A new technique for monitoring plant transpiration under field conditions using leaf optical dendrometry

Abstract

Monitoring plant transpiration (Ec) is important for quantifying irrigation amounts and maintaining optimum soil moisture conditions for maximum growth potential. However, real-time quantification of Ec dynamics under field conditions is still challenging due to technical limitations. Here, we present a new approach for monitoring plant water use using continuous measurement of stem water potential dynamics (Ψstem) under non-limiting water conditions. According to Darcy's law, if daytime root to stem hydraulic conductance (Kr-s) remains constant and the contribution of plant capacitance to Ec is minimal or accounted for, then daytime Ec could potentially be continuously inferred from Ψstem dynamics. We investigated the viability of this approach by first quantifying the dynamics of daytime Kr-s and the transient buffering effects of plant capacitance in irrigated plants of two crop species with contrasting physiology and life history: the perennial herbaceous Tanacetum cinerariifolium and the annual monocot Triticum aestivum. Daytime Kr-s dynamics and the effects of plant capacitance were determined by continuously and simultaneously measuring Ψstem with optical dendrometers and Ec with gravimetric method under naturally variable conditions of light, temperature and humidity. Kr-s remained relatively stable throughout the day and the effects of plant capacitance were negligible in both species under field conditions implying that the dynamics of Ec can be inferred from Ψstem. This was clearly supported by the close agreement between measured Ec and that predicted from the optically derived Ψstem. We conclude that optical dendrometry has the potential to continuously monitor both Ec and plant hydration status at high accuracy and temporal resolution under naturally variable atmospheric conditions and optimum water supply. Combining these two aspects of the optically derived Ψstem should create new opportunities to monitor crop water use and maintain plant hydration at optimum levels for maximum productivity in various crops under field conditions.