The relative impacts of daytime and night-time warming on photosynthetic capacity in Populus deltoides

In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night-time respiration and daytime photosynthesis of leaves in canopies of 4-m-tall cottonwood ( Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 ∞ C) and either (i) a constant nocturnal temperature of 20 ∞ C or (ii) increasing nocturnal temperatures (15, 20 or 25 ∞ C). In the first (day warming only) experiment, rates of night-time leaf dark respiration ( R dark ) remained constant and leaves displayed a modest increase (11%) in light-saturated photosynthetic capacity ( A max ) during the day (1000–1300 h) over the 6 ∞ C range. In the second (dual night and day warming) experiment, R dark increased by 77% when nocturnal temperatures were increased from 15 ∞ C (0·36 m mol m 2 s 1 ) to 25 ∞ C (0·64 m mol m 2 s 1 ). A max responded positively to the additional nocturnal warming, and increased by 38 and 64% in the 20/28 and 25/31 ∞ C treatments, respectively, compared with the 15/25 ∞ 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. Key-words : carbohydrate content; global warming; net carbon exchange; photosynthesis.