In C3 plants, carbon isotope discrimination (?) 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 ? variation in different organs of the plant and its possible association with biomass or grain yield is still scarce. In the present study, ? 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 ? was higher than ear and silk ?. Drought stress significantly increased ? in all organs. Under drought, tolerant inbred lines showed significantly higher ? than susceptible ones. There was a significant positive correlation between leaf, ear and silk ?, 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 ? than the used checks. No correlation was found, however, between ? and grain yield within tolerant hybrids. The use of ? 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 ? variation and precise which organ sampled and time of sampling may allow the better precision in the screening process.
