Oxygen isotope enrichment (Δ¹⁸O) as a measure of time-averaged transpiration rate

Abstract

Experimental evidence is presented to show that the ¹⁸O enrichment in the leaf biomass and the mean (time-averaged) transpiration rate are positively correlated in groundnut and rice genotypes. The relationship between oxygen isotope enrichment and stomatal conductance (g_s) was determined by altering gs through ABA and subsequently using contrasting genotypes of cowpea and groundnut. The Peclet model for the ¹⁸O enrichment of leaf water relative to the source water is able to predict the mean observed values well, while it cannot reproduce the full range of measured isotopic values. Further, it fails to explain the observed positive correlation between transpiration rate and ¹⁸O enrichment in leaf biomass. Transpiration rate is influenced by the prevailing environmental conditions besides the intrinsic genetic variability. As all the genotypes of both species experienced similar environmental conditions, the differences in transpiration rate could mostly be dependent on intrinsic g_s. Therefore, it appears that the Δ¹⁸O of leaf biomass can be used as an effective surrogate for mean transpiration rate. Further, at a given vapour pressure difference, Δ¹⁸O can serve as a measure of stomatal conductance as well.