Intrinsic water use efficiency estimate: an isotopic method

Protocol summary and key steps

This protocol describes in detail how to calculate the intrinsic water use efficiency (W_iEi) fro¹³13C measurements in phloem samples collected at breast height in trees. Basically, the isotopic signature of photosynthetized carbohydrates¹³13¹²12C) in the phloem sap inform on the assimilation condition: a higher¹³13¹²12C) is associated with a lower transpiration rate, increasing heavy isotope¹³13C) use by the plant (Dawson et al. 2002). A highe¹³13¹²12C in the phloem content reflect a higher proportion of assimilated C compared to transpiration rate, in other words, a higher W_iEi. The method described here is based on the isotopic discrimination of¹³13C in trees and considers ecophysiological processes of stomatal and mesophyll conductance _ss and_mgm, respectively). The protocol aims to provide the operator with a step-by-step method from the collection of phloem samples in the field to the

final calculation. The protocol is already described in the scientific literature (Klein et al. 2016; Seibt et al. 2008; Vernay et al. 2020) but this document addresses more practical and technical details that a practitioner might expect. WUE (water use efficiency) corresponds to plant productivity or the productivity of an organ of the plant regarding the amount of water used to produce this fresh matter (Théroux Rancourt 2014). At an individual scale, WUE considers a ratio of the biomass produced compared to the amount of water used to produce this biomass. At the leaf level, _iUEi (intrinsic WUE) determines the ratio of₂CO2 assimilation per amount of water transpired and can take into account the micro-environment of the leaf, by considerin_s g_mgm to be more precise (Vialet-Chabrand 2013).

WUEi_Protocol_2023.pdf