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