Oxygen isotope ratio of leaf and grain material correlates with stomatal conductance and grain yield in irrigated wheat

Theory (Craig and Gordon 1965; Dongmann et al. 1974; Sternberg et al. 1986; Farquhar and Lloyd 1993) suggests that the oxygen isotope ratio (δ¹⁸O) of plant material should reflect the evaporative conditions under which the material was formed, so that differences in stomatal conductance should show up in plant δ¹⁸O. To test this theory we measured the oxygen isotope ratio of organic matter from flag leaves at anthesis and grain at harvest from eight cultivars of spring wheat (Triticum aestivum L.) grown under irrigation in each of three seasons in Mexico. The cultivars ranged widely in stomatal conductance and in average grain yield, with which conductance was positively correlated. Supporting theory, the oxygen isotope ratio of flag leaves (δ¹⁸O₁) was found to correlate negatively with stomatal conductance for two of the three seasons. The significant correlations are consistent with high conductance cultivars having lower leaf temperatures and kinetic fractionation factors, and higher vapour pressure fractionation factors and Peclet numbers, all of which combine to result in less enriched δ¹⁸O₁. Yield (grain weight per square meter) was also found to be significantly negatively correlated with δ¹⁸O₁ in two of the three seasons. δ¹⁸O₁ was as good a predictor of yield as stomatal conductance, and significantly better than carbon isotope discrimination. Correlations between grain δ¹⁸O(δ¹⁸O(g)) and physiological parameters were less clear. Significant negative correlations between δ¹⁸O(g) and stomatal conductance, leaf temperature and yield were found only during the first season. By measuring the oxygen isotope ratio of cellulose extracted from leaf samples, the difference in fractionation factors (ε(cp)) for cellulose and whole leaf tissue was assessed. ε(cp) was found to be variable, and more negative when δ¹⁸O(c) and δ¹⁸O₁ were lower. Cultivar means for δ¹³C and δ¹⁸O of whole leaf material were found to be significantly positively related, and the factors required to produce such a relationship are discussed.