ALBERT

Notes: Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO2 concentrations (350 and 700 μmol mol−1), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha−1 and 87 kg ha−1). Leaves grown at 700 μmol mol−1 CO2 had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO2 concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long-term exposure to elevated CO2 concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO2 concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO2 concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.

Notes: Abstract. Only a small proportion of elevated CO2 studies on crops have taken place in the field. They generally confirm results obtained in controlled environments: CO2 increases photosynthesis, dry matter production and yield, substantially in C3 species, but less in C4, it decreases stomatal conductance and transpiration in C3 and C4 species and greatly improves water-use efficiency in all plants. The increased productivity of crops with CO2 enrichment is also related to the greater leaf area produced. Stimulation of yield is due more to an increase in the number of yield-forming structures than in their size. There is little evidence of a consistent effect of CO2 on partitioning of dry matter between organs or on their chemical composition, except for tubers. Work has concentrated on a few crops (largely soybean) and more is needed on crops for which there are few data (e.g. rice). Field studies on the effects of elevated CO2 in combination with temperature, water and nutrition are essential; they should be related to the development and improvement of mechanistic crop models, and designed to test their predictions.

Notes: Effects on sugar beet (Beta vulgaris L.) of current and elevated CO2 and temperature alone and in combination and their interactions with abundant and deficient nitrogen supply (HN and LN, respectively) have been studied in three experiments in 1993, 1994 and 1995. Averaged over all experiments, elevated CO2 (600 μmol mol–1 in 1993 and 700 μmol mol–1 in 1994 and 1995) increased total dry mass at final harvest by 21% (95% confidence interval (CI) = 21, 22) and 11% (CI = 6, 15) and root dry mass by 26% (CI = 19, 32) and 12% (CI = 6, 18) for HN and LN plants, respectively. Warmer temperature decreased total dry mass by 11% (CI = – 15, – 7) and 9% (CI = – 15, – 5) and root dry mass by 7% (CI = – 12, – 2) and 7% (CI = – 10, 0) for HN and LN plants, respectively. There was no significant interaction between temperature and CO2 on total or root dry mass. Neither elevated CO2 nor temperature significantly affected sucrose concentration per unit root dry mass. Concentrations of glycinebetaine and of amino acids, measured as α-amino-N, decreased in elevated CO2 in both N applications; glycinebetaine by 13% (CI = – 21, – 5) and 16% (CI = – 24, – 8) and α-amino-N by 24% (CI = – 36, – 11) and 16% (CI = – 26, – 5) for HN and LN, respectively. Warmer temperature increased α-amino-N, by 76% (CI = 50, 107) for HN and 21% (CI = 7, 36) for LN plants, but not glycinebetaine.

Notes: The global environment is changing with increasing temperature and atmospheric carbon dioxide concentration, [CO2]. Because these two factors are concomitant, and the global [CO2] rise will affect all biomes across the full global range of temperatures, it is essential to review the theory and observations on effects of temperature and [CO2] interactions on plant carbon balance, growth, development, biomass accumulation and yield. Although there are sound theoretical reasons for expecting a larger stimulation of net CO2 assimilation rates by increased [CO2] at higher temperatures, this does not necessarily mean that the pattern of biomass and yield responses to increasing [CO2] and temperature is determined by this response. This paper reviews the interactions between the effects of [CO2] and temperature on plants. There is little unequivocal evidence for large differences in response to [CO2] at different temperatures, as studies are confounded by the different responses of species adapted and acclimated to different temperatures, and the interspecific differences in growth form and development pattern. We conclude by stressing the importance of initiation and expansion of meristems and organs and the balance between assimilate supply and sink activity in determining the growth response to increasing [CO2] and temperature.

Notes: Winter wheat (Triticum aestivuin cv. Mercia) was grown in a controlled-environment facility under simulated Held conditions at ambient (360μmol mol−1) and elevated (690 μmol mol−1) CO2 concentrations. Some of the plants were shaded to mimic cloudy conditions during three periods of about 20d duration between terminal spikelet and start of grain-fill, giving 16 treatments in all. Elevated CO2, increased grain yield by about 20%, while shading in any period decreased yield, with the greatest effect in the last period, encompassing anthesis. No interactions between these effects were significant for grain yield, but there were complex interactions for mean grain size. Observed effects of shading and elevated CO2 on biomass production were well predicted by a simulation model. Observed effects of treatments on yield could be related to effects on biomass using a simple model which assumes that yield is proportional to biomass production, with coefficients of 0.42 (g grain yield g−1 biomass) for the first two periods and 0.74 for the last period. Wheat models should therefore include developmental changes in sensitivity of yield to biomass production, but biomass changes induced by different CO2 concentrations or light environments can be treated as having equivalent effects on grain yield.

Notes: Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO2 and O2 concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO2 is suggested to explain the discrepancy. The model suggests that resistance to CO2 fixation is mainly in the carboxylation reactions, not in CO2 transport.Using the steady state model, the kinetics of 14C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of 14C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO2 released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.

Notes: The effects of extreme phosphate (Pi) deficiency during growth on the contents of adenylates and pyridine nucleotides and the in vivo photochemical activity of photosystem II (PSII) were determined in leaves of Helianthus annuus and Zea mays grown under controlled environmental conditions. Phosphate deficiency decreased the amounts of ATP and ADP per unit leaf area and the adenylate energy charge of leaves. The amounts of oxidized pyridine nucleotides per unit leaf area decreased with Pi deficiency, but not those of reduced pyridine nucleotides. This resulted in an increase in the ratio of reduced to oxidized pyridine nucleotides in Pi-deficient leaves. Analysis of chlorophyll a fluorescence at room temperature showed that Pi deficiency decreased the efficiency of excitation capture by open PSII reaction centres (φe), the in vivo quantum yield of PSII photochemistry (φPSII) and the photochemical quenching co-efficient (qP), and increased the non-photochemical quenching co-efficient (qN) indicating possible photoinhibitory damage to PSII. Supplying Pi to Pi-deficient sunflower leaves reversed the long-term effects of Pi-deficiency on PSII photochemistry. Feeding Pi-sufficient sunflower leaves with mannose or FCCP rapidly produced effects on chlorophyll a fluorescence similar to long-term Pi-deficiency. Our results suggest a direct role of Pi and photophosphorylation on PSII photochemistry in both long-and short-term responses of photosynthetic machinery to Pi deficiency. The relationship between φPSII and the apparent quantum yield of CO2 assimilation determined at varying light intensity and 21 kPa O2 and 35 Pa CO2 partial pressures in the ambient air was linear in Pi-sufficient and Pi-deficient leaves of sunflower and maize. Calculations show that there was relatively more PSII activity per mole of CO2 assimilated by the Pi-deficient leaves. This indicates that in these leaves a greater proportion of photosynthetic electrons transported across PSII was used for processes other than CO2 reduction. Therefore, we conclude that in vivo photosynthetic electron transport through PSII did not limit photosynthesis in Pi-deficient leaves of sunflower and maize and that the decreased CO2 assimilation was a consequence of a smaller ATP content and lower energy charge which restricted production of ribulose, 1-5, bisphosphate, the acceptor for CO2.

Notes: The requirements for the experimental study of the effects of global climate change conditions on plants are outlined. A semi-controlled plant growth facility is described which allows the study of elevated CO2 and temperature, and their interaction on the growth of plants under radiation and temperature conditions similar to the field. During an experiment on winter wheat (cv. Mercia), which ran from December 1990 through to August 1991, the facility maintained mean daytime CO2 concentrations of 363 and 692 cm3 m−3 for targets of 350 and 700 cm3 m−3 respectively. Temperatures were set to follow outside ambient or outside ambient +4°C, and hourly means were within 0.5°C of the target for 92% of the time for target temperatures greater than 6°C. Total photosynthetically active radiation incident on the crop (solar radiation supplemented by artifieal light with natural photoperiod) was 2% greater than the total measured outside over the same period.

Notes: Abstract. A compact, portable lighting system, developed for the measurement of photosynthesis in the field with portable chambers and applicable to the laboratory, is described. The system consists of a miniature 50-W, 12-V tungsten-halogen lamp and a light guide which is constructed with randomized fibre bundles and mounted above the window of a portable assimilation chamber. The light guide distributes light uniformly across the leaf surface; the photon irradiance at the leaf surface is monitored by measuring the irradiant emitted from one fibre-optic bundle using a sensor connected to the body of the light guide. Up to 1600 μmol quanta photosynthetic active radiation m−2s−1 may be achieved at the leaf surface; irradiance may be varied by wire mesh screens of different densities. Leaf temperature follows air temperature outside the chamber to within ±2 °C over the range 10–30 °C and within the range of photon irradiances from 0 to 1600/μmol quanta m −2s. The power supply for the lamp is a 12-V, 24-A h lead-acid battery and the photon irradiance at the leaf surface gradually decreases by c. 3% over 2.5 h of measurement. With this system, response of photosynthetic rate to irradiance and to CO2 partial pressure at constant irradiance may be measured in the field, independent of natural variations in solar irradiance.

Notes: Experimental studies on CO2 assimilation of mesophytic C3 plants in relation to relative water content (RWC) are discussed. Decreasing RWC slows the actual rate of photosynthetic CO2 assimilation (A) and decreases the potential rate (Apot). Generally, as RWC falls from c. 100 to c. 75%, the stomatal conductance (gs) decreases, and with it A. However, there are two general types of relation of Apot to RWC, which are called Type 1 and Type 2. Type 1 has two main phases. As RWC decreases from 100 to c. 75%, Apot is unaffected, but decreasing stomatal conductance (gs) results in smaller A, and lower CO2 concentration inside the leaf (Ci) and in the chloroplast (Cc), the latter falling possibly to the compensation point. Down-regulation of electron transport occurs by energy quenching mechanisms, and changes in carbohydrate and nitrogen metabolism are considered acclimatory, caused by low Ci and reversible by elevated CO2. Below 75% RWC, there is metabolic inhibition of Apot, inhibition of A then being partly (but progressively less) reversible by elevated CO2; gs regulates A progressively less, and Ci and CO2 compensation point, Γ rise. It is suggested that this is the true stress phase, where the decrease in Apot is caused by decreased ATP synthesis and a consequent decreased synthesis of RuBP. In the Type 2 response, Apot decreases progressively at RWC 100 to 75%, with A being progressively less restored to the unstressed value by elevated CO2. Decreased gs leads to a lower Ci and Cc but they probably do not reach compensation point: gs becomes progressively less important and metabolic limitations more important as RWC falls. The primary effect of low RWC on Apot is most probably caused by limited RuBP synthesis, as a result of decreased ATP synthesis, either through inhibition of Coupling Factor activity or amount due to increased ion concentration. Carbohydrate synthesis and accumulation decrease. Type 2 response is considered equivalent to Type 1 at RWC below c. 75%, with Apot inhibited by limited ATP and RuBP synthesis, respiratory metabolism dominates and Ci and Γ rise. The importance of inhibited ATP synthesis as a primary cause of decreasing Apot is discussed. Factors determining the Type 1 and Type 2 responses are unknown. Electron transport is maintained (but down-regulated) in Types 1 and 2 over a wide range of RWC, and a large reduced/oxidized adenylate ratio results. Metabolic imbalance results in amino acid accumulation and decreased and altered protein synthesis. These conditions profoundly affect cell functions and ultimately cause cell death. Type 1 and 2 responses may reflect differences in gs and in sensitivity of metabolism to decreasing RWC.