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Water availability in semi-arid regions is increasingly becoming threatened by erratic rains and frequent droughts leading to over-reliance on irrigation to meet food demand. Improving crop water use efficiency (WUE) has become a priority but direct measurements remain a challenge. There is therefore a need to identify reliable proxies and screening traits for WUE. Carbon isotope discrimination (Delta C-13) offers potential as a proxy for WUE, but its application is hindered by environmental factors and thus varies greatly among different studies. A two-year study was carried out with four moisture levels, ranging from well-watered (430-450 mm) to severe stress (SS) (220-250 mm), combined with four commercial triticale genotypes grown under field conditions in a hot, arid, steppe climate of Limpopo in South Africa. The study tested the use of Delta C-13 as a proxy of intrinsic WUE and grain yield of triticale. Second, delta C-13 and delta O-18 in combination with measured gas exchanges were used to test the functionality of the dual isotope model to interpret causes of variation in carbon isotope composition. Third, grain filling carbon assimilate sources were inferred from measured flag leaf and grain Delta C-13.

The results showed that moisture level significantly influenced grain yield, intrinsic WUE and Delta C-13 in triticale. Well-watered conditions increased grain yield, which ranged from 3.5 to 0.8 t ha(-1) and 4.9-1.8 t ha(-1) in 2013 and 2014 respectively. Delta C-13 was also high under well-watered conditions and decreased with decreasing moisture level while WUEintrinsic increased with decreasing moisture level. The relationship between Delta C-13 and grain yield was positive (P < 0.01), but only significant under water stressed conditions, indicating dependence of the relationship on moisture level. The relationship between Delta C-13 and WUEintrinsic did not depend on, the moisture level but showed a negative relationship when data for all moisture levels was combined. delta C-13 showed a negative relationship with photosynthetic rate (A), while the relationship between stomatal conductance (gs) and delta O-18 varied with season. Hence, the dual isotope model could only predict that variation observed in Delta C-13 and thus intrinsic water use efficiency was due to a concomitant decrease in both A and gs when transpiration was not limited by evaporative demand. Flag leaf Delta C-13 measured under SS at GS71 in the 2014 growing season, was significantly higher (2.2-3.6%0) than grain Delta C-13, also measured under SS, suggesting minimal contribution of flag leaf photosynthesis to grain filling. No genotypic differences were observed in Delta C-13, grain yield and WUEininnsic, indicating a probable lack of diversity in the studied genotypes.

The results of this study show that carbon isotope discrimination could be useful as a predictor of triticale grain yield in drought prone areas. Delta C-13 also offers potential as a proxy for WUEintrinsic and breeding for lower Delta C-13 values could result in varieties with higher WUEIntrinsic in triticale. Flag leaf photosynthesis and pre-anthesis assimilates contribute much less carbon to grain filling under water stress than previously thought. Lastly, our results show that the dual isotope model is operational, but is not all encompassing but depends on evaporative demand. (C) 2016 Elsevier B.V. All rights reserved.