Transient shade and drought have divergent impacts on the temperature sensitivity of dark respiration in leaves of Geum urbanum

The respiratory response of plants to temperature is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated the effects of environmental stresses on the short-term temperature response of dark respiration (R-dark) at the leaf level. We investigated the effect of shade and transient drought on the temperature sensitivity (Q(10); the proportional increase in respiration per 10 degrees C increase in temperature) of R-dark of Geum urbanum L. in controlled experiments. Shade effects were most pronounced following sustained, near-darkness, when rates of leaf Rdark at a set measuring temperature (25 degrees C) and the Q(10) of R-dark were both reduced. By contrast, rates of leaf R-dark and the Q(10) of R-dark both increased in response to the onset of severe water stress. Water stress was associated with a rapid (but reversible) decline in rates of light-saturated photosynthesis (P-sat), stomatal closure (g(s)) and progressive wilting. Re-watering resulted in a rapid recovery of P-sat, g(s) and a decline in the Q(10) of R-dark (due to larger proportional reductions in the rate of R-dark measured at 25 degrees C compared with those measured at 14 degrees C). The concentration of soluble sugars in leaves did not decline during drought (5-7 day cycles) or shading, but during drought the starch concentration dropped, suggesting starch to sugar conversion helped to maintain homeostatic concentrations of soluble sugars. Thus, the drought and shade induced changes in R-dark were unlikely to be due to stress-induced changes in substrate supply. Collectively, the data highlight the dynamic responses of respiratory Q(10) values to changes in water supply and sustained reductions in growth irradiance. If widespread, such changes in the Q(10) of leaf respiration could have important implications for predicted rates of ecosystem carbon exchange in the future, particularly in areas that experience more frequent droughts.