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Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf–air vapour pressure difference (D). We expected smaller gs, but greater Anet, responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs. The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions. AG gratefully acknowledges a studentship provided by the John Horseman Trust and the University of Birmingham. The BIFoR FACE facility is supported by the JABBS Foundation, the University of Birmingham and the John Horseman Trust. ARMK acknowledges support from the UK Natural Environment Research Council through grant NE/S015833/1. MJ and BEM acknowledge funding from the Australian Research Council (DE210101654, FL190100003).

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterised. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (Anet ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (gm ) and its effect on the estimation of carboxylation capacity (VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters, and quantified the biochemical and diffusional controls alongside the environmental controls over Anet . The seasonality of Anet was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and fall but increased to 31% in June. In contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited Anet in the spring, while daily temperatures and the gradual reduction of light availability limited Anet in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between gm and Anet , but not between gm and gs .

Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate. Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient. Heat tolerance acclimated to maximum leaf temperature (Tleaf ) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest Tleaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest Tleaf . The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling. More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.

The sensitivity of photosynthetic metabolism to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial feedback on future climate change. While temperature responses of photosynthetic capacities have been comparatively well investigated in temperate species, the responses of tropical tree species remain unexplored. We compared the responses of seedlings of native cold-adapted tropical montane rainforest tree species with those of exotic warm-adapted plantation species, all growing in an intermediate temperature common garden in Rwanda. Leaf gas exchange responses to carbon dioxide (CO2 ) at different temperatures (20-40°C) were used to assess the temperature responses of biochemical photosynthetic capacities. Analyses revealed a lower optimum temperature for photosynthetic electron transport rates than for Rubisco carboxylation rates, along with lower electron transport optima in the native cold-adapted than in the exotic warm-adapted species. The photosynthetic optimum temperatures were generally exceeded by daytime peak leaf temperatures, in particular in the native montane rainforest climax species. This study thus provides evidence of pronounced negative effects of high temperature in tropical trees and indicates high susceptibility of montane rainforest climax species to future global warming.

SUMMARY Cuttings of two clones of Norway spruce, Picea abies (L.) Karst., were exposed to three different ozone concentrations in open-top chambers during four months in the summer of 1990. The treatments were charcoal-filtered air, non-filtered air and non-filtered air with extra ozone (c. 30 ppb) added daily between 1100–1800 local time. During the last 4 wk of the exposure period, half the seedlings were drought-stressed, while the remainder were well watered. Biomass, gas exchange and water potential were measured during the drought stress period. The ozone treatments affected the two clones very differently. High ozone reduced the rate of biomass increase in the faster-growing clone (clone M) whereas ozone generally stimulated the rate of biomass increase in the slower-growing clone (clone L). At the end of the measuring period, the high-ozone treatments reduced the rate of root biomass increase of the well watered seedlings of the M-clone, but it had no effect on the drought-stressed seedlings of the same clone, probably because the root growth was already to a large extent inhibited by the drought stress. The treatment with the highest concentration of ozone partly protected the seedlings of the M clone against the drought stress, presumably by delaying shoot growth and thus delaying the increase in the total transpiring needle area. As a result, stomata tended to close less during the drought period in this treatment. The results are discussed in relation to the suggested critical level for ozone effects on trees.

SUMMARY A model for the impact of ozone on grain yield in spring wheat (Triticum aestivum L.) was constructed. The mode was based on the observations in the literature as well as in the present experiment, that the productivity of the above-ground biomass and the carbon partitioning of the above-ground biomass do not respond to ozone above the same threshold concentrations. The model was applied to a data set in which spring wheat was exposed to different ozone concentrations in open-top chambers. It was concluded that the nonlinearity of a simple dose-response curve for grain yield can be explained in terms of the product of two linear responses: (1) the response of total above-ground biomass to an ozone dose above 10 nl 1−1, and (2) the response of the harvest index to an ozone dose above 40 nl 1−1 The nitrogen content of the grain increased linearly with the dose above 40 nl 1−1.It is hypothesized that the effect of comparatively small ozone concentrations on total above-ground biomass is caused by a reduction of leaf duration by ozone, and that the effect on carbon partitioning is caused by ozone affecting the sink capacity or activity of the growing grain. The maximum coefficients of determination (r2) for the linear regression between grain yield and accumulated ozone dose above thresholds in the range of 0–70 nl 1−1, were never as great as those for total above-ground biomass and harvest index. Nevertheless, statistically significant linear regressions were obtained for the thresholds 20, 30 and 40 nl 1−1, the highest correlation being with 30 nl 1−1 Thus, it seems acceptable to use linear regressions between grain yield and the accumulated dose, although these do not take into direct account the complexity of the effects of ozone on wheat.

summary Rates of net photosynthesis were measured on Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for three seasons (1985–7) in open-top chambers. The treatments were non-filtered air + ozone (NF +), non-filtered air (NF) and charcoal-filtered air (CF). Ozone was added daily between 11.00–18.00 local time. The study was performed during September and October, 1987. The rate of net photosynthesis was determined using an infrared gas analyser (IRGA) connected to a small cuvette, which was equipped with an artificial light source for unilateral illumination. The measurements were made on shoots of three different age classes: current year, 1 year old and 2 years old. Measurements took place at saturating light intensity (1150 μmol m1s1), at a temperature of 16.6 ±0.7°C, and at a relative humidity of 67.5 ± 5.3%. In current-year shoots the rate of net photosynthesis expressed on a needle-area basis was lowest in the CF treatment and highest in the NF treatment. In 1- and 2-year-old shoots, a decreased rate of the net photosynthetic assimilation rate, expressed on a needle area basis, was observed in trees from the NF and NF+ treatment compared with that of trees from the CF treatment. The effect increased with increasing ozone concentration and shoot age. A significant increase in the rate of respiration, expressed on a dry-weight basis, was observed in current year shoots from trees exposed to NF+. A significant decrease of the leaf conductance to carbon dioxide was obtained in 2-year-old shoots exposed to NF and NF+. A decrease in the content of chlorophyll was observed in 1- and 2-year-old needles from trees exposed to NF+.

Rates of photosynthesis at different concentrations of CO2 (0-1850 μmol mol-1 ) were measured using one clone of Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for four seasons (1985-8) in open-top chambers. The treatments were non-filtered air+ozone (NF+), non-filtered air (NF) and charcoal-filtered air (CF). The measurements were made on shoots of four different age classes. No significant effects were found on photosynthesis or on leaf conductance to CO2 in current year shoots. In 1- to 3-yr-old shoots, leaf conductance to CO2 and rates of net photosynthesis at both 330 μmol mol-1 CO2 (Pn 330), and saturating concentrations of CO2 (Pn max), decreased with increasing shoot age and ozone concentration, although this was only significant for Pn 330 in 3-yr-old shoots. In current year shoots the intercellular concentration of CO2 was significantly higher in NF+-treated shoots compared with CF- and NF-treated shoots, while in the 2- and 3-yr-old shoots it was significantly higher in NF- and NF +-treated shoots compared with CF-treated shoots. The carboxylation efficiency significantly decreased in 2- and 3-yr-old shoots from the NF and NF+ treatments compared with shoots from the CF treatment. The gas phase limitation of photosynthesis decreased with increasing shoot age and ozone concentration.