Drought responses of C4 plants: resolving the effects of physiological pathway from phylogenetic history

在温暖的气候中，利用C4光合作用途径的植物主导着草原生态系统和谷物生产。这一群体生态优势的多重进化起源和突然的地质变化都与大气二氧化碳和气候的变化有关。因此，C4植物对全球变化的反应对于人为气候变化下的生态系统资源管理和了解地球系统至关重要，这两个目标都是国家环境研究中心的关键科学目标。近年来，人们的注意力已经从大气中的二氧化碳转移到水的可获得性上，这是C4植物对过去和未来大气变化做出反应的关键驱动因素，恢复了关于C4植物-干旱关系的重大悬而未决的问题作为研究重点。至关重要的是，如果C4光合作用在利用水分方面比C3型更有效，为什么具有NADP-ME C4亚型的物种在草本植物区系中的比例随着降雨量的减少而下降，而具有NAD-ME亚型的物种则表现出相反的模式？C4光合作用的某些亚型是否具有抗旱性，而其他亚型是否与干旱敏感性有关？或者，这些相关性与C4光合作用的固有属性无关，而是与C4途径起源的独立植物谱系的特征有关？该项目旨在通过在新的分子遗传分类框架内将实验、模型和田间调查联系起来，解决C4植物水分关系的这些生理和系统发育成分。我们的第一个假设，即C4生理对植物抗旱性的直接影响，将通过使用同属C3/C4种对的系统发育控制实验来检验，每个种对代表单子叶和优子叶类群中独立的C4光合作用起源。在一种新的气孔控制机制模型的框架内设计了受控环境实验，以形成干旱条件下C4生理本身如何影响植物水分关系的综合图景。我们的第二个假设，即C4牧草中存在一个重要的耐旱性系统发育成分，将在PACCAD分支中进行测试，该分支包括世界上所有的C4牧草和至少四个独立的C4途径起源。通过将筛选分析扩展到嵌套在同一分支中的C3组，生理和系统发育假说的相互作用将被量化。实验将针对决定水分吸收、运输和损失以及脱水耐性的关键植物性状，以测试C3和C4类型、NADP-ME和NAD-ME亚型、独立草枝及其相互作用之间的对比。南非的一项多因素共同花园实验将使用NADP-ME C4和C3种的一个小样本，研究这些相互作用对自然气候和土壤条件下的生产力和水分利用的影响，这两个物种共同构成了南部非洲牧草植物群的主要部分。

Plants using the C4 photosynthetic pathway dominate grassland ecosystems and cereal production in warm climates. Both the multiple evolutionary origins and abrupt geological shifts in the ecological dominance of this group have been linked with changes in atmospheric CO2 and climate. C4 plant responses to global change are therefore of fundamental importance for ecosystem resource management under anthropogenic climate change and for understanding the Earth System, both key scientific objectives for NERC. In recent years, attention has shifted from atmospheric CO2 towards water availability as a key driver of C4 plant responses to past and future atmospheric change, reinstating major unanswered questions about C4 plant-aridity relationships as research priorities. Crucially, if C4 photosynthesis is more efficient in its use of water than the C3 type, why does the fraction of species with the NADP-ME C4 sub-type decline in grass floras as rainfall decreases, whereas species with the NAD-ME sub-type show the opposite pattern? Do some sub-types of C4 photosynthesis confer drought tolerance, whilst others are linked with drought sensitivity? Or are these correlations unrelated to the inherent properties of C4 photosynthesis, but instead linked with the traits characterising the independent plant lineages where the C4 pathway originated? The proposed project aims to resolve these physiological and phylogenetic components of C4 plant water relations by linking experimental, model and field investigations within the framework of new molecular genetic classifications. Our first hypothesis, that there is a direct effect of C4 physiology on plant drought tolerance, will be tested with phylogenetically controlled experiments using congeneric C3/C4 species pairs, each representing independent C4 photosynthetic origins across monocot and eudicot groups. Controlled environment experiments have been designed within the framework of a new mechanistic model of stomatal control to develop an integrated picture of how C4 physiology per se influences plant water relations under drought. Our second hypothesis, that there is an important phylogenetic component to drought tolerance in the C4 grasses, will be tested within the PACCAD Clade, which encompasses all of the world's C4 grasses and at least four independent origins for the C4 pathway. The interaction of physiological and phylogenetic hypotheses will be quantified by extending the screening analysis to C3 groups nested within the same clade. Experiments will target key plant traits determining water uptake, transport and loss, and desiccation tolerance to test the contrasts between C3 and C4 types, NADP-ME and NAD-ME sub-types, independent grass clades, and their interactions. A multi-factorial common garden experiment in South Africa will examine the implications of these interactions for productivity and water-use in natural climate and soil conditions, using a sub-sample of the NADP-ME C4 and C3 species which together comprise a major part of the southern African grass flora.