Relationships between stable isotope natural abundances (δ(13)C and δ(15)N) and water use efficiency in rice under alternate wetting and drying irrigation in soils with high clay contents

Natural abundance of the stable isotope (δ(13)C and δ(15)N) in plants is widely used to indicate water use efficiency (WUE). However, soil water and texture properties may affect this relationship, which remains largely elusive. Therefore, the purpose of this study was to evaluate δ(13)C as affected by different combinations of alternate wetting and drying irrigation (AWD) with varied soil clay contents in different organs and whole plant and assess the feasibility of using δ(13)C and δ(15)N as a physiological indicator of whole-plant water use efficiency (WUE(whole-plant)). Three AWD regimes, I(100) (30 mm flooded when soil reached 100% saturation), I(90) (30 mm flooded when reached 90% saturation) and I(70) (30 mm flooded when reached 70% saturation) and three soil clay contents, 40% (S(40)), 50% (S(50)), and 60% (S(60)), were included. Observed variations in WUE(whole-plant) did not conform to theoretical expectations of the organs δ(13)C (δ(13)C(organs)) of plant biomass based on pooled data from all treatments. However, a positive relationship between δ(13)C(leaf) and WUE(ET) (dry biomass/evapotranspiration) was observed under I(90) regime, whereas there were no significant relationships between δ(13)C(organs) and WUE(ET) under I(100) or I(70) regimes. Under I(100), weak relationships between δ(13)C(organs) and WUE(ET) could be explained by (i) variation in C allocation patterns under different clay content, and (ii) relatively higher rate of panicle water loss, which was independent of stomatal regulation and photosynthesis. Under I(70), weak relationships between δ(13)C(organs) and WUE(ET) could be ascribed to (i) bigger cracks induced by water-limited irrigation regime and high clay content soil, and (ii) damage caused by severe drought. In addition, a negative relationship was observed between WUE(whole-plant) and shoot δ(15)N (δ(15)N(shoot)) across the three irrigation treatments, indicating that WUE(whole-plant) is tightly associated with N metabolism and N isotope discrimination in rice. Therefore, δ(13)C should be used cautiously as an indicator of rice WUE(whole-plant) at different AWD regimes with high clay content, whereas δ(15)N could be considered an effective indicator of WUE(whole-plant).