Wood rays represent the most important living tissue in xylem of trees and have two key functions: radial transport and storage. Radial transport is a bi-directional exchange process between living bark and sapwood that involves water, osmotica and other substrates. Storage concerns the role of rays in accumulating and releasing non-structural carbohydrates (NSC).
The source-sink paradigm of phloem function is widely accepted among plant physiologists. In tree stems gradients in phloem turgor are usually small. It was suggested that uptake and release of NSC by rays is responsible to maintain NSC supply to all sinks. How water limitation affects ray function as mediator among sinks remains undescribed.
With increasing water limitation plants progressively rely on internally stored water. Utilising stored water requires shifting pressure gradients and osmotic potential using compounds like NSC. However, water limitation reduces de-novo NSC synthesis, whilst increasingly steeper gradients of osmotica are required to mobilise and recharge stored water sources. This leads to conflicts between maintaining source-sink balance and availability of NSC for osmotic regulation during water limitations. Moreover, NSC have been suggested to play an important role in restoring tree hydraulic functioning, further highlighting the role of ray physiology during water limitation.
Results/Conclusions
Physiological and hydraulic functioning of rays is essential for tree health. However, the multifaceted role of rays during water limitation remains understudied. Importantly, this lack of knowledge prevents accounting for changes in ray physiology due to water limitation in long-distance transport models for plants, impeding our capacity to predict effects of water availability on long-term survival of trees. We review how water limitation will affect the multiple links between fluxes of water and carbon mediated by rays. A set of novel experiments will be introduced that help progress our understanding of the important roles wood rays play during water limitation.