Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, Gustaf Hällströmin katu 2b, University of Helsinki, Helsinki, Finland.
Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, Latokartanonkaari 7, University of Helsinki, Helsinki, Finland.
Tree Physiol. 2019 Feb 1;39(2):173-191. doi: 10.1093/treephys/tpy153.
On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.
持续的气候变化正在增加全球大片地区干旱压力的风险。此类干旱事件会降低生态系统的生产力，而且与树木死亡的关系越来越密切。了解树木如何应对缺水是预测生态系统功能未来的关键。韧皮部是树木功能的核心，将非结构性碳水化合物、养分、防御和信息分子等资源在整株植物中移动。韧皮部的功能和运输资源的能力受到树木内部碳和水通量平衡的严格控制。因此，干旱预计会通过减少可用水量和新的光合同化物来影响韧皮部功能。然而，在过去几十年中，干旱对韧皮部的影响却很少受到关注。在这里，我们回顾了现有的关于干旱对韧皮部运输影响的知识，从细胞水平上的装载和卸载过程到对长距离运输的可能影响以及通过生态生理反馈对生态系统的影响。我们还指出了需要探索的新研究前沿，以提高我们对干旱下韧皮部功能的理解。特别是，我们展示了韧皮部运输是如何因干旱强度的增加而受到不同影响的，从没有反应到减缓，并探讨了严重干旱实际上是如何破坏韧皮部运输，从而威胁到树木的生存。由于资源运输会影响与树木相互作用的其他生物体，我们还回顾了韧皮部对干旱的反应的生态后果，特别是捕食、互利和竞争关系。最后，由于韧皮部是碳从源到汇的主要途径，我们展示了干旱如何通过韧皮部运输的变化来影响生物地球化学循环。总的来说，现有的知识与韧皮部对干旱的反应对理解树木和生态系统功能很重要的假设是一致的。然而，迫切需要对大范围的物种和生态系统进行未来研究，以全面了解这一问题。
