植物干旱胁迫下水分代谢、碳饥饿与死亡机理

植物在生长发育过程中受众多环境因子共同作用。随着全球气候变化,气温升高、降水量下降等问题频繁出现。目前气象学家一致预测未来环境变暖会使干旱更加频繁剧烈,这一环境改变使植物死亡更加严重。植物在水分胁迫、特别是干旱胁迫条件下,体内水分代谢与碳代谢会发生失衡现象:光合速率降低、蒸腾速率降低,带来生长降低;为维持植物新陈代谢,植物呼吸作用必然下调。在长期干旱胁迫条件下植物体内碳水化合物储存发生失衡现象,这种失衡使植物陷入碳饥饿现象。另外,由于水分失衡而出现的木质部栓塞和空穴会进一步加剧水分运输障碍,而修复空穴则需要大量非结构性碳水化合物(NSC),这使植物陷入两难选择。总结了植物干旱胁迫下,碳饥饿与水分代谢、植物死亡关系的相关研究,对未来的研究方向和重点提出建议,以期对未来的植物死亡研究提供帮助。

Relationship among drought, hydraulic metabolic, carbon starvation and vegetation mortality

Plants would be affected by different environmental factors during the whole life cycles. Events of regional-scale vegetation mortality appear more and more frequently with the global change which is associated with increased temperatures, droughts, and often (but not always) with biotic agents (such as insects and pathogens) outbreaks. In recent years, consistent predictions of a warming climate went with more frequent and severe droughts, coupled with effects of heat and drought on vegetation mortality, indicate that widespread mortality events are a likely future phenomenon. These harsh factors and predictions required us have to understand plant mortality mechanism, although these precise physiological mechanisms were poorly understood. The difficulties of these problems were about interaction among environmental factors and interaction among plant physiological action. For example, drought was always appeared coupled with heat. When heat appeared, plants would have some physiological reaction response to heat and drought. If stress lasted for a long time, plants would die. But the signal mechanism to understand of drought or heat death was hard to achieve. On the other hand, even drought was artificial controlled, plants physiological processes were interacted. When plants were under drought, stomata would close as humidity was low. Because of gas exchange decreasing, photosynthesis and respiration were down-regulated. As a result, carbon sink and storage were damaged under drought. But we cannot understand precisely about where, when and how vegetation die in each physiological process. As discussed above, plants would down-regulated photosynthesis, transpiration and therefore impede plant growth. In order to maintain metabolism, plants have to decrease respiration. Long lasting drought will lead to imbalance of carbohydrate storage which would eventually cause carbon starvation (failure to maintain metabolism, carbon supply from photosynthesis and the mobilization of nonstructural carbohydrate (NSC) is less than carbon use for respiration, growth and defense). Plants need NSC to maintain osmotic adjustment when water stress made soil water content is low, but this subsequently elevates the minimum survivable NSC content. On the other hand, drought would induce another adverse affect-xylem embolism and cavitation (phloem column was filled with gas or air caused by drought). Because of xylem embolism, water and nutrient were hard to transport to the top of the leaves. This can cut off organs that require NSC from storage or new NSC assimilated. Refilling of embolized xylem is an energy-requiring process in many species that depends on conversion of starch to sugars for aquaporin regulation and for the creation of an osmotic gradient within embolized conduits. If drought blocked refilling cavitation, xylem embolism would become more severe and subsequently decrease hydraulic conductivity, photosynthesis and increase turgor loss. This is a negative feedback loop that makes plants have to choose whether refilling the cavitation or storage for living. No matter how plants choose, carbon starvation would lead to a further hydraulic failure and push plants faster toward the threshold for hydraulic failure or carbon starvation. In this review, we discussed about the mechanisms of drought-induced mortality from carbon and hydraulic metabolism. Xylem embolism and cavitation which were induced by drought were also mentioned. Plants physiological mechanisms of mortality were complex and difficult to measure accurately. We could forecast these events and prevent vegetation mortality only if we understand these mechanisms deeper and deeper.