Abstract
Most investigations of plant responses to changes in temperature have focused on a constant increase in mean day/night temperature without considering how differences in temperature cycles can affect physiological processes and growth. To test the effects of changes in growth temperature on foliar carbon balance and plant growth, we repeatedly exposed poplar saplings (Populus deltoides × nigra) to temperature cycles consisting of 5 days of a moderate (M, +5 °C) or extreme (E, +10 °C) increase in temperature followed by 5 days of a moderate (M, -5 °C) or extreme (E, -10 °C) decrease in temperature, with respect to a control treatment (C, 23.4 °C). The temperature treatments had the same mean temperature over each warm and cool cycle and over the entire study. Our goal was to examine the influence of recurring temperature shifts on growth. Net photosynthesis (A) was relatively insensitive to changes in growth temperature (from 20 to 35 °C), suggesting a broad range of optimum temperature for photosynthesis. Leaf respiration (R) exhibited substantial acclimation to temperature, having nearly the same rate at 13 °C as at 33 °C. There was no evidence that preconditioning through temperature cycles affected the response of A or R to treatment temperature fluctuations. Averaged across the complete warm/cool temperature cycle, the A : R ratio did not differ among the temperature treatments. While foliar carbon balance was not affected, the temperature treatments significantly affected growth. Whole-plant biomass was 1.5 times greater in the M treatment relative to the C treatment. Carbon allocation was also affected with shoot volume and biomass greater in the M and E treatments than in the C treatment. Our findings indicate that temperature fluctuations can have important effects on growth, though there were few effects on leaf gas exchange, and can help explain differences in growth that are not correlated with mean growth temperature.
Highlights
Along with increases in mean global temperature (Meehl et al 2007), global climate models predict future increases in the frequency and magnitude of sudden episodes of extreme temperature (Coumou and Rahmstorf 2012)
We found that A did not differ significantly between the E and S treatments, indicating that it was not altered by temperature acclimation, or stress, after exposure to repeated temperature cycles, compared with plants that had never been exposed to temperature shifts
Volume and biomass growth increased in response to repeated cycles of fluctuating temperatures, even though there was little change in leaf respiration and net photosynthesis
Summary
Along with increases in mean global temperature (Meehl et al 2007), global climate models predict future increases in the frequency and magnitude of sudden episodes of extreme temperature (Coumou and Rahmstorf 2012). A short-term shift to a higher temperature caused a change in growth in herbaceous communities (De Boeck et al 2011; Dreesen et al 2012) and in foliar gas exchange of tree seedlings (Bolstad et al 2003; Ameye et al 2012). Tree biomass accumulation was positively correlated with mean growth temperature (Usami et al 2001; Allen and Vu 2009; Ghannoum et al 2010; Way and Oren 2010; Duan et al 2013), though this response was not universal and some experiments reported no increase in biomass accumulation when mean growth temperature was increased (De Lucia et al 1994; Maherali and DeLucia 2000). The effect of temperature on growth will most likely be determined by a combination of factors, including the thermal sensitivity of growth (Atkin et al 2006a), the temperature optimum for photosynthesis (Berry and Bjorkman 1980), the acclimation of respiration (Atkin et al 2005) and, in some cases, the acclimation of photosynthesis (Ghannoum et al 2010; Gunderson et al 2010)
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