Carbon Isotope Discrimination and Productivity in Winter Wheat¹)

Abstract

Abstract The recent discovery that carbon isotope discrimination (Δ) relates negatively to leaf transpiration efficiency (CER/g s , defined as leaf CO 2 exchange rate/leaf conductance) has led to suggest the trait may be useful in breeding programs. However, the tendency of wheat yields to relate positively to Δ has raised the question as to whether plant scientists should be selecting for more negative (i.e., greater transpiration use efficiency) or more positive (greater yield potential) Δ. Two studies were undertaken, one to examine how Δ relates to yield and yield components in 15 winter wheat genotypes, and the other to study how Δ varies among two wheat genotypes from the USA and two from Italy known to differ in drought resistance. Variation in Δ among the fifteen genotypes was positively associated (r = 0.71**) with grain yield under irrigated conditions, but only weakly associated (r = 0.43) under non‐irrigated conditions. Δ was not significantly associated with shoot weight under both irrigated and non‐irrigated conditions. Significant correlations of Δ with tiller number and grains per spike under both irrigated and non‐irrigated environments suggest Δ influences wheat yields primarily through variations in grain number per land area. Variation in Δ among drought resistant and susceptible genotypes from the USA and Italy, grown in Northern Italy, did not relate simply to drought susceptibility, but appeared more related to the apparent improved adaptation of the Italian genotypes in their native environment. Δ of grain tissue was lower than in peduncle tissue, reflecting the improved water use efficiency of the later‐formed grain tissue which developed in a more dessicating environment. Lower Δ in grain tissue from non‐irrigated compared to irrigated environments was also attributed to increased transpiration use efficiency.