Using carbon isotope discrimination to select maize (Zea mays L.) inbred lines and hybrids for drought tolerance

In C3 plants, carbon isotope discrimination (Δ) shows large environmental and genetic variations and is linearly positively related to Ci/Ca, the internal to atmospheric CO2 ratio. It has been proven to be an accurate predictor of yield under drought conditions. In C4 crops, information concerning Δ variation in different organs of the plant and its possible association with biomass or grain yield is still scarce. In the present study, Δ was first analyzed at flowering, under drought and well-watered conditions, in different organs of maize (Zea mays L.) inbred lines differing for drought tolerance. Leaf Δ was higher than ear and silk Δ. Drought stress significantly increased Δ in all organs. Under drought, tolerant inbred lines showed significantly higher Δ than susceptible ones. There was a significant positive correlation between leaf, ear and silk Δ, and ear dry weight at flowering, a trait closely associated to grain yield. Carbon isotope discrimination was then analyzed at flowering in leaves of a set of drought tolerant maize hybrids and checks. Drought tolerant hybrids had significantly higher grain yield and Δ than the used checks. No correlation was found, however, between Δ and grain yield within tolerant hybrids. The use of Δ appears consequently accurate for a first screening of lines or hybrids highly contrasting for drought tolerance, but not for a more advanced selection among tolerant hybrids. More studies are needed in maize and other C4 crops to establish the causes of Δ variation and precise which organ sampled and time of sampling may allow the better precision in the screening process.