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.