Carbon Isotope Discrimination and Water-Use Efficiency in Crotalaria Cover Crops under Moderate Water Deficit

The legume species Crotalaria juncea and Crotalaria spectabilis, used as cover crops (CC) in tropical and temperate zones, are being evaluated in Uruguay where their productive potential may be limited by periods of summer water deficit. In this study, C. juncea and C. spectabilis were sown in pots with soil as a substrate. After substantial growth (20 days), two different treatments of water (80% or 50% of field capacity) were imposed on the plants during 30 days, and later water was restored during 40 days. Afterward, the dry matter produced and total evapotranspired water was determined in all plants. Also, water-use efficiency (WUE) was determined using a gravimetric technique and by carbon isotope analysis (Δ13C). Dry matter production of C. spectabilis was 1.7 times higher than that of C. juncea and both legumes species tolerated a moderate water deficit, which did not affect the redistribution of dry matter between the shoot and root. A negative linear relationship (r = 0.8; p < 0.0001) was found between the WUE and Δ13C in both species. In this study, the use of 13C to estimate WUE in Crotalaria spectabilis and Crotalaria juncea was validated for the first time. This strategy allowed us to recommend the use of Crotalaria spectabilis as a cover crop in conditions of moderate water deficit in a soil with a shallow A horizon.