respectively).     

Low nitrogen stress regulates chlorophyll fluorescence in coordination with photosynthesis and Rubisco efficiency of rice

Nitrogen (N) is the basis of plant growth and development and, is considered as one of the priming agents to elevate a range of stresses. Plants use solar radiations through photosynthesis, which amasses the assimilatory components of crop yield to meet the global demand for food. Nitrogen is the main regulator in the allocation of photosynthetic apparatus which changes of the photosynthesis (P_n) and quantum yield (F_v/F_m) of the plant. In the present study, dynamics of the photosynthetic establishment, N-dependent relation with chlorophyll fluorescence attributes and Rubisco efficacy was evaluated in low-N tolerant (cv. CR Dhan 311) and low-N sensitive (cv. Rasi) rice cultivars under low-N and optimum-N conditions. There was a decrease in the stored leaf N under low-N condition, resulting in the decreased P_n and F_v/F_m efficiency of the plants through depletion in the activity and content of Rubisco. The P_n and F_v/F_m followed the parallel trend of leaf N content during low-N condition along with depletion of intercellular CO₂ concentration and overall conductance under low-N condition. Photosynthetic saturation curve cleared abrupt decrease of effective quantum yield in the low-N sensitive rice cultivar than the low-N tolerant rice. Also, the rapid light curve highlighted the unacclimated regulation of photochemical and non-photochemical quenching in the low-N condition. The low-N sensitive rice cultivar triumphed non-photochemical quenching, whereas the low-N tolerant rice cultivar rose gradually during the light curve. Our study suggested that the quantum yield is the key limitation for photosynthesis in low-N condition. Regulation of Rubisco, photochemical and non-photochemical quenching may help plants to grow under low-N level.

     

Differences in photosynthetic behaviour and leaf senescence of soybean (Glycine max [L.] Merrill) dependent on N2 fixation or nitrate supply

Biological N₂ fixation can fulfil the N demand of legumes but may cost as much as 14% of current photosynthate. This photosynthate (C) sink strength would result in loss of productivity if rates of photosynthesis did not increase to compensate for the costs. We measured rates of leaf photosynthesis, concentrations of N, ureides and protein in leaves of two soybean cultivars ( Glycine max [L.] Merrill) differing in potential shoot biomass production, either associated with Bradyrhizobium japonicum strains, or amended with nitrate. Our results show that the C costs of biological N₂ fixation can be compensated by increased photosynthesis. Nodulated plants shifted N metabolism towards ureide accumulation at the start of the reproductive stage, at which time leaf N concentration of nodulated plants was greater than that of N-fertilized plants. The C sink strength of N₂ fixation increased photosynthetic N use efficiency at the beginning of plant development. At later stages, although average protein concentrations were similar between the groups of plants, maximum leaf protein of nodulated plants occurred a few days later than in N-fertilized plants. The chlorophyll content of nodulated plants remained high until the pod-filling stage, whereas the chlorophyll content of N-fertilized plants started to decrease as early as the flowering stage. These results suggest that, due to higher C sink strength and efficient N₂ fixation, nodulated plants achieve higher rates of photosynthesis and have delayed leaf senescence.     

Modification of Seed Growth in Soybean by Physical Restraint: Effect on Leaf Senescence

Crafts-Brandner, S. J. and Egli, D. B. 1987. Modification ofseed growth in soybean by physical restraint. Effect on leafsenescence.—J. exp. Bot. 38: 2043–2049. The effect of total plant sink size on leaf senescence in soybean[Glycine max (L.) Merrill] was investigated by using a simple,non-destructive method to decrease seed growth rate and totalplant fruit sink size without altering fruit or seed number.The treatment consisted of placing plastic pod restriction devices(PPRD), which were made from plastic drinking straws (6·35mm diameter), over the fruits so that all of the seeds werecontained within the PPRD's. The treatment did not alter thetime of initiation of leaf senescence for two cultivars (McCalland Maple Amber), but decreased the rate of leaf senescencebased on declines in chlorophyll, ribulose-l,5-hi'sphosphatecarboxylase/oxygenase level and carbon dioxide exchange rate.The treatment also delayed seed maturation. At the time of seedmaturation, the plants still retained green leaves. In a separate experiment, one seed in each fruit (40% of theseeds on the plant) was not restrained by the PPRD's. This treatmentled to an intermediate rate of leaf senescence compared to controland complete seed restriction treatments. The results indicatedthat, for the cultivars examined (1) leaf senescence was initiatedat the same time regardless of sink size (2) the rate of leafsenescence could be modified by altering sink size and (3) seedmaturation could occur without complete leaf yellowing and leafabscission. The effect of the PPRD treatments on leaf senescencewere similar to results obtained when fruits were physicallyremoved, which indicated that physical removal of fruits doesnot lead to artefacts due to wounding of the plants. Key words: Glycine max L, senescence, source-sink     

Sugar repression of photosynthesis: the role of carbohydrates in signalling nitrogen deficiency through source:sink imbalance

The aim of this work was to examine whether carbohydrates are involved in signalling N deficiency through source:sink imbalance. Photosynthetic metabolism in tobacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated by shading leaves on some of the plants. In N-sufficient plants over this time-scale, there was a small decline in photosynthetic rate, Rubisco protein and amino acid content, with a larger decrease in carbohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO₂), which was caused by a large decrease in the amount of Rubisco protein (62% after 8 d) and Rubisco activity. Higher amounts; of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the sourcersink ratio by shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.     

The impact of decreased Rubisco on photosynthesis, growth, allocation and storage in tobacco plants which have been transformed with antisense rbcS

Transgenic tobacco plants tranformed with antisense to rbcS to decrease expression of ribulose-1,5–bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate (a) whether Rubisco is limiting for photosynthesis and plant growth and (b) whether biomass allocation and storage of carbohydrate and nitrogen are regulated in response to decreased rate of photosynthesis. The rate of photosynthesis (measured in growth conditions) and plant growth were not strongly inhibited until almost half of the Rubisco was removed. When Rubisco was decreased further there was a large decrease of photosynthesis and plant growth. When photosynthesis decreased in the ‘antisense’ plants there was an increase in the shoot/root ratio and the specific leaf area. As a result, the leaf area ratio (leaf area per g plant dry weight) increased 3–4–fold. This shows that tobacco compensates for decreased photosynthesis by maximizing leaf area. The decrease of photosynthesis also resulted in lower starch and free hexose in the leaf, but the volume of the diurnal starch turnover was largely maintained. This indicates that partitioning to starch is regulated to decrease non-productive accumulation of starch, but still maintain a pool of transient starch for export during the night. The decrease of photosynthesis was also accompanied by a large increase of the nitrogen/ carbon balance, due to a large accumulation of nitrate in the leaf. This shows that assimilation of nitrate is inhibited in response to low availability of photo-synthate.     

Nonstructural carbohydrate metabolism during leaf ageing in tobacco (Nicotiana tabacum)

Nonstructural carbohydrate status and activities of ADP-glucose pyrophosphorylase (EC 2.7.7.27, ADPG pyrophosphorylase) and sucrose phosphate synthase (EC 2.4.1.14, SPS) were determined during ageing of tobacco ( Nicotiana tabacum L., cvs KY 14 and Speight G28) leaves sampled from control plants and from plants that had the apical meristem and subsequent axillary growth removed (detopped plants). Over the 30-day period shoot growth increased much more for control compared to detopped plants, but the increase in root growth was similar for both treatments. Dry matter and leaf area of the individual leaf used for enzyme and metabolite analysis were constant over time for controls but increased 5-fold for detopped plants. Ageing of control leaves was indicated by a progressive loss of chlorophyll and ribulose 1, 5-bisphosphate carboxylase (EC 4.1.1.39, Rubisco) activity; loss of these components was diminished for detopped plants. In contrast to chlorophyll and Rubisco activity, activities of ADPG pyrophosphorylase and SPS remained relatively constant over time for controls. Thus, under normal ageing conditions, changes in activities of ADPG pyrophosphorylase and SPS were not closely associated with changes in the standard senescence indicators chlorophyll and Rubisco activity. The activities of ADPG pyrophosphorylase and SPS were enhanced, relative to controls, within 6 days after applying the detopping treatment and activities remained high for the duration of the 30-day period. Detopping also led to increased concentrations of starch and sucrose, but the increases were not well correlated with changes in enzyme activities. The data indicated that the leaves of detopped plants functioned as both source leaves, with enhanced ability to synthesize carbohydrate, and sink leaves, with enhanced growth. Therefore, activities of ADPG pyrophosphorylase and SPS were more responsive to changes within an individual leaf than to changes in whole plant growth.     

Adjustments of net photosynthesis in Solanum tuberosum in response to reciprocal changes in ambient and elevated growth CO2 partial pressures

Single leaf photosynthetic rates and various leaf components of potato ( Solanum tuberosum L.) were studied 1–3 days after reciprocally transferring plants between the ambient and elevated growth CO₂ treatments. Plants were raised from individual tuber sections in controlled environment chambers at either ambient (36 Pa) or elevated (72 Pa) CO₂. One half of the plants in each growth CO₂ treatment were transferred to the opposite CO₂ treatment 34 days after sowing (DAS). Net photosynthesis (P_n) rates and various leaf components were then measured 34, 35 and 37 DAS at both 36 and 72 Pa CO₂. Three-day means of single leaf P_n rates, leaf starch, glucose, initial and total Rubisco activity, Rubisco protein, chlorophyll ( a + b ), chlorophyll ( a/b ), α -amino N, and nitrate levels differed significantly in the continuous ambient and elevated CO₂ treatments. Acclimation of single leaf P_n rates was partially to completely reversed 3 days after elevated CO₂-grown plants were shifted to ambient CO₂, whereas there was little evidence of photosynthetic acclimation 3 days after ambient CO₂-grown plants were shifted to elevated CO₂. In a four-way comparison of the 36, 72, 36 to 72 (shifted up) and 72 to 36 (shifted down) Pa CO₂ treatments 37 DAS, leaf starch, soluble carbohydrates, Rubisco protein and nitrate were the only photosynthetic factors that differed significantly. Simple and multiple regression analyses suggested that negative changes of P_n in response to growth CO₂ treatment were most closely correlated with increased leaf starch levels.     

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.     

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].     

Does Decrease in Ribulose-1,5-Bisphosphate Carboxylase by Antisense RbcS Lead to a Higher N-Use Efficiency of Photosynthesis under Conditions of Saturating CO2 and Light in Rice Plants?

Rice (Oryza sativa L.) plants with decreased ribulose-1,5-bisphosphate carboxylase (Rubisco) were obtained by transformation with the rice rbcS antisense gene under the control of the rice rbcS promoter. The primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO2 partial pressures for photosynthesis under conditions of high irradiance and 25[deg]C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. The R1 selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R1 population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO2), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO2, (100-115 Pa CO2) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO2 and high irradiance.     

The effects of flower removal on the seed yield of pigeonpeas (Cajanus cajan)

In field experiments carried out at Hyderabad, India with early and mediumduration cultivars of Cajanus cajan sown at the normal time, in July, removal of all flowers and young pods for up to 5 wk had little or no effect on final yield. The flowering period of the deflowered plants was extended and their senescence delayed. The plants compensated for the loss of earlier-formed flowers by setting pods from later-formed flowers; there was relatively little effect of the deflowering treatments on the number of seeds per pod or weight per seed. The plants were also able to compensate for the repeated removal of all flowers and young pods from alternate nodes by setting more pods at the other nodes.
The removal of flowers from pigeonpeas grown as a winter crop resulted in yield reductions roughly proportional to the length of the deflowering period, probably because maturation of these plants was delayed and occurred under increasingly unfavourable conditions as the weather became hotter.     

Effect of sulphate on photosynthesis in greenhouse–grown tomato plants

Sulphate accumulates in the rhizosphere of plants grown in hydroponic systems. To avoid such sulphate accumulation and promote the use of environmentally sound hydroponic systems, we examined the effects of four sulphate concentrations (0.1, 5,2, 10.4 and 20.8 m M ) on photosynthesis, ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activities and related physiological processes in greenhouse–grown tomato plants ( Lycopersicon esculentum Mill. cv. Trust). The lowest sulphate concentration (0.1 m M ) significantly decreased photosynthetic capacity (P_c) and Rubisco activities on a leaf area basis. This result was supported by our data for dry matter per plant, which was low for plants in the 0.1 m M treatment. The photosynthesis-related variables such as leaf conductance, chlorophyll and soluble protein were lowest for the 0.1 m M treatment. Both total Rubisco activity and the activated ratio were reduced with this treatment. However, Rubisco activities expressed per g of protein or per g of chlorophyll were not significantly affected. These results suggest that sulphur deficiency depressed P_c– by reducing the amount of both Rubisco and chlorophyll and by causing an inactivation of Rubisco. The ratio of organic sulphur vs organic nitrogen (S/N) in plants of the 0.1 m M treatment was far below the normal values. This low S/N ratio might be accountable for the negative effect of low sulphate on P_c and plant growth. P_c and dry matter were not affected until sulphate concentration in the nutrient solution reached a high level of 20.8 m M .     

The Effect of Elevated Partial Pressures of CO2 on the Relationship between Photosynthetic Capacity and N Content in Rice Leaves

The effects of growth CO2 levels on the photosynthetic rates; the amounts of ribulose-1,5-bisphosphate carboxylase (Rubisco), chlorophyll (Chl), and cytochrome f; sucrose phosphate synthase activity; and total N content were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under two CO2 partial pressures of 36 and 100 Pa at three N concentrations. The light-saturated photosynthesis at 36 Pa CO2 was lower in the plants grown in 100 Pa CO2 than those grown in 36 Pa CO2. Similarly, the amounts of Rubisco, Chl, and total N were decreased in the leaves of the plants grown in 100 Pa CO2. However, regression analysis showed no differences between the two CO2 treatments in the relationship between photosynthesis and total N or in the relationship between Rubisco and Chl and total N. Although a relative decrease in Rubisco to cytochrome f or sucrose phosphate synthase was found in the plants grown in 100 Pa CO2, this was the result of a decrease in total N content by CO2 enrichment. The activation state of Rubisco was also unaffected by growth CO2 levels. Thus, decreases in the photosynthetic capacity of the plants grown in 100 Pa CO2 could be simply accounted for by a decrease in the absolute amount of leaf N.     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

Growth and photosynthetic response of three soybean cultivars to simultaneous increases in growth temperature and CO2

Three soybean ( Glycine max L. Merr.) cultivars (Maple Glen, Clark and CNS) were exposed to three CO₂ concentrations (370, 555 and 740 μmol mol⁻¹) and three growth temperatures (20/15°, 25/20° and 31/26°C, day/night) to determine intraspecific differences in single leaf/whole plant photosynthesis, growth and partitioning, phenology and final biomass. Based on known carboxylation kinetics, a synergistic effect between temperature and CO₂ on growth and photosynthesis was predicted since elevated CO₂ increases photosynthesis by reducing photorespiration and photorespiration increases with temperature. Increasing CO₂ concentrations resulted in a stimulation of single leaf photosynthesis for 40–60 days after emergence (DAE) at 20/15°C in all cultivars and for Maple Glen and CNS at all temperatures. For Clark, however, the onset of flowering at warmer temperatures coincided with the loss of stimulation in single leaf photosynthesis at elevated CO₂ concentrations. Despite the season-long stimulation of single leaf photosynthesis, elevated CO₂ concentrations did not increase whole plant photosynthesis except at the highest growth temperature in Maple Glen and CNS, and there was no synergistic effect on final biomass. Instead, the stimulatory effect of CO₂ on growth was delayed by higher temperatures. Data from this experiment suggest that: (1) intraspecific variation could be used to select for optimum soybean cultivars with future climate change; and (2) the relationship between temperature and CO₂ concentration may be expressed differently at the leaf and whole plant levels and may not solely reflect known changes in carboxylation kinetics.