ABSTRACT∞ C treatment. These increases in photosynthetic capacity were associated with strong increases in the maximum carboxylation rate of rubisco ( V cmax ) and ribulose-1,5-bisphosphate (RuBP) regeneration capacity mediated by maximum electron transport rate ( J max ). Leaf soluble sugar and starch concentration, measured at sunrise, declined significantly as nocturnal temperature increased. The nocturnal temperature manipulation resulted in a significant inverse relationship between A max and pre-dawn leaf carbohydrate status. Independent measurements of the temperature response of photosynthesis indicated that the optimum temperature ( T opt ) acclimated fully to the 6 ∞ ∞ ∞ ∞ C range of temperature imposed in the daytime warming. Our findings are consistent with the hypothesis that elevated night-time temperature increases photosynthetic capacity during the following light period through a respiratory-driven reduction in leaf carbohydrate concentration. These responses indicate that predicted increases in night-time minimum temperatures may have a significant influence on net plant carbon uptake.

To further our understanding of the influence of global climate change on isoprene production we studied the effect of elevated [CO2] and vapour pressure deficit (VPD) on isoprene emission rates from leaves of Populus deltoides Bartr. during drought stress. Trees, grown inside three large bays with atmospheres containing 430, 800, or 1200 μmol mol–1 CO2 at the Biosphere 2 facility, were subjected to a period of drought during which VPD was manipulated, switching between low VPD (approximately 1 kPa) and high VPD (approximately 3 kPa) for several days. When trees were not water-stressed, elevated [CO2] inhibited isoprene emission and stimulated photosynthesis. Isoprene emission was less responsive to drought than photosynthesis. As water-stress increased, the inhibition of isoprene emission disappeared, probably as a result of stomatal closure and the resulting decreases in intercellular [CO2] (Ci). This assumption was supported by increased isoprene emission under high VPD. Drought and high VPD dramatically increased the proportion of assimilated carbon lost as isoprene. When measured at the same [CO2], leaves from trees grown at ambient [CO2] always had higher isoprene emission rates than the leaves of trees grown at elevated [CO2], demonstrating that CO2 inhibition is a long-term effect.