Relationship of carbon isotope discrimination with biomass and water use efficiency for alfalfa in northwestern China

Abstract

Carbon isotope discrimination (∆) has been proposed as a method for evaluating transpiration efficiency in C 3 plants. It is important to study the association of ∆ in different parts with the long-term water use efficiency (WUE). In this paper, plant height, ∆ in leaf water soluble carbohydrates (∆ WSC ), photosynthetic gas exchange parameters, ∆ in aboveground shoot (∆ AGS ), stem/leaf weight ratio, aboveground biomass (AGB), evapotranspiration, and WUE were determined in a collection of 10 alfalfa (Medicago sativa L.) genotypes under three water treatments (230 [T 1 ], 460 [T 2 ], and 700 mm irrigation [T 3 ]) in a Ningxia central desert steppe in 2012 and 2013. Significant interactions were found for AGB and WUE among year, treatment, and genotype, whereas significant interactions of year ☓ treatment and year ☓ genotype were only found for ∆ AGS but not for ∆ WSC . Generally, more water input increased the values of the traits tested except for WUE and single-leaf intrinsic WUE (P n /G s ). More irrigated water input resulted in increase in AGB, ∆ AGS , and ∆ WSC and decrease in P n /G s . The greatest WUE was recorded under T 2 in 2012. Significant and positive correlations were observed between AGB and Δ AGS (r = 0.588, 0.574, and 0.386 under T 1 , T 2 , and T 3 , respectively). Under T 2 and T 3 , ∆ WSC correlated negatively with WUE (r = −0.383 and 0.602, respectively). Ningmu No. 1 was found to have the greatest WUE (18.34 kg ha −1 mm −1 ) and yield (17.71 Mg ha −1 ) across water treatments and years. The findings suggest that in the semiarid areas applying irrigation of 230 to 700 mm yr −1 , ∆ in aboveground shoots or in leaf water-soluble carbohydrates at the flowering stage seems to be a useful indicators reflecting aboveground biomass and integrated WUE for alfalfa.