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Summary Galdieria sulphuraria is a cosmopolitan microalga found in volcanic hot springs and calderas. It grows at low pH in photoautotrophic (use of light as a source of energy) or heterotrophic (respiration as a source of energy) conditions, using an unusually broad range of organic carbon sources. Previous data suggested that G. sulphuraria cannot grow mixotrophically (simultaneously exploiting light and organic carbon as energy sources), its photosynthetic machinery being repressed by organic carbon.Here, we show that G. sulphuraria SAG21.92 thrives in photoautotrophy, heterotrophy and mixotrophy. By comparing growth, biomass production, photosynthetic and respiratory performances in these three trophic modes, we show that addition of organic carbon to cultures (mixotrophy) relieves inorganic carbon limitation of photosynthesis thanks to increased CO2 supply through respiration. This synergistic effect is lost when inorganic carbon limitation is artificially overcome by saturating photosynthesis with added external CO2.Proteomic and metabolic profiling corroborates this conclusion suggesting that mixotrophy is an opportunistic mechanism to increase intracellular CO2 concentration under physiological conditions, boosting photosynthesis by enhancing the carboxylation activity of Ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (Rubisco) and decreasing photorespiration.We discuss possible implications of these findings for the ecological success of Galdieria in extreme environments and for biotechnological applications.

Summary Much of humanity relies on rice (Oryza sativa) as a food source, but cultivation is water intensive and the crop is vulnerable to drought and high temperatures. Under climate change, periods of reduced water availability and high temperature are expected to become more frequent, leading to detrimental effects on rice yields.We engineered the high‐yielding rice cultivar ‘IR64’ to produce fewer stomata by manipulating the level of a developmental signal. We overexpressed the rice epidermal patterning factor OsEPF1, creating plants with substantially reduced stomatal density and correspondingly low stomatal conductance.Low stomatal density rice lines were more able to conserve water, using c. 60% of the normal amount between weeks 4 and 5 post germination. When grown at elevated atmospheric CO 2, rice plants with low stomatal density were able to maintain their stomatal conductance and survive drought and high temperature (40°C) for longer than control plants. Low stomatal density rice gave equivalent or even improved yields, despite a reduced rate of photosynthesis in some conditions.Rice plants with fewer stomata are drought tolerant and more conservative in their water use, and they should perform better in the future when climate change is expected to threaten food security.

A combined increase in seawater [CO2 ] and [H(+) ] was recently shown to induce a shift from photosynthetic HCO3 (-) to CO2 uptake in Emiliania huxleyi. This shift occurred within minutes, whereas acclimation to ocean acidification (OA) did not affect the carbon source. To identify the driver of this shift, we exposed low- and high-light acclimated E. huxleyi to a matrix of two levels of dissolved inorganic carbon (1400, 2800 μmol kg(-1) ) and pH (8.15, 7.85) and directly measured cellular O2 , CO2 and HCO3 (-) fluxes under these conditions. Exposure to increased [CO2 ] had little effect on the photosynthetic fluxes, whereas increased [H(+) ] led to a significant decline in HCO3 (-) uptake. Low-light acclimated cells overcompensated for the inhibition of HCO3 (-) uptake by increasing CO2 uptake. High-light acclimated cells, relying on higher proportions of HCO3 (-) uptake, could not increase CO2 uptake and photosynthetic O2 evolution consequently became carbon-limited. These regulations indicate that OA responses in photosynthesis are caused by [H(+) ] rather than by [CO2 ]. The impaired HCO3 (-) uptake also provides a mechanistic explanation for lowered calcification under OA. Moreover, it explains the OA-dependent decrease in photosynthesis observed in high-light grown phytoplankton.

The global climate is changing mostly because of the anthropogenic emission of greenhouse gases, especially CO2, into the atmosphere. The rate of atmospheric CO2 growth in the future will depend on the balance between emissions and sink strengths. The importance of land photosynthesis in dampening the CO2 increase in the atmosphere is now well recognized and both climate and global carbon (C) models incorporate photosynthesis- climate feedbacks. However, we still lack robust tools for partitioning different component fluxes (e.g. photosynthesis, respiration, decomposition) and for assessing the effects of climate change and atmospheric CO2 increase on the C-sequestration potential of the biosphere. The paper by Stimler and coworkers in this issue of New Phytologist (pp. 869-878) shows that carbonyl sulfide (COS) represents a useful tracer of gross photosynthesis. This offers the perspective of an additional independent tool to study the terrestrial C cycle and to investigate ecosystem responses to global change.

Branch bags were used to expose branches on mature Sitka spruce trees to either ambient [CO2] (A) or elevated [CO2] (E) for 4 yr. This paper reports the effects of this treatment on the growth, development and phenology of the branches, including shoot expansion, shoot numbers, needle dimensions, needle numbers and stomatal density. The effect of elevated [CO2] on the relationship between leaf area and sapwood area was investigated. Exposure to elevated [CO2] doubled photosynthetic rates in current-year shoots and, despite some down-regulation, 1-yr-old E shoots also had higher rates of photosynthesis than their A counterparts. Thus, the amount of assimilate fixed by E branches was substantially more than that fixed by A branches; however, this increase in the local production of assimilate did not lead to an increase in non-structural carbohydrate or stimulate growth or meristematic activity within the E branches. There was a very consistent relationship between leaf area and stem cross-sectional area that was not influenced by [CO2]. However, unbagged branches had thicker stems than bagged branches, resulting in a slightly lower ratio of leaf area to cross-sectional area. The implications of the results for the modelling of growth and allocation and the potential utility of the branch bag technique are discussed.

summary Theoretical developments in our understanding of leaf gas exchange processes and carbon isotope composition (δ13C) mean that it should now be possible to model their responses to global environmental change. Such a model would be of use for process-based interpretations of historical changes in leaf δ13C and for understanding the global stable carbon isotope balance. This paper describes the development and validation of a model towards this aim. The resulting model is used to simulate changes in leaf photosynthesis, stomatal conductance and δ13C of limber pine (Pinus flexilis) in response to the past 30000 y of global environmental change. The predictions of needle δ13C are in line with reported measurements of δ13C from fossilized Pinus flexilis needles preserved in packrat middens in western USA. Leaf gas exchange predictions show that the increased water use efficiency (WUE) of these trees growing in present-day environments, relative to the past, was brought about through an increase in photosynthetic rates and a decrease in stomatal conductance. This contrasts with the explanation of the recent (past 200 y) increase in the WUE of temperate and Mediterranean ecosystems inferred from δ13C measurements which are predicted by the model to have arisen largely by a decrease in stomatal conductance in response to increases in the concentration of atmospheric CO2 since the pre-industrial era. The model as described offers the potential to contribute to our understanding of vegetation effects on the global carbon isotope balance during the glacial periods, and therefore to provide a further constraint on the carbon cycle models used to explain the low concentrations of atmospheric CO2 at these times.

Ecophysiological information on the responses of plants to past global environmental changes may be obtained from Quaternary fossil leaves by measurements of (i) stomatal density, (ii) stomatal dimensions and (iii) 13 C discrimination (Δ13 C). The stomatal density and stomatal dimensions of leaves can be used to calculate stomatal conductance, while leaf Δ13 C values provide independent information on stomatal conductance and plant water use efficiency. In this paper, stomatal conductance is calculated for a sequence of radiocarbon dated fossil leaves of Salix herbacea L. which, together with herbarium and fresh material, represents a time-series spanning from the Last Glacial Maximum (LGM) (16500 yr BP) to the present day. The calculated values were then tested against leaf Δ13 C values previously reported for the same material. Our calculations show that stomatal conductance is negatively correlated with increases in atmospheric CO2 concentration over the last 16500 yr. This represents the first evidence of long-term response of stomatal conductance to increases in atmospheric CO2 concentration and confirms the response observed in experimental systems exposing plants to lower-than-present CO2 concentrations in controlled environments. The calculated decrease in conductance was positively correlated with leaf Δ13 C values, supporting this interpretation. The mean leaf Δ13 C value for the 18th and 19th centuries was significantly (P≥ 0.05) lower than the mean for the interval LGM-Holocene (10000 yr BP) implying an increase in plant water-use-efficiency over this time. These two lines of evidence, together with the stomatal density record from a glacial cycle, and experimental studies growing C3 plants in glacial-to-present CO2 concentrations, strongly imply that the water use efficiency of vegetation during the LGM was lower than at present and that it has increased since that time. Further evidence in support of this conclusion comes from the pattern of world vegetation types present during the LGM previously reconstructed using palaeoecological data. This evidence demonstrates that the distribution of vegetation types during the LGM was significantly different from that of the present day and showed a contraction in the area of rain forest and a major expansion of desert areas.

This paper presents the results of 2 yr of CO(2) flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 degrees C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO(2) fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. While CO(2) outputs through respiration were largely unchanged under warming, CO(2) inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO(2) fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. This study shows that in a future warmer climate the CO(2) sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness.

Experimental work on respiration and phosphate absorption by beech mycorrhiza has demonstrated a general similarity between these complex salt-absorbing organs and normal root systems. Not only may the rates of salt absorption be dependent upon the concentration of salt applied, upon the temperature and upon the availability of oxygen in the medium; but also mycorrhizas may be brought, by storage in water, into a condition where large respiratory responses to the application of salts are observable. In a previous paper (Harley, McCready and Geddes, I954) a general account was given of the development of respiratory response to salts during storage in distilled water. The response was shown to be developed by both fungal and host tissues and to result from the application of numerous nutritive salts and sugars. Of these, salts of ammonium and phosphate elicited the greatest increases of oxygen consumption and carbon dioxide emission by stored roots. In this present paper a more detailed study of the connexion between 'the respiration and phosphate absorption of excised mycorrhizas, both fresh and stored, is presented. It is based upon the results of experiments in which various oxygen concentrations, phosphate concentrations and inhibitors have been used. Its aim has been both to provide the necessary background knowledge for the interpretation of the functioning of mycorrhizal roots and also to show how experiments with mycorrhizas may contribute to the general problems of salt absorption by plant material. The paper is divided into two parts. The first describes experiments upon freshly excised roots using different oxygen concentrations and inhibitors to study their effects on respiration and phosphate absorption. In the second part, the results of experiments with roots stored for several days in water are used in an attempt to solve the problems raised. METHODS

T'he metabolic effects of water shortage or of artificial wilting on plants have been little studied. A recent review by Levitt (I95i) described the long-term effects and a brief statement of short-term effects was given by Wager (I954). In developing seeds of the garden pea (Pisum sativum) wilting brings about large changes in the rate of carbon dioxide evolution (Wager, 1954) and it was to investigate these changes that the work described below was undertaken. An analysis of the physiological effects of wilting on respiration has been attempted by a study of the carbon dioxide output during a period of anaerobiosis, the method first used by Blackman and Parija (1928) in their study of the respiration of apples. In peas the pattern of response changes with maturity and similar experiments should, ideally, be performed at all maturities while at the same time a wide range of wilting at each maturity should also be studied. This involves too much replication and as a simplification all maturities (except young) were studied but at only one level of wilting while different levels of wilting were mainly studied at one maturity. As the work progressed and the pattern of the changes in rate of carbon dioxide production emerged, it became clear that no interpretation of results would be of much value without knowledge of what was occurring at the biochemical level of organization, and the determination of the changes in certain metabolic products during a period of anaerobiosis was accordingly undertaken. The present paper describes the broad physiological pattern of behaviour of turgid and wilted maturing peas under aerobic and anaerobic conditions and later papers will attempt to interpret the results in the light of changes in concentration of metabolites.

HE experiments reported in this paper have all been carried out with barley grains, var. Plumage Archer, supplied annually by Messrs Sutton and Sons. 1LThe aim was to investigate the drift of carbon dioxide emission and oxygen absorption resulting from the breakdown of the grains' reserves. They were, therefore, kept continuously in the dark from the moment of first coming into contact with water until death occurred. A constant temperature of 2I? C. and the normal atmospheric gas mixture were employed throughout. In each experiment grain of the most recent harvest was used.

T HE purpose of the work, the results of which are dealt with in this paper, was to study the influence of oxygen and of carbon dioxide upon the progressive changes in chemical composition that take place during the storage of mature Bramley's Seedling apples after gathering. Two temperatures of storage were used, i and 50 C., because it was hoped that the results might afford some clue as to why functional breakdown and death of the tissue occur prematurely when these apples are stored at io C., although they remain healthy when stored at 50 C. It is known that Bramley's Seedling apples will not tolerate long exposures to temperatures lower than about 30 C. in air and that carbon dioxide accelerates the onset of functional breakdown at temperatures below this (Kidd & West, I930).