Growth temperature can alter the temperature dependent stimulation of photosynthesis by elevated carbon dioxide in Albutilon theophrasti

Stimulation of photosynthesis in response to elevated carbon dioxide concentration [CO2] in the short-term (min) should be highly temperature dependent at high photon flux. However, it is unclear if long-term (days, weeks) adaptation to a given growth temperature alters the temperature-dependent stimulation of photosynthesis to [CO2]. In velveltleaf (Albutilon theophrasti), the response of photosynthesis, determined as CO2 assimilation, was measured over a range of internal CO2 concentrations at 7 short-term measurement (12, 16, 20, 24, 28, 32, 36 degrees C) temperatures for each of 4 long-term growth (16, 20, 28 and 32 degrees C) temperatures. In vivo estimates of VCmax, the maximum RuBP saturated rate of carboxylation, and Jmax, the light-saturated rate of potential electron transport, were determined from gas exchange measurements for each temperature combination. Overall, previous exposure to a given growth temperature adjusted the optimal temperatures of Jmax and VCmax with subsequently greater enhancement of photosynthesis at elevated [CO2] (i.e., a greater enhancement of photosynthesis at elevated [CO2] was observed at low measurement temperatures for A. theophrasti grown at low growth temperatures compared with higher growth temperatures, and vice versa for plants grown and measured at high temperatures). Previous biochemical based models used to predict the interaction between rising [CO2] and temperature on photosynthesis have generally assumed no growth temperature effect on carboxylation kinetics or no limitation by Jmax. In the current study, these models over predicted the temperature dependence of the photosynthetic response to elevated [CO2] at temperatures above 24 degrees C. If these models are modified to include long-term adjustments of Jmax and VCmax to growth temperature, then greater agreement between observed and predicted values was obtained.     

Fingerprinting the impacts of global change on tropical forests

Recent observations of widespread changes in mature tropical forests such as increasing tree growth, recruitment and mortality rates and increasing above-ground biomass suggest that ''global change'' agents may be causing predictable changes in tropical forests. However, consensus over both the robustness of these changes and the environmental drivers that may be causing them is yet to emerge. This paper focuses on the second part of this debate. We review (i) the evidence that the physical, chemical and biological environment that tropical trees grow in has been altered over recent decades across large areas of the tropics, and (ii) the theoretical, experimental and observational evidence regarding the most likely effects of each of these changes on tropical forests. Ten potential widespread drivers of environmental change were identified: temperature, precipitation, solar radiation, climatic extremes (including El Niño-Southern Oscillation events), atmospheric CO2 concentrations, nutrient deposition, O3/acid depositions, hunting, land-use change and increasing liana numbers. We note that each of these environmental changes is expected to leave a unique ''fingerprint'' in tropical forests, as drivers directly force different processes, have different distributions in space and time and may affect some forests more than others (e.g. depending on soil fertility). Thus, in the third part of the paper we present testable a priori predictions of forest responses to assist ecologists in attributing particular changes in forests to particular causes across multiple datasets. Finally, we discuss how these drivers may change in the future and the possible consequences for tropical forests.     

The Effect of Leaf Temperature and Photorespiratory Conditions on Export of Sugars during Steady-State Photosynthesis in Salvia splendens

Export and photosynthesis in leaves of Salvia splendens were measured concurrently during steady-state 14CO2 labeling conditions. Under ambient CO2 and O2 conditions, photosynthesis and export rates were similar at 15 and 25[deg]C, but both declined as leaf temperature was raised from 25 to 40[deg]C. Suppressing photorespiration between 15 and 40[deg]C by manipulating CO2 and O2 levels resulted in higher rates of leaf photosynthesis, total sugar synthesis, and export. There was a linear relationship between the rate of photosynthesis and the rate of export between 15 and 35[deg]C. At these temperatures, 60 to 80% of the carbon fixed was readily exported with sucrose, raffinose, and stachyose, which together constituted over 90% of phloem mobile assimilates. Above 35[deg]C, however, export during photosynthesis was inhibited both in photorespiratory conditions, which inhibited photosynthesis, and in nonphotorespiratory conditions, which did not inhibit photosynthesis. Sucrose and raffinose but not stachyose accumulated in the leaf at 40[deg]C. When leaves were preincubated at 40[deg]C and then cooled to 35[deg]C, export recovered more slowly than photosynthesis. These data are consistent with the view that impairment of export processes, rather than photosynthetic processes associated with light trapping, carbon reduction, and sucrose synthesis, accounted for the marked reduction in export between 35 and 40[deg]C. Taken together, the data indicated that temperature changes between 15 and 40[deg]C had two effects on photosynthesis and concurrent export. At all temperatures, suppressing photorespiration increased both photosynthesis and export, but above 35[deg]C, export processes were more directly inhibited independent of changes in photorespiration and photosynthesis.     

No evidence for consistent long‐term growth stimulation of 13 tropical tree species: results from tree‐ring analysis

The important role of tropical forests in the global carbon cycle makes it imperative to assess changes in their carbon dynamics for accurate projections of future climate–vegetation feedbacks. Forest monitoring studies conducted over the past decades have found evidence for both increasing and decreasing growth rates of tropical forest trees. The limited duration of these studies restrained analyses to decadal scales, and it is still unclear whether growth changes occurred over longer time scales, as would be expected if CO₂‐fertilization stimulated tree growth. Furthermore, studies have so far dealt with changes in biomass gain at forest‐stand level, but insights into species‐specific growth changes – that ultimately determine community‐level responses – are lacking. Here, we analyse species‐specific growth changes on a centennial scale, using growth data from tree‐ring analysis for 13 tree species (~1300 trees), from three sites distributed across the tropics. We used an established (regional curve standardization) and a new (size‐class isolation) growth‐trend detection method and explicitly assessed the influence of biases on the trend detection. In addition, we assessed whether aggregated trends were present within and across study sites. We found evidence for decreasing growth rates over time for 8–10 species, whereas increases were noted for two species and one showed no trend. Additionally, we found evidence for weak aggregated growth decreases at the site in Thailand and when analysing all sites simultaneously. The observed growth reductions suggest deteriorating growth conditions, perhaps due to warming. However, other causes cannot be excluded, such as recovery from large‐scale disturbances or changing forest dynamics. Our findings contrast growth patterns that would be expected if elevated CO₂ would stimulate tree growth. These results suggest that commonly assumed growth increases of tropical forests may not occur, which could lead to erroneous predictions of carbon dynamics of tropical forest under climate change.     

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

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

Effect of High Temperature on Photosynthesis in Beans (II. CO2 Assimilation and Metabolite Contents)

The effect of high temperatures on CO2 assimilation, metabolite content, and capacity for reducing power production in non-photorespiratory conditions has been assessed in two different bean (Phaseolus vulgarus L.) varieties, Blue Lake (commercially available in the United Kingdom) and Barbucho (a noncommercially bred Chilean variety), which are known to differ in their resistance to extreme high temperatures. Barbucho maintains its photosynthetic functions for a longer period of time under extreme heat compared with Blue Lake. The CO2 assimilation rate was increased by increases in temperature, with a decrease in ratio of rates of temperatures differing by 10[deg]C. It is suggested that limitations to CO2 assimilation are caused by metabolic restrictions that can be differentiated between those occurring in the range of 20 to 30[deg]C and 30 to 35[deg]C. It is likely that changes in the capacity for Calvin cycle regeneration and starch synthesis affect photosynthesis in the range of 20 to 30[deg]C. But following an increase in temperature from 30 to 35[deg]C, the supply of reducing power becomes limiting. From analysis of adenylate concentration, transthylakoid energization, and, indirectly, NADPH/NADP+ ratio, it was concluded that the limitation in the assimilatory power was due to an oxidation of the NADPH/NADP+ pool. In the range of 30 to 35[deg]C, the photosystem I quantum yield increased and photosystem II maintained its value. We conclude that the reorganization of thylakoids observed at 30 to 35[deg]C increased the excitation of photosystem I, inducing an increase in cyclic electron transport and a decrease in the supply of NADPH, limiting carbon assimilation.     

Direct and indirect climate change effects on photosynthesis and transpiration

Climate change affects plants in many different ways. Increasing CO(2) concentration can increase photosynthetic rates. This is especially pronounced for C(3) plants, at high temperatures and under water-limited conditions. Increasing temperature also affects photosynthesis, but plants have a considerable ability to adapt to their growth conditions and can function even at extremely high temperatures, provided adequate water is available. Temperature optima differ between species and growth conditions, and are higher in elevated atmospheric CO(2). With increasing temperature, vapour pressure deficits of the air may increase, with a concomitant increase in the transpiration rate from plant canopies. However, if stomata close in response to increasing CO(2) concentration, or if there is a reduction in the diurnal temperature range, then transpiration rates may even decrease. Soil organic matter decomposition rates are likely to be stimulated by higher temperatures, so that nutrients can be more readily mineralised and made available to plants. This is likely to increase photosynthetic carbon gain in nutrient-limited systems. All the factors listed above interact strongly so that, for different combinations of increases in temperature and CO(2) concentration, and for systems in different climatic regions and primarily affected by water or nutrient limitations, photosynthesis must be expected to respond differently to the same climatic changes.     

High temperature acclimation of C4 photosynthesis is linked to changes in photosynthetic biochemistry

With average global temperatures predicted to increase over the next century, it is important to understand the extent and mechanisms of C4 photosynthetic acclimation to modest increases in growth temperature. To this end, we compared the photosynthetic responses of two C4 grasses (Panicum coloratum and Cenchrus ciliaris) and one C4 dicot (Flaveria bidentis) to growth at moderate (25/20 degrees C, day/night) or high (35/30 degrees C, day/night) temperatures. In all three C4 species, CO2 assimilation rates (A) underwent significant thermal acclimation, such that when compared at growth temperatures, A increased less than what would be expected given the strong response of A to short-term changes in leaf temperature. Thermal photosynthetic acclimation was further manifested by an increase in the temperature optima of A, and a decrease in leaf nitrogen content and leaf mass per area in the high- relative to the moderate-temperature-grown plants. Reduced photosynthetic capacity at the higher growth temperature was underpinned by selective changes in photosynthetic components. Plants grown at the higher temperature had lower amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase and cytochrome f and activity of carbonic anhydrase. The activities of photosystem II (PSII) and phosphoenolpyruvate carboxylase were not affected by growth temperature. Chlorophyll fluorescence measurements of F. bidentis showed a corresponding decrease in the quantum yield of PSII (phi(PSII)) and an increase in non-photochemical quenching (phi(NPQ)). It is concluded that through these biochemical changes, C4 plants maintain the balance between the various photosynthetic components at each growth temperature, despite the differing temperature dependence of each process. As such, at higher temperatures photosynthetic nitrogen use efficiency increases more than A. Our results suggest C4 plants will show only modest changes in photosynthetic rates in response to changes in growth temperature, such as those expected within or between seasons, or the warming anticipated as a result of global climate change.     

Variations in Amazon forest productivity correlated with foliar nutrients and modelled rates of photosynthetic carbon supply

The rate of above-ground woody biomass production, W(P), in some western Amazon forests exceeds those in the east by a factor of 2 or more. Underlying causes may include climate, soil nutrient limitations and species composition. In this modelling paper, we explore the implications of allowing key nutrients such as N and P to constrain the photosynthesis of Amazon forests, and also we examine the relationship between modelled rates of photosynthesis and the observed gradients in W(P). We use a model with current understanding of the underpinning biochemical processes as affected by nutrient availability to assess: (i) the degree to which observed spatial variations in foliar [N] and [P] across Amazonia affect stand-level photosynthesis; and (ii) how these variations in forest photosynthetic carbon acquisition relate to the observed geographical patterns of stem growth across the Amazon Basin. We find nutrient availability to exert a strong effect on photosynthetic carbon gain across the Basin and to be a likely important contributor to the observed gradient in W(P). Phosphorus emerges as more important than nitrogen in accounting for the observed variations in productivity. Implications of these findings are discussed in the context of future tropical forests under a changing climate.     

Panicum milioides (C(3)-C(4)) does not have improved water or nitrogen economies relative to C(3) and C(4) congeners exposed to industrial-age climate change

The physiological implications of C(3)-C(4) photosynthesis were investigated using closely related Panicum species exposed to industrial-age climate change. Panicum bisulcatum (C(3)), P. milioides (C(3)-C(4)), and P. coloratum (C(4)) were grown in a glasshouse at three CO(2) concentrations ([CO(2)]: 280, 400, and 650?μl l(-1)) and two air temperatures [ambient (27/19?°C day/night) and ambient + 4?°C] for 12 weeks. Under current ambient [CO(2)] and temperature, the C(3)-C(4) species had higher photosynthetic rates and lower stomatal limitation and electron cost of photosynthesis relative to the C(3) species. These photosynthetic advantages did not improve leaf- or plant-level water (WUE) or nitrogen (NUE) use efficiencies of the C(3)-C(4) relative to the C(3) Panicum species. In contrast, the C(4) species had higher photosynthetic rates and WUE but similar NUE to the C(3) species. Increasing [CO(2)] mainly stimulated photosynthesis of the C(3) and C(3)-C(4) species, while high temperature had no or negative effects on photosynthesis of the Panicum species. Under ambient temperature, increasing [CO(2)] enhanced the biomass of the C(3) species only. Under high temperature, increasing [CO(2)] enhanced the biomass of the C(3) and C(3)-C(4) species to the same extent, indicating increased CO(2) limitation in the C(3)-C(4) intermediate at high temperature. Growth [CO(2)] and temperature had complex interactive effects, but did not alter the ranking of key physiological parameters amongst the Panicum species. In conclusion, the ability of C(3)-C(4) intermediate species partially to recycle photorespired CO(2) did not improve WUE or NUE relative to congeneric C(3) or C(4) species grown under varying [CO(2)] and temperature conditions.     

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].     

Contrasting crop species responses to CO2 and temperature: rice,soybean and citrus

The continuing increase in atmospheric carbon dioxide concentration ([CO₂]) and projections of possible future increases in global air temperatures have stimulated interest in the effects of these climate variables on plants and, in particular, on agriculturally important food crops. Mounting evidence from many different experiments suggests that the magnitude and even direction of crop responses to [CO₂] and temperature is almost certain to be species dependent and very likely, within a species, to be cultivar dependent. Over the last decade, [CO₂] and temperature experiments have been conducted on several crop species in the outdoor, naturally-sunlit, environmentally controlled, plant growth chambers by USDA-ARS and the University of Florida, at Gainesville, Florida, USA. The objectives for this paper are to summarize some of the major findings of these experiments and further to compare and contrast species responses to [CO₂] and temperature for three diverse crop species: rice (Oryza sativa, L.), soybean (Glycine max, L.) and citrus (various species). Citrus had the lowest growth and photosynthetic rates but under [CO₂] enrichment displayed the greatest percentage increases over ambient [CO₂] control treatments. In all three species the direct effect of [CO₂] enrichment was always an increase in photosynthetic rate. In soybean, photosynthetic rate depended on current [CO₂] regardless of the long-term [CO₂] history of the crop. In rice, photosynthetic rate measured at a common [CO₂], decreased with increasing long-term [CO₂] growth treatment due to a corresponding decline in RuBP carboxylase content and activity. Rice specific respiration decreased from subambient to ambient and superambient [CO₂] due to a decrease in plant tissue nitrogen content and a decline in specific maintenance respiration rate. In all three species, crop water use decreased with [CO₂] enrichment but increased with increases in temperature. For both rice and soybean, [CO₂] enrichment increased growth and grain yield. Rice grain yields declined by roughly 10 % per each 1 °C rise in day/night temperature above 28/21 °C.     

西伯利亚蝗气门结构及呼吸代谢对高温胁迫的响应

【目的】掌握西伯利亚蝗Gomphocerus sibiricus的气门结构和呼吸代谢应对高温胁迫的响应策略。【方法】运用扫描电镜观察西伯利亚蝗气门超显微结构,多通道昆虫呼吸仪测定18~42℃处理下西伯利亚蝗的O_2吸收率、CO_2释放率、代谢率和呼吸商。【结果】西伯利亚蝗共有10对气门,胸部2对,腹部8对,呈圆形或椭圆形,内侧着生有筛板,呈刺凸状,分布密集。18~42℃范围内,随温度升高,西伯利亚蝗呼吸代谢呈先上升后下降的趋势。18℃时西伯利亚蝗雌雄成虫的O_2吸收率、CO_2释放率和代谢率值显著小于其他温度下的相应值(P<0.05),雌虫分别为0.0022 m L/min、0.0019 m L/min和0.0210 m L/(g·min),雄虫分别为0.0016 m L/min、0.0016 m L/min和0.0236 m L/(g·min);21~27℃范围内,西伯利亚蝗雌雄成虫的呼吸代谢变化平稳,超过30℃呼吸代谢各指标值迅速上升,至36℃时雌雄成虫的O_2吸收率和代谢率值均显著大于其他温度下的相应值(P<0.05);雌虫分别为0.0071 m L/min和0.0592 m L/(g·min),雄虫分别为0.0089 m L/min和0.1108 m L/(g·min),39℃时CO_2释放率显著大于其他温度下的相应值(P<0.05),雌雄成虫分别为0.0074 m L/min和0.0067 m L/min。【结论】依据西伯利亚蝗呼吸代谢随温度变化特征可以判断,随气候持续变暖,西伯利亚蝗仍将是新疆高山、亚高山草原的重要致灾类群。     

Cold Hardening of Spring and Winter Wheat and Rape Results in Differential Effects on Growth,Carbon Metabolism,and Carbohydrate Content

The effect of long-term (months) exposure to low temperature (5[deg]C) on growth, photosynthesis, and carbon metabolism was studied in spring and winter cultivars of wheat (Triticum aestivum) and rape (Brassica napus). Cold-grown winter rape and winter wheat maintained higher net assimilation rates and higher in situ CO2 exchange rates than the respective cold-grown spring cultivars. In particular, the relative growth rate of spring rape declined over time at low temperature, and this was associated with a 92% loss in in situ CO2 exchange rates. Associated with the high photosynthetic rates of cold-grown winter cultivars was a 2-fold increase per unit of protein in both stromal and cytosolic fructose-1,6-bisphosphatase activity and a 1.5- to 2-fold increase in sucrose-phosphate synthase activity. Neither spring cultivar increased enzyme activity on a per unit of protein basis. We suggest that the recovery of photosynthetic capacity at low temperature and the regulation of enzymatic activity represent acclimation in winter cultivars. This allow these overwintering herbaceous annuals to maximize the production of sugars with possible cryoprotective function and to accumulate sufficient carbohydrate storage reserve to support basal metabolism and regrowth in the spring.     

Tropical forest carbon balance in a warmer world: a critical review spanning microbial- to ecosystem-scale processes

Tropical forests play a major role in regulating global carbon (C) fluxes and stocks, and even small changes to C cycling in this productive biome could dramatically affect atmospheric carbon dioxide (CO(2) ) concentrations. Temperature is expected to increase over all land surfaces in the future, yet we have a surprisingly poor understanding of how tropical forests will respond to this significant climatic change. Here we present a contemporary synthesis of the existing data and what they suggest about how tropical forests will respond to increasing temperatures. Our goals were to: (i) determine whether there is enough evidence to support the conclusion that increased temperature will affect tropical forest C balance; (ii) if there is sufficient evidence, determine what direction this effect will take; and, (iii) establish what steps should to be taken to resolve the uncertainties surrounding tropical forest responses to increasing temperatures. We approach these questions from a mass-balance perspective and therefore focus primarily on the effects of temperature on inputs and outputs of C, spanning microbial- to ecosystem-scale responses. We found that, while there is the strong potential for temperature to affect processes related to C cycling and storage in tropical forests, a notable lack of data combined with the physical, biological and chemical diversity of the forests themselves make it difficult to resolve this issue with certainty. We suggest a variety of experimental approaches that could help elucidate how tropical forests will respond to warming, including large-scale in situ manipulation experiments, longer term field experiments, the incorporation of a range of scales in the investigation of warming effects (both spatial and temporal), as well as the inclusion of a diversity of tropical forest sites. Finally, we highlight areas of tropical forest research where notably few data are available, including temperature effects on: nutrient cycling, heterotrophic versus autotrophic respiration, thermal acclimation versus substrate limitation of plant and microbial communities, below-ground C allocation, species composition (plant and microbial), and the hydraulic architecture of roots. Whether or not tropical forests will become a source or a sink of C in a warmer world remains highly uncertain. Given the importance of these ecosystems to the global C budget, resolving this uncertainty is a primary research priority.     

CO2浓度升高与氮沉降对南亚热带森林生态系统植物生物量积累及分配格局的影响

CO2浓度升高与氮沉降增加对陆地生态系统的耦合作用已成为全球变化的研究热点。应用大型开顶箱 (OTC) 人工控制手段研究了人工生态系统在1) 高CO2 (700±20μmol·mol-1) +高氮沉降 (100kg N·hm-2·a-1) (CN) ;2) 高CO2 (700±20μmol·mol-1) +背景氮沉降 (C+) ;3) 高氮沉降 (100kg N·hm-2·a-1) +背景CO2 (N+) ;4) 背景CO2+背景氮沉降处理 (CK) 4种处理条件下荷木 (Schima superba) 、红锥 (Castanopsis hystrix) 、海南红豆 (Ormosia pinnata) 、肖蒲桃 (Acmena acuminatissima) 、红鳞蒲桃 (Syzygium hancei) 等主要南亚热带森林植物的生物量积累模式及其分配格局。连续近3年的实验结果表明:不同处理条件下, 各参试植物生物量积累具有不同的响应特征, N+处理显著促进荷木、肖蒲桃及红鳞蒲桃生物量的积累;C+处理显著促进肖蒲桃、海南红豆生物量的积累;CN处理显著促进除红锥外其他物种生物量的积累, 并且具有两者单独处理的叠加效应。不同处理改变物种生物量的分配模式, N+处理降低植物的根冠比, 促进地上部分生物量的积累;C+处理增加红锥和红鳞蒲桃地下部分生物量的分配, 却促进荷木和海南红豆地上部分的积累;CN处理仅促进红磷蒲桃地下部分的积累。群落生物量的积累与分配格局取决于优势物种的生物量及其分配格局在群落中所 占的权重。     

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.     

The capacity to emit isoprene differentiates the photosynthetic temperature responses of tropical plant species

Experimental research shows that isoprene emission by plants can improve photosynthetic performance at high temperatures. But whether species that emit isoprene have higher thermal limits than non‐emitting species remains largely untested. Tropical plants are adapted to narrow temperature ranges and global warming could result in significant ecosystem restructuring due to small variations in species' thermal tolerances. We compared photosynthetic temperature responses of 26 co‐occurring tropical tree and liana species to test whether isoprene‐emitting species are more tolerant to high temperatures. We classified species as isoprene emitters versus non‐emitters based on published datasets. Maximum temperatures for net photosynthesis were ~1.8°C higher for isoprene‐emitting species than for non‐emitters, and thermal response curves were 24% wider; differences in optimum temperatures (T_opt) or photosynthetic rates at T_opt were not significant. Modelling the carbon cost of isoprene emission, we show that even strong emission rates cause little reduction in the net carbon assimilation advantage over non‐emitters at supraoptimal temperatures. Isoprene emissions may alleviate biochemical limitations, which together with stomatal conductance, co‐limit photosynthesis above T_opt. Our findings provide evidence that isoprene emission may be an adaptation to warmer thermal niches, and that emitting species may fare better under global warming than co‐occurring non‐emitting species.     

Stable carbon isotopes in tree rings indicate improved water use efficiency and drought responses of a tropical dry forest tree species

Understanding the responses of tropical trees to increasing [CO₂] and climate change is important as tropical forests play an important role in carbon and hydrological cycles. We used stable carbon isotopes (δ¹³C) in tree rings to study the physiological responses of a tropical dry forest tree species in southern Mexico, Mimosa acantholoba to changes in atmospheric [CO₂] and variation in climate. Based on annual records of tree ring δ¹³C, we calculated intrinsic water use efficiency (W _i) and intercellular [CO₂] (c _i). Our results showed that trees responded strongly to the increase in atmospheric [CO₂] over the last four decades; W _i increased dramatically by 40%, while c _i remained largely constant. The maintenance of a constant c _i indicates that photosynthetic rates are unlikely to have increased in response to higher [CO₂], and that improvements in W _i are probably due to a reduction in stomatal conductance. This may have large consequences for the hydrological cycle. Inter-annual variation in c _i was strongly correlated with total annual rainfall (r = 0.70), and not influenced by temperature, solar radiation or cloud cover. Our results show that δ¹³C in tree rings of tropical dry forest trees may be a powerful tool to evaluate long-term responses of trees to increasing [CO₂] and to variation in climate.     

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.