ABSTRACT
The increasing demand for food production and predicted climate change scenarios highlight the need for improvements in crop sustainability. The efficient use of water will become increasingly important for rain-fed agricultural crops even in fertile regions that have historically received ample precipitation. Improvements in water-use efficiency in Zea mays have been limited, and warrant a renewed effort aided by molecular breeding approaches. Progress has been constrained by the difficulty of measuring water-use in a field environment. The stable carbon isotope composition (δ13 C) of the leaf has been proposed as an integrated signature of carbon fixation with a link to stomatal conductance. However, additional factors affecting leaf δ13 C exist, and a limited number of studies have explored this trait in Z. mays. Here we present an extensive characterization of leaf δ13 C in Z. mays. Significant variation in leaf δ13 C exists across diverse lines of Z. mays, which we show to be heritable across several environments. Furthermore, we examine temporal and spatial variation in leaf δ13 C to determine the optimum sampling time to maximize the use of leaf δ13 C as a trait. Finally, our results demonstrate the relationship between transpiration and leaf δ13 C in the field and the greenhouse. Decreasing transpiration and soil moisture are associated with decreasing leaf δ13 C. Taken together these results outline a strategy for using leaf δ13 C and reveal its usefulness as a measure of transpiration efficiency under well-watered conditions rather than a predictor of performance under drought.