Abstract
Water-transport pathways through the leaf are complex and include several checkpoints. Some of these checkpoints exhibit dynamic behavior that may be regulated by aquaporins (AQPs). To date, neither the relative weight of the different water pathways nor their molecular mechanisms are well understood. Here, we have collected evidence to support a putative composite model of water pathways in the leaf and the distribution of water across those pathways. We describe how water moves along a single transcellular path through the parenchyma and continues toward the mesophyll and stomata along transcellular, symplastic and apoplastic paths. We present evidence that points to a role for AQPs in regulating the relative weight of each path in the overall leaf water-transport system and the movement of water between these paths as a result of the integration of multiple signals, including transpiration demand, water potential and turgor. We also present a new theory, the hydraulic fuse theory, to explain effects of the leaf turgor-loss-point on water paths alternation and the subsequent reduction in leaf hydraulic conductivity. An improved understating of leaf water-balance management may lead to the development of crops that use water more efficiently, and responds better to environmental changes.
Highlights
A Composite Model of Water Transport in the LeafLand plants evolved 450 million years ago from aquatic algae [1]. Vascular plants have evolved adaptations to extreme differences in environmental conditions while competing for light
Water-transport pathways through the leaf are complex and include several checkpoints
Higher crop yields have been correlated with increased stomatal conductance [10], high leaf hydraulic conductivity (Kleaf) [5] and greater water loss
Summary
Land plants evolved 450 million years ago from aquatic algae [1]. Vascular plants have evolved adaptations to extreme differences in environmental conditions while competing for light. The leaf hydraulic resistance is not constant, but dynamic, and varies nonlinearly with water potential This dynamic capability is controlled by aquaporins (AQPs) that are part of the plasma membrane (PIPs). We will try to use Steudle’s composite transport model to describe the movement of water through the leaf, emphasizing the different compartments and factors involved in the regulation of leaf hydraulic conductance and the possible role of AQPs in the dynamic regulation of the movement of water through the leaf
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