Photosynthetic acclimation and photosynthate partitioning in soybean leaves in response to carbon dioxide enrichment

Photosynthetic rates and photosynthate partitioning were studied in three-week-old soybean [Glycine max (L.) Merr. cv. Williams] plants exposed to either ambient (35 Pa) or elevated (70 Pa) CO₂ in controlled environment chambers. Ambient CO₂-grown plants also were given a single 24 h treatment with 70 Pa CO₂ 1 d prior to sampling. Photosynthetic rates of ambient CO₂-grown plants initially increased 36% when the measurement CO₂ was doubled from 35 to 70 Pa. Photosynthetic rates of the third trifoliolate leaf, both after 1 and 21 d of elevated CO₂ treatment, were 30 to 45% below those of ambient CO₂-grown plants when measured at 35 Pa CO₂. These reduced photosynthetic rates were not due to increased stomatal resistance and were observed for 2 to 8 h after plants given 1 d of CO₂ enrichment were returned to ambient CO₂. Initial and total ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities, percent activation, Rubisco protein, soluble protein and leaf chlorophyll content were similar in all CO₂ treatments. Quantum yields of photosynthesis, determined at limiting irradiances and at 35 Pa CO₂, were 0.049±0.003 and 0.038±0.005 mol CO₂ fixed per mol quanta for ambient and elevated CO₂-grown plants, respectively (p<0.05). Leaf starch and sucrose levels were greater in plants grown at 70 than at 35 Pa CO₂. Starch accumulation rates during the day were greater in ambient CO₂-grown plants than in plants exposed to elevated CO₂ for either 1 or 21 d. However, the percentage of C partitioned to starch relative to total C fixed was unaffected by 1 d of CO₂ enrichment. The above results showed that both photosynthetic and starch accumulation rates of soybean leaflets measured at 35 Pa CO₂ were temporarily reduced after 1 and 21 d of CO₂ enrichment. The biochemical mechanism affecting these responses was not identified.Abbreviations SLW- specific leaf weight (g m^–2) - Rubisco- ribulose 1,5-bisphosphate carboxylase/oxygenase - Rul- 5bisP, ribulose 1,5 bisphosphate - DAP- days after planting - SAR- starch accumulation rate - C_i- intercellular CO₂ concentration     

Effects of elevated temperature and carbon dioxide on seed-set and yield of kidney bean (Phaseolus vulgaris L.)

It is important to quantify and understand the consequences of elevated temperature and carbon dioxide (CO₂) on reproductive processes and yield to develop suitable agronomic or genetic management for future climates. The objectives of this research work were (a) to quantify the effects of elevated temperature and CO₂ on photosynthesis, pollen production, pollen viability, seed‐set, seed number, seeds per pod, seed size, seed yield and dry matter production of kidney bean and (b) to determine if deleterious effects of high temperature on reproductive processes and yield could be compensated by enhanced photosynthesis at elevated CO₂ levels. Red kidney bean cv. Montcalm was grown in controlled environments at day/night temperatures ranging from 28/18 to 40/30 °C under ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ levels. There were strong negative relations between temperature over a range of 28/18–40/30 °C and seed‐set (slope, ? 6.5% °C^?1) and seed number per pod (? 0.34 °C^?1) under both ambient and elevated CO₂ levels. Exposure to temperature > 28/18 °C also reduced photosynthesis (? 0.3 and ? 0.9 µmol m^?2 s^?1 °C^?1), seed number (? 2.3 and ? 3.3 °C^?1) and seed yield (? 1.1 and ? 1.5 g plant^?1 °C^?1), at both the CO₂ levels (ambient and elevated, respectively). Reduced seed‐set and seed number at high temperatures was primarily owing to decreased pollen production and pollen viability. Elevated CO₂ did not affect seed size but temperature > 31/21 °C linearly reduced seed size by 0.07 g °C^?1. Elevated CO₂ increased photosynthesis and seed yield by approximately 50 and 24%, respectively. There was no beneficial interaction of CO₂ and temperature, and CO₂ enrichment did not offset the negative effects of high temperatures on reproductive processes and yield. In conclusion, even with beneficial effects of CO₂ enrichment, yield losses owing to high temperature (> 34/24 °C) are likely to occur, particularly if high temperatures coincide with sensitive stages of reproductive development.     

Effects of elevated CO2 and temperature on the growth,elemental composition,and cell size of two marine diatoms: potential implications of global climate change

Two pennate diatoms, Amphora coffeaeformis and Nitzschia ovalis, were used to evaluate potential responses to the future CO₂ and temperature increases with respect to cell-specific growth rate, elemental composition, size, population growth rate, and carrying capacity. Diatoms were subjected to four different treatments over a 2 week period (approximately 4 generations): a control (28°C and present-day CO₂, ~400 ppm), high CO₂ (28°C with high CO₂, ~750 ppm), high temperature (31°C and present-day CO₂, ~400 ppm), and greenhouse-effect treatment (31°C with high CO₂, ~750 ppm). The results indicated that both the cell-specific growth rates and the carrying capacity of A. coffeaeformis decreased at the higher temperature treatment, whereas N. ovalis did not differ among all treatments. No significant difference was found in either species’ elemental cell composition, but higher C:N and C:P ratios were observed for A. coffeaeformis and N. ovalis, respectively, in high CO₂ and greenhouse-effect treatments. Smaller cell sizes were observed for both species under the greenhouse-effect treatment, a phenomenon that could alter benthic food webs in the future.     

Effects of elevated CO2 and temperature on photosynthesis and Rubisco in rice and soybean

Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO₂, were grown for a season in sunlight at ambient and twice-ambient [CO₂], and under daytime temperature regimes ranging from 28 to 40°C. The objectives of the study were to test whether CO₂ enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C₃ species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO₂], were increased by CO₂ enrichment, but decreased by supraoptimal temperatures. However, CO₂ enrichment more than compensated for the temperature-induced decline in A. For soybean, this CO₂ enhancement of A increased in a linear manner by 32–95% with increasing growth temperatures from 28 to 40°C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38°C. Both elevated CO₂ and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO₂ enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO₂] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (K_cat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO₂] and temperature increased the apparent K_cat by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO₂] in rice, but by high temperature in soybean, suggesting that [CO₂] and growth temperature influenced the metabolism of 2-carboxyarabinitol-1-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO₂ enhancement of A at supraoptimal temperatures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO₂] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C₃ plants species-specific differences will be encountered as a result of future increases in global [CO₂] and air temperatures.     

Rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) growth and nitrogen distribution under elevated CO<Subscript>2</Subscript> concentration and air temperature

A field experiment was conducted to investigate the effects of elevated atmospheric CO₂ concentration and temperature, singly and in combination, on grain yield and the distribution of nitrogen (N) in different rice organs. The rice ‘Wuyunjing 23’ was planted under four treatments: ambient CO₂ and temperature (ACT), elevated CO₂ (200 μmol mol^?1 higher than ambient CO₂) (EC), elevated temperature (1 °C above the ambient temperature) (ET), and the combination of elevated CO₂ and temperature (ECT) under T-FACE (temperature and CO₂ free air controlled enrichment) system. CO₂-induced increment and temperature-induced reduction in grain yield was 6.0 and 25.2% in 2013, and that was 9.8 and 10.8% in 2014, respectively. Dry matter (DM) production in different organs increased under EC at vegetative stage but decreased under ET at reproductive stage. The negative effects of temperature on grain yield and DM was weakened when combined with CO₂ enrichment. And the trends of decrease for yield and DM under ET and ECT in 2013 were more obvious than those in 2014 due to the annual temperature differences. Furthermore, ET led to greater distribution of N in root and stem but not for panicle than that under ACT. These mainly demonstrated that the rice production would be suffered varying degree of loss under global warming in future although the CO₂ enrichment could alleviate the effects of high temperature on rice growth.     

Impact of elevated temperatures on specific leaf weight,stomatal density,photosynthesis and chlorophyll fluorescence in soybean

High-temperature stress is a major environmental stress and there are limited studies elucidating its impact on soybean (Glycine max L. Merril.). The objectives of present study were to quantify the effect of high temperature on changes in leaf thickness, number of stomata on adaxial and abaxial leaf surfaces, gas exchange, chlorophyll fluorescence parameters and seed yield in soybean. Twelve soybean genotypes were grown at day/night temperatures of 30/22, 34/24, 38/26 and 42/28?°C with an average temperature of 26, 29, 32 and 35?°C, respectively, under greenhouse conditions. One set was also grown under ambient temperature conditions where crop season average maximum, minimum and mean temperatures were 28.0, 22.4 and 25.2?°C, respectively. Significant negative effect of temperature was observed on specific leaf weight (SLW) and leaf thickness. Rate of photosynthesis, stomatal conductance and water use efficiency declined as the growing temperatures increased; whereas, intercellular CO₂ and transpiration rate were increased. With the increase in temperature chlorophyll fluorescence parameters such as Fv/Fm, qP and PhiPSII declined while there was increase in qN. Number of stomata on both abaxial and adaxial surface of leaf increased significantly with increase in temperatures. The rate of photosynthesis, PhiPSII, qP and SPAD values were positively associated with leaf thickness and SLW. This indicated that reduction in photosynthesis and associated parameters appears to be due to structural changes observed at higher temperatures. The average seed yield was maximum (13.2 g/pl) in plants grown under ambient temperature condition and declined by 8, 14, 51 and 65% as the temperature was increased to 30/22, 34/24, 38/26 and 42/28?°C, respectively.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth) and the feeding behaviour of the weevil Phyllobius maculicornis

Abstract 1 The effect of elevated CO₂ and temperature on the foliar chemistry Betula pendula Roth and the feeding performance of polyphagous weevils Phyllobius maculicornis Germ. was studied. Birch seedlings were grown during one growing season in chamber‐less field conditions and in closed‐top chambers exposed to four different treatments: ambient CO₂ (350 p.p.m) and temperature, elevated atmospheric CO₂ (700 p.p.m) and ambient temperature, elevated temperature +3 °C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. 2 In leaves under CO₂ enrichment, the concentration of nitrogen and some flavonol glycosides significantly decreased, whereas the concentration of total phenolics, condensed tannins and (+)‐catechin significantly increased. The total concentration of cinnamoylquinic acids was significantly increased by CO₂ and decreased by temperature. The concentration of salidroside increased under elevated temperature. 3 Weevil‐feeding experiments were carried out in a five‐choice arrangement, one leaf from each of the five treatments (chamber‐less field controls and four different treatments in chambers) being placed in random order in a plastic box. The weevils preferred the leaves grown under elevated CO₂, which had low nitrogen, high phenolics and the highest condensed tannin concentrations. Whether the reason for this trend is due to the stimulating effect of condensed tannins and/or a change in other secondary compounds, remains unknown. The weevils did not show any obviously different response in feeding performance to temperature and the combination of elevated CO₂ and temperature.     

Rapidly diminishing mangrove forests in Myanmar (Burma): a review

IPCC predictions indicate an increase in temperatures by 1.5–7°C in some Amazonian regions during the twenty-first century. These changes could disrupt the present distribution patterns of organisms, including wetland plant species. In this work, we determined in microcosms the effects of scenarios combining elevated temperature and atmospheric CO₂ concentration on the germination and initial growth of the arborescent Amazonian aquatic macrophyte Montrichardia arborescens. Seeds were germinated, and seedlings produced were monitored over a 5-month period in four microcosms: Control: ambient temperature and CO₂ level; Mild: Control + 1.5°C and + 200 ppm CO₂; intermediate: control + 2.5°C and + 400 ppm CO₂; Extreme: Control + 4.5°C and + 850 ppm of CO₂. Rapid light response curves and Fv/Fm values taken in seedlings showed a decrease in electron transportation rate with CO₂ and temperature elevation. Mild and Intermediate treatments stimulated biomass production; Extreme treatment and Control produced similar results. The severe climatic changes expected in the future may negatively influence carbon accumulation in M. arborescens. Since aquatic macrophytes in Amazonian wetlands and wetlands worldwide are key plant species, further studies are needed to predict their fate in a global change perspective.     

Interactive effects of increased temperature and CO2 on the growth of Quercus myrsinaefolia saplings

The interactive effects of increased temperature and CO₂ enrichment on the growth of 2‐year‐old saplings of Quercus myrsinaefolia, an evergreen broad‐leaved oak, were studied throughout an entire year in the vicinity of their northernmost distribution. Saplings were grown under different conditions in two chambers: (1) a temperature gradient chamber at ambient temperature, 3 and 5 °C warmer conditions with an ambient CO₂ concentration, and (2) in a CO₂ temperature gradient chamber at 3 °C warmer conditions with 1·5 times the normal CO₂ concentration, and 5 °C warmer conditions with doubled CO₂ concentration. The 3 and 5 °C warmer conditions enhanced the relative growth rate during almost the entire year, producing 53 and 47% increases in annual biomass production, 27 and 44% enhancement of root growth during shoot dormancy and 3 and 5 week prolongation of the shoot growing period, respectively. However, a daily mean air temperature exceeding 30 °C under the 5 °C warmer condition caused a marked reduction in net assimilation rate (NAR) from July to September. The CO₂ enrichment further enhanced the positive effects of warming in spring and the resulting increases in NAR almost completely compensated for the negative effect of warming during summer. From autumn to winter, attenuation of the effects of CO₂ was compensated by the increased sink strength produced by the warming. The annual biomass production was more than doubled by the combination of temperature elevation and CO₂ enrichment.     

Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa

Methane emissions from rice grown within Temperature Gradient Greenhouse Tunnels under doubled CO₂ concentrations were 10–45 times less than emissions from control plants grown under ambient CO₂. For two cultivars of rice (cvs. Lemont and IR-72), methane emissions increased with a temperature increase of 2°, from outdoor ambient temperatures to the first cell of the ambient CO₂ tunnel (ambient temperature + 2 °C). Within both tunnels and for both cultivars methane emissions decreased with further temperature increases (from 2° to 5 °C above ambient). Carbon dioxide enrichment stimulated both above- and below-ground production. Our original hypothesis was that increased CO₂ would stimulate plant productivity and therefore stimulate methane emission, since direct linkages between these parameters have been observed. We hypothesize that CO₂ enrichment led to the attenuation of methane production due to increased delivery of oxygen to the rhizosphere because of increased root biomass and porosity. The increased root biomass due to elevated CO₂ may have more effectively aerated the soil, suppressing methane production. However, this study may be unique because the low organic content (< 1%) of the sandy soils in which the rice was grown created very little oxygen demand.     

Effects of climate change scenarios on Tempranillo grapevine (Vitis vinifera L.) ripening: response to a combination of elevated CO2 and temperature, and moderate drought

Greenhouse experiments were conducted to investigate the impact of predicted climate change (elevated CO₂, 700 μmol CO₂ mol^?1 air vs. ambient; elevated temperature, 28/18°C vs. 24/14°C, day/night; and partial irrigation, 40% of field capacity vs. well-irrigated) on grape berry quality characteristics during ripening. Grapevine (Vitis vinifera L. cv. Tempranillo) fruiting cuttings were used as experimental plant material. Climate change shortened the time between grape veraison and full maturity. At harvest time, many of the grape quality parameters determined were affected by the different grape maturity. The data were re-grouped according to total soluble solids to factor out changes due to the shortened time to maturity, and the effects on grape quality were then re-examined. Under current CO₂ and temperature conditions, partial irrigation decreased berry malic acid concentration and facilitated anthocyanins extractability. Elevated CO₂ and temperature decreased berry malic acid and total anthocyanins potential in well-irrigated plants and increased tonality index, irrespective of water availability. In partial irrigation conditions, elevated CO₂ and temperature hindered the anthocyanins extractability. In summary, results indicate that climate change (elevated CO₂, high temperature and partial irrigation) affects phenology and berry quality.     

Elevated CO2 and temperature increase soil C losses from a soybean–maize ecosystem

Warming temperatures and increasing CO₂ are likely to have large effects on the amount of carbon stored in soil, but predictions of these effects are poorly constrained. We elevated temperature (canopy: +2.8 °C; soil growing season: +1.8 °C; soil fallow: +2.3 °C) for 3 years within the 9th–11th years of an elevated CO₂ (+200 ppm) experiment on a maize–soybean agroecosystem, measured respiration by roots and soil microbes, and then used a process‐based ecosystem model (DayCent) to simulate the decadal effects of warming and CO₂ enrichment on soil C. Both heating and elevated CO₂ increased respiration from soil microbes by ~20%, but heating reduced respiration from roots and rhizosphere by ~25%. The effects were additive, with no heat × CO₂ interactions. Particulate organic matter and total soil C declined over time in all treatments and were lower in elevated CO₂ plots than in ambient plots, but did not differ between heat treatments. We speculate that these declines indicate a priming effect, with increased C inputs under elevated CO₂ fueling a loss of old soil carbon. Model simulations of heated plots agreed with our observations and predicted loss of ~15% of soil organic C after 100 years of heating, but simulations of elevated CO₂ failed to predict the observed C losses and instead predicted a ~4% gain in soil organic C under any heating conditions. Despite model uncertainty, our empirical results suggest that combined, elevated CO₂ and temperature will lead to long‐term declines in the amount of carbon stored in agricultural soils.     

Elevated [CO2] and growth temperature have a small positive effect on photosynthetic thermotolerance of Pinus taeda seedlings

Key message

Growth temperature had little effect on the response of net photosynthesis to high temperatures (up to 47 °C). On the other hand, elevated [CO ₂ ] increased net photosynthesis at high temperatures.

Abstract

We investigated whether Pinus taeda seedlings grown under elevated CO₂-concentration ([CO₂]) and temperature would be able to maintain positive net photosynthesis (A _net) longer than seedlings grown under ambient conditions when exposed to temperatures up to 47 °C. Additionally, we investigated whether a locally applied temperature increase would yield the same short-term gas exchange response to temperatures up to 47 °C as a naturally occurring latitudinal temperature increase of equal magnitude. Growth conditions were applied for 7 months (February to August) in treatment chambers constructed at two sites in the native range of P. taeda in the southern US. The sites were located 300 km apart along a north–south axis with a natural temperature difference of 2.1 °C. Seedlings were grown under ambient temperature and [CO₂] conditions at both sites. At the northern site, we also applied a temperature increase of 2 °C (T _E), ensuring that this treatment equalled the mean temperature at the southern site. Additionally, at the northern site, we applied a treatment of elevated [CO₂] (C _E). Gas exchange was measured on all plants in walk-in environmentally controlled chambers. Under C _E, there was no difference in A _net of seedlings grown in ambient or ambient +2 °C temperatures at any measurement temperature, while differences were present under ambient [CO₂]. Furthermore, A _net was higher under C _E than under ambient [CO₂]. At 47 °C, A _net was negative in all seedlings except those in the C _E and ambient temperature treatment combination. Seedlings at the northern site in the T _E treatment showed no significant differences in A _net compared with seedlings grown at ambient temperature at the southern site, indicating that the plants responded equally to a manipulated temperature increase and a latitudinal increase in temperature. Our results suggest that elevated [CO₂] increases photosynthetic thermotolerance at high temperature (>41 °C), but this effect diminishes as temperature increases further. Temperature manipulations could provide accurate information on the effect of latitudinal differences in temperature on leaf gas exchange of P. taeda.     

Effects of increased CO2 concentration and temperature on growth and yield of winter wheat at two levels of nitrogen application

Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO₂ concentration (seasonal means: 361 and 692 μmol mol^?1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha^?1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO₂ enrichment at both levels of the temperature and N treatments. At high N, there was a CO₂-induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO₂ have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO₂ treatments. This could be explained by the temperature-dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (-37%) than on yield (-18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO₂ did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO₂ enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.     

The effect of heat waves,elevated [CO2] and low soil water availability on northern red oak (Quercus rubra L.) seedlings

The frequency and intensity of heat waves are predicted to increase. This study investigates whether heat waves would have the same impact as a constant increase in temperature with the same heat sum, and whether there would be any interactive effects of elevated [CO₂] and soil moisture content. We grew Quercus rubra seedlings in treatment chambers maintained at either ambient or elevated [CO₂] (380 or 700 μmol CO₂ mol^?1) with temperature treatments of ambient, ambient +3 °C, moderate heat wave (+6 °C every other week) or severe heat wave (+12 °C every fourth week) temperatures. Averaged over a 4‐week period, and the entire growing season, the three elevated temperature treatments had the same average temperature and heat sum. Half the seedlings were watered to a soil water content near field capacity, half to about 50% of this value. Foliar gas exchange measurements were performed morning and afternoon (9:00 and 15:00 hours) before, during and after an applied heat wave in August 2010. Biomass accumulation was measured after five heat wave cycles. Under ambient [CO₂] and well‐watered conditions, biomass accumulation was highest in the +3 °C treatment, intermediate in the +6 °C heat wave and lowest in the +12 °C heat wave treatment. This response was mitigated by elevated [CO₂]. Low soil moisture significantly decreased net photosynthesis (A_net) and biomass in all [CO₂] and temperature treatments. The +12 °C heat wave reduced afternoon A_net by 23% in ambient [CO₂]. Although this reduction was relatively greater under elevated [CO₂], A_net values during this heat wave were still 34% higher than under ambient [CO₂]. We concluded that heat waves affected biomass growth differently than the same amount of heat applied uniformly over the growing season, and that the plant response to heat waves also depends on [CO₂] and soil moisture conditions.     

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.     

Photosynthetic responses of two eucalypts to industrial‐age changes in atmospheric [CO2] and temperature

The unabated rise in atmospheric [CO₂] is associated with increased air temperature. Yet, few CO₂‐enrichment studies have considered pre‐industrial [CO₂] or warming. Consequently, we quantified the interactive effects of growth [CO₂] and temperature on photosynthesis of faster‐growing Eucalyptus saligna and slower‐growing E. sideroxylon. Well‐watered and ‐fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO₂] (290, 400, and 650 µL L^?1), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO₂] and temperature treatments with few interactive effects. Light‐saturated photosynthesis (A_sat) and light‐ and [CO₂]‐saturated photosynthesis (A_max) increased by ～50% and ～10%, respectively, with each step‐increase in growth [CO₂], underpinned by a corresponding 6–11% up‐regulation of maximal electron transport rate (J_max). Maximal carboxylation rate (V_cmax) was not affected by growth [CO₂]. Thermal photosynthetic acclimation occurred such that A_sat and A_max were similar in ambient‐ and high‐temperature‐grown plants. At high temperature, the thermal optimum of A_sat increased by 2–7 °C across [CO₂] treatments. These results are the first to suggest that photosynthesis of well‐watered and ‐fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO₂] in a future, warmer climate.     

The influence of increasing growth temperature and CO2 concentration on the ratio of respiration to photosynthesis in soybean seedlings

Using controlled environmental growth chambers, whole plants of soybean, cv. ‘Clark’, were examined during early development (7–20 days after sowing) at both ambient (≈ 350 μL L^–1) and elevated (≈ 700 μL L^–1) carbon dioxide and a range of air temperatures (20, 25, 30, and 35 °C) to determine if future climatic change (temperature or CO₂ concentration) could alter the ratio of carbon lost by dark respiration to that gained via photosynthesis. Although whole-plant respiration increased with short-term increases in the measurement temperature, respiration acclimated to increasing growth temperature. Respiration, on a dry weight basis, was either unchanged or lower for the elevated CO₂ grown plants, relative to ambient CO₂ concentration, over the range of growth temperatures. Levels of both starch and sucrose increased with elevated CO₂ concentration, but no interaction between CO₂ and growth temperature was observed. Relative growth rate increased with elevated CO₂ concentration up to a growth temperature of 35 °C. The ratio of respiration to photosynthesis rate over a 24-h period during early development was not altered over the growth temperatures (20–35 °C) and was consistently less at the elevated relative to the ambient CO₂ concentration. The current experiment does not support the proposition that global increases in carbon dioxide and temperature will increase the ratio of respiration to photosynthesis; rather, the data suggest that some plant species may continue to act as a sink for carbon even if carbon dioxide and temperature increase simultaneously.     

Super-optimal temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide

Continuing increases in atmospheric carbon dioxide concentration (CO₂) will likely be accompanied by global warming. Our research objectives were (a) to determine the effects of season‐long exposure to daytime maximum/nighttime minimum temperatures of 32/22, 36/26, 40/30 and 44/34°C at ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ on reproductive processes and yield of peanut, and (b) to evaluate whether the higher photosynthetic rates and vegetative growth at elevated CO₂ will negate the detrimental effects of high temperature on reproductive processes and yield. Doubling of CO₂ increased leaf photosynthesis and seed yield by 27% and 30%, respectively, averaged across all temperatures. There were no effects of elevated CO₂ on pollen viability, seed‐set, seed number per pod, seed size, harvest index or shelling percentage. At ambient CO₂, seed yield decreased progressively by 14%, 59% and 90% as temperature increased from 32/22 to 36/26, 40/30 and 44/34°C, respectively. Similar percentage decreases in seed yield occurred at temperatures above 32/22°C at elevated CO₂ despite greater photosynthesis and vegetative growth. Decreased seed yields at high temperature were a result of lower seed‐set due to poor pollen viability, and smaller seed size due to decreased seed growth rates and decreased shelling percentages. Seed harvest index decreased from 0.41 to 0.05 as temperature increased from 32/22 to 44/34°C under both ambient and elevated CO₂. We conclude that there are no beneficial interactions between elevated CO₂ and temperature, and that seed yield of peanut will decrease under future warmer climates, particularly in regions where present temperatures are near or above optimum.