The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

These authors contributed equally to this work. SummaryWe describe the identi®cation and functional characterization of two Arabidopsis mitochondrial basic amino acid carriers (BAC), AtmBAC1 and AtmBAC2, which are related to the yeast ornithine (Orn) carrier Ort1p, also known as Arg11p. The arg11 mutant requires arginine (Arg) supplementation because it fails to export suf®cient ornithine from the mitochondrion to the cytosol where it is converted to arginine. Atm-BAC1 and, to a lesser extent, AtmBAC2 partially replaced the function of Ort1p in yeast arg11. The more ef®cient putative carrier, AtmBAC1, was expressed in E. coli, puri®ed, and reconstituted into phospholipid vesicles, where it transported the basic L-amino acids arginine, lysine, ornithine and histidine (in order of decreasing af®nity). AtmBAC1 recognized L-histidine whereas both yeast Ort1p and the mammalian ortholog ORNT1p do not. Also different from ORNT1p, AtmBAC1 did not transport citrulline. AtmBAC1 appeared to be more stereospeci®c than the yeast and mammalian ornithine carriers, exhibiting greater preference for the L-forms of arginine, lysine and ornithine. By RT-PCR, both AtmBAC1 and AtmBAC2 transcripts were detected in stems, leaves,¯owers, siliques, and seedlings. Expression of AtmBAC1 in seedlings is consistent with its involvement in Arg breakdown in early seedling development, i.e. delivery of Arg to mitochondrial arginase. The K m (0.19 mM) for Arg uptake by AtmBAC1 was close to the value we previously determined for the saturable component of Arg uptake into intact mitochondria from soybean seedling cotyledons.

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 (Anet ) and biomass in all [CO2 ] and temperature treatments. The +12 °C heat wave reduced afternoon Anet by 23% in ambient [CO2 ]. Although this reduction was relatively greater under elevated [CO2 ], Anet 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.

Stomatal pores on leaf surfaces respond to environmental and physiological signals to regulate leaf gas exchange. Mathematical models can predict stomatal conductance (g ), with one parameter (m or g) reflecting the sensitivity of g to the photosynthetic rate (A), atmospheric carbon dioxide concentration and atmospheric humidity, and a second parameter (g) representing the minimum g . Such models are solved iteratively with a photosynthesis model to form the core of many models of crop or ecosystem carbon and water fluxes. For three decades, g models have frequently been used assuming fixed parameter values for m or g and g across species and major plant functional types. This study of temperate tree species reveals significant interspecific variation in stomatal function. Applying species-specific parameterizations substantially reduced error in model predictions of g by 34 to 64% and A by 52 to 60% and resulted in significant correlation between modelled and measured values. This work challenges the long-held assumption of fixed parameter values and, in doing so, suggests an approach for reducing modelling error across a wide range of ecological and agricultural applications.

SummaryHere, we investigated the effect of different heat-wave intensities applied at two atmospheric CO 2 concentrations ([CO 2 ]) on seedlings of two tree species, loblolly pine (Pinus taeda) and northern red oak (Quercus rubra).Seedlings were assigned to treatment combinations of two levels of [CO 2 ] (380 or 700 lmol mol À1 ) and four levels of air temperature (ambient, ambient +3°C, or 7-d heat waves consisting of a biweekly +6°C heat wave, or a monthly +12°C heat wave). Treatments were maintained throughout the growing season, thus receiving equal heat sums. We measured gas exchange and fluorescence parameters before, during and after a mid-summer heat wave. The +12°C heat wave, significantly reduced net photosynthesis (A net ) in both species and [CO 2 ] treatments but this effect was diminished in elevated [CO 2 ]. The decrease in A net was accompanied by a decrease in F v ′/F m ′ in P. taeda and Φ PSII in Q. rubra.Our findings suggest that, if soil moisture is adequate, trees will experience negative effects in photosynthetic performance only with the occurrence of extreme heat waves. As elevated [CO 2 ] diminished these negative effects, the future climate may not be as detrimental to plant communities as previously assumed.

Dryland ecosystems represent >40% of the terrestrial landscape and support over two billion people; consequently, it is vital to understand how drylands will respond to climatic change. However, while arid and semiarid ecosystems commonly experience extremely hot and dry conditions, our understanding of how further temperature increases or altered precipitation will affect dryland plant communities remains poor. To address this question, we assessed plant physiology and growth at a long-term (7-year) climate experiment on the Colorado Plateau, USA, where the community is a mix of shallow-rooted C3 and C4 grasses and deep-rooted C4 shrubs. The experiment maintained elevated-temperature treatments (+2 or +4 °C) in combination with altered summer monsoonal precipitation (+small frequent precipitation events or +large infrequent events). Increased temperature negatively affected photosynthesis and growth of the C3 and C4 grasses, but effects varied in their timing: +4 °C treatments negatively affected the C3 grass early in the growing season of both years, while the negative effects of temperature on the C4 grass were seen in the +2 and +4 °C treatments, but only during the late growing season of the drier year. Increased summer precipitation did not affect photosynthesis or biomass for any species, either in the year the precipitation was applied or the following year. Although previous research suggests dryland plants, and C4 grasses in particular, may respond positively to elevated temperature, our findings from a cool desert show marked declines in C3 and C4 photosynthesis and growth, with temperature effects dependent on the degree of warming and growing-season precipitation.

Ozone is the most damaging air pollutant to crops, currently reducing Midwest US maize production by up to 10%, yet there has been very little effort to adapt germplasm for ozone tolerance. Ozone enters plants through stomata, reacts to form reactive oxygen species in the apoplast and ultimately decreases photosynthetic C gain. In this study, 10 diverse inbred parents were crossed in a half‐diallel design to create 45 F1 hybrids, which were tested for ozone response in the field using free air concentration enrichment (FACE). Ozone stress increased the heritability of photosynthetic traits and altered genetic correlations among traits. Hybrids from parents Hp301 and NC338 showed greater sensitivity to ozone stress, and disrupted relationships among photosynthetic traits. The physiological responses underlying sensitivity to ozone differed in hybrids from the two parents, suggesting multiple mechanisms of response to oxidative stress. FACE technology was essential to this evaluation because genetic variation in photosynthesis under elevated ozone was not predictable based on performance at ambient ozone. These findings suggest that selection under elevated ozone is needed to identify deleterious alleles in the world's largest commodity crop.

The influence of elevated temperature, elevated atmospheric CO 2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern, central and southern sites in its native range Abstract We investigated the effect of elevated [CO 2 ] (700 lmol mol À1 ), elevated temperature ( 1 2 1C above ambient) and decreased soil water availability on net photosynthesis (A net ) and water relations of one-year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north-south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO 2 ] and ambient temperature, elevated [CO 2 ] and ambient temperature, ambient [CO 2 ] and elevated temperature, or elevated [CO 2 ] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered).Measurements of net photosynthesis (A net ), stomatal conductance (g s ), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A net in response to elevated [CO 2 ] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 1C had no significant effect on A net at any of the sites in June or September despite over a 6 1C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A net in all treatments at each site in June. The effects of elevated [CO 2 ] and temperature on g s followed a similar trend. The temperature, [CO 2 ] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO 2 ] will significantly increase A net , while predicted increases in air temperature will have little effect on A net across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A net , though this may be mitigated by increased [CO 2 ].