Influence of Plant Growth at High CO2 Concentrations on Leaf Content of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Intracellular Distribution of Soluble Carbohydrates in Tobacco,Snapdragon, and Parsley

We have examined the possible role of leaf cytosolic hexoses and the expression of mannitol metabolism as mechanisms that may affect the repression of photosynthetic capacity when plants are grown at 1000 versus 380 [mu]L L-1 CO2. In plants grown at high CO2, leaf ribulose-1,5-bisphosphate carboxylase/oxygenase content declined by [greater than or equal to]20% in tobacco (Nicotiana sylvestris) but was not affected in the mannitol-producing species snapdragon (Antirrhinum majus) and parsley (Petroselinum hortense). In the three species mesophyll glucose and fructose at midday occurred almost entirely in the vacuole (>99%), irrespective of growth CO2 levels. The estimated cytosolic concentrations of glucose and fructose were [less than or equal to]100 [mu]M. In the three species grown at high CO2, total leaf carbohydrates increased 60 to 100%, but mannitol metabolism did not function as an overflow mechanism for the increased accumulation of carbohydrate. In both snapdragon and parsley grown at ambient or high CO2, mannitol occurred in the chloroplast and cytosol at estimated midday concentrations of 0.1 M or more each. The compartmentation of leaf hexoses and the metabolism of alternate carbohydrates are further considered in relation to photosynthetic acclimation to high levels of CO2.     

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase,Electron Transport Components,and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice,and Their Relationships to Photosynthesis

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.     

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.     

根际CO2浓度升高对网纹甜瓜光合特性及产量和品质的影响

采用雾培植株根际通CO₂处理方式,研究了开花结果期根际CO₂浓度升高对网纹甜瓜光合作用及产量和品质的影响.结果表明：在网纹甜瓜果实发育期间,与350 μL·L^-1（对照）处理相比,根际2500和5000 μL CO₂·L^-1处理的叶片光合色素含量、净光合速率（P_n）、气孔导度（g_s）、胞间CO₂浓度（C_i）及PSⅡ最大光化学效率（F_v/F_m）均不同程度降低,而气孔限制值（L_s）显著提高,且5000 μL CO₂·L^-1处理的变化幅度高于2500 μL CO₂·L^-1处理;单株产量、果实中维生素C和可溶性糖含量显著降低,有机酸含量显著提高.可见,网纹甜瓜果实发育期间根际CO₂浓度超过2500 μL·L^-1时,其光合作用及果实发育会受到显著抑制,从而导致产量和品质降低.
     

Photosynthesis during leaf ontogeny in field-grown Nicotiana tabacum L. lines selected by survival at low CO2 concentrations

Leaf photosynthesis, stomatal conductance, internal CO₂ concentrationand leaf composition (photosynthetic pigments, total solubleprotein and Rubisco content) during leaf ontogeny of field grownNicotiana tabacum L. lines selected for survival at low atmosphericCO₂ concentrations are described. Selection at low CO₂ concentrations resutted in lines with highertotal dry matter production than their parent cultivar, butthis could not be related to improved photosynthesis which theselection method was designed to achieve (shown in previouswork). In the present work we report higher rates of photosynthesisin the selected lines in mature and old leaves, not found inyoung leaves when the capacity for photosynthesis was maximum.Differences in the regulation of the photosynthetic carbon reductioncycle as well as differences in the diffusive characteristicsof the mesophyll, due to changes in the size of the cells, maybe the cause for the higher rates of photosynthesis during leafsenescence in the selected lines. Key words: Photosynthesis, leaf ontogeny, tobacco, lines     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

Photosynthetic Acclimation in Pea and Soybean to High Atmospheric CO2 Partial Pressure

Nonnodulated pea (Pisum sativum L. cv Frosty) and soybean (Glycine max [L.] Merr. cv Wye) plants were grown under artificial lights from germination with ample nutrients, 600 [mu]mol photons m-2 s-1, and either 34 to 36 (control) or 64 to 68 Pa (enriched) CO2. For soybean, pod removal and whole-plant shading treatments were used to alter the source-sink balance and carbohydrate status of the plants. Growth of both species was substantially increased by CO2 enrichment despite some down-regulation of photosynthesis rate per unit leaf area ("acclimation"). Acclimation was observed in young pea leaves but not old and in old soybean leaves but not young. Acclimation was neither evident in quantum yield nor was it related to triose phosphate limitation of net photosynthesis. A correlation between levels of starch and sugars in the leaf and the amount of acclimation was apparent but was loose and only weakly related to the source-sink balance of the plant. A consistent feature of acclimation was reduced ribulose bisphosphate carboxylase (RuBPCase) content, although in vivo RuBPCase activity was not necessarily diminished by elevated growth CO2 owing to increased percentage of activation of the enzyme. A proposal is discussed that the complexity of photosynthetic acclimation responses to elevated CO2 is as an expression of re-optimization of deployment of within-plant resources at three levels of competition.     

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.     

Photosynthetic Characteristics of Segregates from Hybrids between Flaveria brownii (C4 Like) and Flaveria linearis (C3-C4)

Characteristics related to C4 photosynthesis were studied in reciprocal F1 hybrids and F2 plants from Flaveria brownii (C4 like) and Flaveria linearis (C3-C4). The reciprocal F1 plants differed in 13C/12C ratios of leaves and the percentage of 14C initially incorporated into C4 acids, being more like the pollen parents in these traits. They did not differ in apparent photosynthesis or in O2 inhibition of apparent photosynthesis and differed only slightly in CO2 compensation concentration at 175 [mu]mol quanta m-2 s-1 and 400 mL L-1 O2. The 13C/12C ratios of 78 F2 progeny from the two F1 plants exhibited a normal distribution centered between those of the parents, with a few values slightly higher and lower than the parents. Apparent photosynthesis at 130 [mu]L L-1 CO2 and inhibition of photosynthesis by O2 was nearly normally distributed in the F2 population, but no values for F2 plants approached those for F. brownii (15.4 [mu]mol m-2 s-1 and 7.8%, respectively). Distribution of the CO2 compensation concentration measured at 1000 [mu]mol quanta m-2 s-1 and 400 mL L-1 of O2 in the F2 population was skewed toward F. brownii with 72% of the progeny having values <9 [mu]L of CO2 L-1 compared to 1.5 and 27.2 [mu]L L-1 for F. brownii and F. linearis, respectively. Correlations among traits of F2 plants were low (coefficients of 0.30 to -0.49), indicating that the C4- related traits are not closely linked in segregating populations. Plants in the F2 population selected for high or low apparent photosynthesis at 130 [mu]L of CO2 L-1 (six each) did not rank consistently high or low for 13C/12C ratios, O2 inhibition of apparent photosynthesis, CO2 compensation concentration, or activities of phosphoenolpyruvate carboxylase or NADP-malic enzyme. This study confirms results of earlier work that indicates independent segregation of C4 traits and also shows that the C4-like parental type can be recovered, at least for some characteristics (13C/12C ratio), in segregating populations. Recovery of fully functional C4 plants awaits further experimentation with C4 x C3 or C4 x C3-C4 hybrid plants that produce fertile progeny.     

Acclimation of rice photosynthesis to irradiance under field conditions

Acclimation to irradiance was measured in terms of light-saturated photosynthetic carbon assimilation rates (P(max)), Rubisco, and pigment content in mature field-grown rice (Oryza sativa) plants in tropical conditions. Measurements were made at different positions within the canopy alongside irradiance and daylight spectra. These data were compared with a second experiment in which acclimation to irradiance was assessed in uppermost leaves within whole-plant shading regimes (10% low light [LL], 40% medium light [ML], and 100% high light [HL] of full natural sunlight). Two varieties, japonica (tropical; new plant type [NPT]) and indica (IR72) were compared. Values for Rubisco amount, chlorophyll a/b, and P(max) all declined from the top to the base of the canopy. In the artificial shading experiment, acclimation of P(max) (measured at 350 microL L(-1) CO(2)) occurred between LL and ML for IR72 with no difference observed between ML and HL. The Rubisco amount increased between ML and HL in IR72. A different pattern was seen for NPT with higher P(max) (measured at 350 microL L(-1) CO(2)) at LL than IR72 and some acclimation of this parameter between ML and HL. Rubisco levels were higher in NPT than IR72 contrasting with P(max). Comparison of data from both experiments suggests a leaf aging effect between the uppermost two leaf positions, which was not a result of irradiance acclimation. Results are discussed in terms of: (a) acclimation of photosynthesis and radiation use efficiency at high irradiance in rice, and (b) factors controlling photosynthetic rates of leaves within the canopy.     

SO42- Deprivation Has an Early Effect on the Content of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Photosynthesis in Young Leaves of Wheat

Wheat (Triticum aestivum cv Chinese Spring) supplied with 0.45 mM SO42- for 14 d with relative growth rates (RGR) of 0.22 to 0.24 d-1 was deprived of S for 7 to 8 d. There was no significant effect on RGR or leaf development (leaf 2 length was constant; leaf 3 expanded for 2-4 d; leaf 4 emerged and elongated throughout the experiment) during the S deprivation. In controls the net assimilation rate (A) closely reflected leaf ontogeny. S deprivation affected A in all leaves, particularly leaf 4, in which A remained at 8 to 10 [mu]mol CO2 m-2 s-1, whereas in controls A rose steadily to >20 [mu]mol CO2 m-2 s-1. In leaf 2, with a fully assembled photosynthetic system, A decreased in S-deprived plants relative to controls only at the end of the experiment. Effects on A were not due to altered stomatal conductance or leaf internal [CO2] ([C]i); decreases in the initial slope of A/[C]i curves indicated an effect of S deprivation on the carboxylase efficiency. Measurement of Rubisco activity and large subunit protein abundance paralleled effects on A and A/[C]i in S-deprived leaves. Negative effects on photosynthesis in S-deprived plants are discussed in relation to mobilization of S reserves, including Rubisco, emphasizing the need for continuous S supply during vegetative growth.     

Photosynthetic Acclimation to Elevated CO2 Occurs in Transformed Tobacco with Decreased Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content

Inhibition of net carbon assimilation rates during growth at elevated CO2 was studied in transgenic tobacco (Nicotiana tabacum L.) plants containing zero to two copies of antisense DNA sequences to the small subunit polypeptide (rbcS) gene of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). High- and low-Rubisco tobacco plants were obtained from the selfed progeny of the original line 3 transformant (S.R. Rodermel, M.S. Abbott, L. Bogorad [1988] Cell 55: 673-681). Assimilation rates of high- and low-Rubisco tobacco plants increased 22 and 71%, respectively, when transferred from 35- to 70-Pa CO2 chamber air at 900 [mu]mol m-2 s-1 photon flux density. However, CO2-dependent increases of net carbon assimilation rates of high- and low-Rubisco plants virtually disappeared after 9 d of growth in elevated CO2 chamber air. Total above-ground dry matter production of high- and low-Rubisco plants was 28 and 53% greater, respectively, after 9 d of growth at 70 Pa compared with 35 Pa CO2. Most of this dry weight gain was due to increased specific leaf weight. Rubisco activity, Rubisco protein, and total chlorophyll were lower in both high- and low-Rubisco plants grown in enriched compared with ambient CO2 chamber air. Soluble leaf protein also decreased in response to CO2 enrichment in high- but not in low-Rubisco tobacco plants. Decreased Rubisco activities in CO2-adapted high- and low-Rubisco plants were not attributable to changes in activation state of the enzyme. Carbonic anhydrase activities and subunit levels measured with specific antibodies were similar in high- and low-Rubisco tobacco plants and were unchanged by CO2 enrichment. Collectively, these findings suggested that photosynthetic acclimation to enriched CO2 occurred in tobacco plants either with or without transgenically decreased Rubisco levels and also indicated that the down-regulation of Rubisco in CO2-adapted tobacco plants was related to decreased specific activity of this enzyme.     

Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO2 Enrichment (FACE)

The initial stimulation of photosynthesis observed on elevation of [CO2] in grasslands has been predicted to be a transient phenomenon constrained by the loss of photosynthetic capacity due to other limitations, notably nutrients and sinks for carbohydrates. Legumes might be expected partially to escape these feedbacks through symbiotic N2 fixation. The Free-Air Carbon dioxide Enrichment (FACE) experiment at Eschikon, Switzerland, has been the longest running investigation of the effects of open-air elevation of [CO2] on vegetation. The prediction of a long-term loss of photosynthetic capacity was tested by analysing photosynthesis in Trifolium repens L. (cv. Milkanova) in the spring and autumn of the eighth, ninth and tenth years of treatment. A high and low N treatment also allowed a test of the significance of exogenous N-supply in maintaining a stimulation of photosynthetic capacity in the long-term. Prior work in this Free Air CO2 Enrichment (FACE) experiment has revealed that elevated [CO2] increased both vegetative and reproductive growth of T. repens independent of N treatment. It is shown here that the photosynthetic response of T. repens was also independent of N fertilization under both current ambient and elevated (600 micro mol mol-1) [CO2]. There was a strong effect of season on photosynthesis, with light-saturated rates (Asat) 37% higher in spring than in autumn. Higher Asat in the spring was supported by higher maximum Rubisco carboxylation rates (Vc,max) and maximum rates of electron transport (Jmax) contributing to RuBP regeneration. Elevated [CO2] increased Asat by 37% when averaged across all measurement periods and both N fertilization levels, and decreased stomatal conductance by 25%. In spring, there was no effect of elevated [CO2] on photosynthetic capacity of leaves, but in autumn both Vc,max and Jmax were reduced by approximately 20% in elevated [CO2]. The results show that acclimation of photosynthetic capacity can occur in a nitrogen-fixing species, in the field where there are no artificial restrictions on sink capacity. However, even with acclimation there was a highly significant increase in photosynthesis at elevated [CO2].     

Light Stress and Oxidative Cell Damage in Photoautotrophic Cell Suspension of Euphorbia characias L

A photoautotrophic cell-suspension culture of Euphorbia characias L. grown at 70 [mu]mol photons m-2 s-1 was very sensitive to light stress: the gross photosynthesis measured by using a mass spectrometric 16O2/18O2 isotope technique showed a fast decrease at a rather low light intensity of 100 [mu]mol photons m-2 s-1, far below the photosynthetic saturation level. The contribution of activated oxygen species on photosystem II photoinhibition was examined for a given light intensity. A protective effect on gross photosynthesis was observed with 1% oxygen. When light stress was applied to a methyl viologen-adapted cell suspension, photoinhibition was reduced. When 50 [mu]mol L-1 methyl viologen was added, photoinhibition was slightly enhanced. These responses suggested an involvement of superoxide radicals in the photoinhibition process of E. characias photoautotrophic cells. The long-term (16 h) effects of photoinhibition were then studied. Aldehyde (malondialdehyde and 4-hydroxyalcenals) production resulting from lipid peroxidation was stimulated in long-term stressed cells. When 50 [mu]mol L-1 methyl viologen were added, increased aldehyde production was measured. Under 1% oxygen, the aldehyde production was comparable to that of nonstressed cells. The relationship among lipid peroxidation, light intensity, and net photosynthesis suggests that aldehyde production may result from cell death provoked by a prolonged energy deficit due to the inhibition of photosynthesis.     

Effects of rising temperatures and [CO2] on the physiology of tropical forest trees

Using a mixture of observations and climate model outputs and a simple parametrization of leaf-level photosynthesis incorporating known temperature sensitivities, we find no evidence for tropical forests currently existing "dangerously close" to their optimum temperature range. Our model suggests that although reductions in photosynthetic rate at leaf temperatures (TL) above 30 degrees C may occur, these are almost entirely accountable for in terms of reductions in stomatal conductance in response to higher leaf-to-air vapour pressure deficits D. This is as opposed to direct effects of TL on photosynthetic metabolism. We also find that increases in photosynthetic rates associated with increases in ambient [CO2] over forthcoming decades should more than offset any decline in photosynthetic productivity due to higher D or TL or increased autotrophic respiration rates as a consequence of higher tissue temperatures. We also find little direct evidence that tropical forests should not be able to respond to increases in [CO2] and argue that the magnitude and pattern of increases in forest dynamics across Amazonia observed over the last few decades are consistent with a [CO2]-induced stimulation of tree growth.     

The Effect of Elevated [CO2] on Growth and Photosynthesis of Two Eucalyptus Species Exposed to High Temperatures and Water Deficits

Two species of eucalyptus (Eucalyptus macrorhyncha and Eucalyptus rossii) were grown for 8 weeks in either ambient (350 [mu]L L-1) or elevated (700 [mu]L L-1) CO2 concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45[deg]C, maximum) and high-light (1250 [mu]mol photons m-2 s-1, maximum) stress period. Water-stressed seedlings of E. macrorhyncha had higher leaf water potentials when grown in elevated [CO2]. Growth analysis indicated that increased [CO2] may allow eucalyptus species to perform better during conditions of low soil moisture. A down-regulation of photosynthetic capacity was observed for seedlings grown in elevated [CO2] when well watered but not when water stressed. Well-watered seedlings grown in elevated [CO2] had lower quantum efficiencies as measured by chlorophyll fluorescence (the ratio of variable to maximal chlorophyll fluorescence [Fv/Fm]) than seedlings grown in ambient [CO2] during the high-temperature stress period. However, no significant differences in Fv/Fm were observed between CO2 treatments when water was withheld. The reductions in dark-adapted Fv/Fm for plants grown in elevated [CO2] were not well correlated with increased xanthophyll cycle photoprotection. However, reductions in the Fv/Fm were correlated with increased levels of nonstructural carbohydrates. The reduction in quantum efficiencies for plants grown in elevated [CO2] is discussed in the context of feedback inhibition of electron transport associated with starch accumulation and variation in sink strength.     

Response of Growth Dynamics of Two Spring Geophytes to Light Regime in a Lime-Beech Forest

Blomaee accumulation, leaf longevity and growth rate of two spring forest geophytes, Scllla blfolla L. and Arum maculatum L. were estimated separately for three size groups within each population of these species. Despite the differences in leaf longevity, both species showed a similar pattern of blomass accumulation In relation to their phenologles and reproductive demands. Eco-physlological acclimation to changing light environment was assumed through photosynthetic parameters and dynamics of leaf area Index In the predominant size group of each species. A light response curve was measured under natural light for each species through the continuum of Its phenology to quantify the photosynthetic photon flux density at light saturation, light-saturated photosynthetic rate, light compensation point, and dark respiration. Light-saturated assimilation per leaf area basis, dark respiration rate and light compensation points were significantly higher in S. blfolla relative to A. maculatum. However, the acclimation of photosynthesis that would respond to light changes in environment was not found in S. bifolla. In contrast, In A. maculatum a marked shift In the light dependence of photosynthesis through the season was noticed, which resulted In a strong photosynthetic acclimation to the low-light conditions. Accompanied by significant leaf area Index, this efficient low-light photosynthesis enabled greater leaf longevity, and consequently longer accumulative period to A. maculatum. From the different parameters that we determined （both photosynthetic acclimation and growth strategy） it would appear that these species belong to two distinct subgroups： S. blfolla to the early and A. maculatum to the late vernals.     

烟草叶片发育过程中光合功能衰退与H_2O_2积累的关系

以烟草(NicotianatabacumL.cvNC89)为材料,研究了叶片发育过程中H2O2积累与叶绿体光合功能衰退、抗坏血酸-谷胱甘肽(AsA-GSH)循环的关联。结果表明,光合功能衰退过程中,各光合参数均表现为先缓慢后快速的下降趋势,核酮糖-1,5-二磷酸羧化酶(RuBPCase)活性下降较电子传递活性下降迅速,H2O2含量与叶绿素含量、光合速率、RuBPCase活性、抗坏血酸过氧化物酶(APX)、谷胱甘肽还原酶(GR)活性显著负相关。H2O2的定位染色也证实光合功能衰退与H2O2积累密切相关。APX和GR在光合功能可逆衰退阶段维持较高水平,不可逆衰退阶段下降稍快。烟草叶片光合功能衰退快于AsA-GSH循环运转的下调。     

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.     

Photosynthesis of Helianthus annuus does not acclimate toelevated CO2 regardless of N supply

A number of studies have shown that relatively long-term exposure to elevated levels of CO₂ can lead to the downward acclimation of photosynthesis. Although the exact mechanisms are not clearly understood, it has been suggested that such a downward adjustment may be more common under limited N availability. Here we examined the effect of N supply on the photosynthetic acclimation response of Helianthus annuus L. cv. Teddy Bear plants to elevated CO₂ at three growth stages – 18, 38 and 56 d after emergence corresponding to vegetative, pre-flowering and flowering stages. Plants were grown at CO₂ partial pressures of 37 or 70 Pa, and supplied with 0.5, 2.5 or 5 mol·m^–3 N. After 18 d of treatment, photosynthetic capacity of H. annuus as evaluated by parameters derived from the A-C_i data (Rubisco carboxylation capacity, V_c,max; electron transport capacity, J_max; and capacity for triose phosphate utilization, TPU) showed no acclimation to elevated CO₂. The leaf nitrogen concentration, [N], and total non-structural carbohydrates, [TNC], were also comparable between ambient- and elevated-CO₂-grown plants. However, all these photosynthetic parameters as well as leaf [N], but not [TNC], significantly increased in response to N supply. Similarly, after 38 and 56 d of exposure to CO₂ treatments, photosynthetic capacities, foliar [N] and [TNC] did not significantly differ between ambient- and elevated-CO₂ plants. These results suggest that H. annuus plants maintained their photosynthetic capacity during long-term exposure to elevated CO₂ because of their capacity to maintain leaf N-status. It is further suggested that plant capacity to maintain the balance between C and N acquisition rather than simply N-supply level, may determine whether photosynthetic acclimation in response to elevated CO₂ occurs or not.