Department of Forest and Ecosystem Science, The University of Melbourne, 4 Water Street, Creswick 3363, VIC, Australia.
Tree Physiol. 2013 Oct;33(10):1018-29. doi: 10.1093/treephys/tpt083. Epub 2013 Oct 31.
Current climate change predictions hint to more frequent extreme weather events, including extended droughts, making better understanding of the impacts of water stress on trees even more important. At the individual plant level, stomatal closure as a result of water deficit leads to reduced CO2 availability in the leaf, which can lead to photo-oxidative stress. Photorespiration and the Mehler reaction can maintain electron transport rates under low internal CO2, but result in production of reactive oxygen species (ROS). If electron consumption is decreased, upstream photochemical processes can be affected and light energy is absorbed in excess of photochemical requirements. Trees evolved to cope with excess energy and elevated concentration of ROS by activating photoprotective and antioxidative defence systems. The meta-analysis we present here assessed responses of these defence systems reported in 50 studies. We found responses to vary depending on stress intensity, foliage type and habitat, and on whether experiments were done in the field or in controlled environments. In general, drought increased concentrations of antioxidants and photoprotective pigments. However, severe stress caused degradation of antioxidant concentrations and oxidation of antioxidant pools. Evergreen trees seemed to preferentially reinforce membrane-bound protection systems zeaxanthin and tocopherol, whereas deciduous species showed greater responses in water-soluble antioxidants ascorbic acid and glutathione. Trees and shrubs from arid versus humid habitats vary in their antioxidative and photoprotective defence responses. In field experiments, drought had greater effects on some defence compounds than under controlled conditions.
目前的气候变化预测表明，极端天气事件将更加频繁，包括持续干旱，因此，更好地了解水分胁迫对树木的影响变得尤为重要。在单个植物水平上，由于缺水导致的气孔关闭会导致叶片中 CO2 可用性降低，从而导致光氧化应激。光呼吸和 Mehler 反应可以在低内部 CO2 下维持电子传递速率，但会导致活性氧（ROS）的产生。如果电子消耗减少，上游光化学过程可能会受到影响，并且光能会被过度吸收超过光化学需求。树木通过激活光保护和抗氧化防御系统来应对过量的能量和升高的 ROS 浓度。我们在这里进行的荟萃分析评估了 50 项研究中报告的这些防御系统的反应。我们发现，这些反应取决于胁迫强度、叶片类型和生境，以及实验是在野外还是在受控环境中进行。一般来说，干旱会增加抗氧化剂和光保护色素的浓度。然而，严重的胁迫会导致抗氧化剂浓度的降解和抗氧化剂库的氧化。常绿树种似乎更倾向于强化膜结合保护系统叶黄素和生育酚，而落叶树种在水溶性抗氧化剂抗坏血酸和谷胱甘肽方面表现出更大的反应。来自干旱和潮湿生境的树木和灌木在其抗氧化和光保护防御反应中存在差异。在野外实验中，干旱对某些防御化合物的影响大于受控条件下的影响。
