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 (Δ¹³C₁). Plants grown at low humidity were more enriched in ¹⁸O than those grown at high RH, as theory predicts. Within each humidity environment, increasing ABA concentration increased oxygen isotope enrichment of leaf cellulose (Δ¹⁸O(c)) and whole-leaf tissue (Δ¹⁸O₁). Values of Δ¹³C₁ and Δ¹⁸O₁ predicted by theoretical models were close to those observed, accounting for 79% of the measured variation in Δ¹³C₁ and 95% of the measured variation in Δ¹⁸O₁. Supporting theory, Δ¹³C₁ and Δ¹⁸O₁ in whole-leaf tissue were negatively related.