Relative humidity‐ and ABA‐induced variation in carbon and oxygen isotope ratios of cotton leaves

Cotton ( Gossypium hirsutum L. cv. CS50) plants were grown at two levels of relative humidity (RH) and sprayed daily with abscisic acid (ABA) at four concentrations. Plants grown at lower humidity had higher transpiration rates, lower leaf temperatures and lower stomatal conductance. Plant biomass was also reduced at low humidity. Within each humidity environment, increasing ABA concentration generally reduced stomatal conductance, evaporation rates, superficial leaf density and plant biomass, and increased leaf temperature and specific leaf area. As expected, decreased stomatal conductance resulted in decreased carbon isotope discrimination in leaf material ( Δ 13 C l ). Plants grown at low humidity were more enriched in 18 O than those grown at high RH, as theory predicts. Within each humidity environment, increasing ABA concentration increased oxygen isotope enrichment of leaf cellulose ( Δ 18 O c ) and whole‐leaf tissue ( Δ 18 O l ). Values of Δ 13 C l and Δ 18 O l predicted by theoretical models were close to those observed, accounting for 79% of the measured variation in Δ 13 C l and 95% of the measured variation in Δ 18 O l . Supporting theory, Δ 13 C l and Δ 18 O l in whole‐leaf tissue were negatively related.