Xylem- phloem hydraulic coupling explains multiple osmoregulatory responses to salt-stress.

Abstract

Salinity is known to affect plant productivity by limiting leaf-level carbon exchange, root water uptake, and carbohydrates transport in the phloem. However, the mechanisms through which plants respond to salt-exposure by adjusting leaf gas-exchange and xylem-phloem flow are still mostly unexplored. A physically-based model coupling xylem, leaf, and phloem flows is here developed to explain different osmoregulation patterns across species. Hydraulic coupling is controlled by leaf water potential,ѱI , and determined under four different maximization hypotheses: water uptake (i), carbon assimilation (ii), sucrose transport (iii), or (iv) profit function - i.e., carbon gain minus hydraulic risk. All four hypotheses assume finite transpiration occurs, providing a further constraint on ѱI. With increasing salinity, the model captures different transpiration patterns observed in halophytes (non-monotonic) and glycophytes (monotonically decreasing) by reproducing the species-specific strength of xylem-leaf-phloem coupling. Salt tolerance thus emerges as plant capability of differentiating between salt- and drought-induced hydraulic risk, and to regulate internal flows and osmolytes accordingly. Results are shown to be consistent across optimization schemes (i-iii) for both halophytes and glycophytes. In halophytes, however, profit-maximization (iv) predicts systematically higher ѱI than (i-iii), pointing to the need of an updated definition of hydraulic cost for halophytes under saline conditions. This article is protected by copyright. All rights reserved.