Partial K replacement by Na: strengthener or attenuator of xylem embolism vulnerability in Eucalyptus during drought stress?

Abstract

Eucalyptus is a typical fast-growing hardwood, widely cultivated in tropical and subtropical regions where soils display low water and nutrients availability. Moreover, the increasingly severity and frequency of droughts may strengthen tree mortality worldwide. The xylem embolism consists in the blockage of the plant vascular system, being considered the major cause of plant death during drought stress. Potassium (K) is one of the most limiting nutrients for Eucalyptus growth in regions with low soil-water availability, and can be partially replaced by sodium (Na). The combined use of K and Na leads Eucalyptus to a more conservative water use strategy and improves drought tolerance by increasing water uptake and storage, and also improving osmotic adjustments, such as reduced leaf water potential and better stomatal control. However, the positive impact of partial K substitution by Na on xylem embolism resistance in Eucalyptus plants under drought-induced stress is still unclear. Thus, this present work will provide new insights about the coordination between stomatal regulation and hydraulic plasticity and vulnerability of trees subjected to different conditions of water supply and nutrition. Therefore, this study aims to evaluate the partial K replacement by Na on xylem embolism resistance and the spreading patterns of vascular failure of E. grandis and E. camaldulensis exposed to drought, by means of non-invasive imaging techniques. We will evaluate the vulnerability curve and embolism parameters such as the water potential point at stomatal closure (P50), lethal water potential at 88 % loss of conductance (P88), hydraulic safety margin (HSM), and percentage loss of hydraulic conductivity (PLC). We expect that Eucalyptus with contrasting drought-tolerance varies in embolism vulnerability, and the partial K replacement by Na will increase hydraulic conductance while reduce the threshold recoverable water potential that a plant can withstand before xylem hydraulic dysfunction, allowing plants to operate close to their hydraulic limits. (AU)