Abstract
In the present study, we investigated the responses of date palm (Phoenix dactylifera) to drought and heat as single stressors and in combination. We tested the hypotheses (i) that heat and drought enhance the capacity of the antioxidative system, and (ii) that due to the high stress tolerance of date palm, the plants' redox state will be widely unaffected, and (iii) that heat but not drought changes the plants' fatty acid composition and biosynthesis of isoprene, both contributing to the stabilization of membrane integrity. Photosynthesis was only weakly affected by both stresses, whereas the levels of the antioxidants ascorbate and glutathione in leaves dropped. This drop was, however, over-compensated by increased activities of glutathione reductase, an important enzyme of the antioxidative system. The plants' redox state was unaffected by stress as indicated by unchanged H2O2 levels. Because we do not know the concentration of isoprene at its site of action, isoprene emission might provide indirect hints on its possible functions. Isoprene emission strongly increased due to heat indicating its possible role as an antioxidant and for stabilization of thylakoid membranes. Fatty acids only reacted in response to drought. We conclude that the high heat and drought tolerance of date palm is the consequence of a concerted action of the antioxidative system, mainly based on enzyme activities and the assumed antioxidative effects of isoprene as well as adjustments in the fatty acid composition. (C) 2016 Elsevier B.V. All rights reserved.