Carbon Isotope Discrimination and Transpiration Efficiency in Common Bean

Abstract

Carbon isotope discrimination (Δ) values were measured in diverse genetic lines of common bean ( Phaseolus vulgaris L.) as indicators of water‐use efficiency. Using on‐line Δ measurements, there was a close agreement between observed Δ values and those predicted by C 3 ‐photosynthesis Δ models. Additionally, Δ values were also significantly correlated with transpiration efficiency estimates ( W ) measured in potted plants grown outdoors on a long‐term basis. Based on ancestral race and domestication center, there were no significant differences in W or Δ. However, differences occurred when cultivars were analyzed on the basis of the geographic region for which they had been developed. On average, lines developed for Central and South America exhibited significantly higher W values (0.63 vs. 0.57 mmol mol ‐1 , respectively) and lower Δ values (21.07 vs. 21.51‰, respectively) than did lines developed for North America. There were also small, but significant differences in the slopes of the relations between Δ and W that were dependent on the geographic region for which lines had been developed. North American common bean lines exhibited a steeper relationship between these two parameters than did Central‐South American lines. The basis for these apparent differences in transpiration efficiency is unclear, but may have been associated with variation in leaf conductances or paraheliotropic leaf movements among the two groups.