Factors preventing the performance of oxygen isotope ratios as indicators of grain yield in maize.

Main conclusion This paper provides new insights into source-sink relationships and transpiration processes which will eventually help to interpret δ 18 O as a genotype selection and ecophysiological tool for maize adaptation to drought. Oxygen isotope composition (δ18O) has been proposed as a phenotyping tool to integrate leaf transpiration in C4 crops, such as maize. Within this context we hypothesize that δ18O in leaves may reflect primarily environmental and genetic variability in evaporative processes, but that this signal may become dampened in transit from source to sink tissues. The aim of this study was to assess the relative importance of transpirative or translocation-related factors affecting δ18O in plant tissues of maize. We performed two water regime experiments, one with two varieties under semi-controlled conditions, and another in the field with 100 genotypes during two consecutive years. The δ18O in organic matter at the leaf base was strongly correlated with the δ18O in stem water, indicating that it could be a good proxy for source water in extensive samplings. Compared to leaves, we observed an 18O depletion in silks and grains, but not in stem-soluble organic matter. We interpret this as evidence of exchange with unenriched water from source to sink, but mainly occurring within sink tissues. Although grain yield (GY) and physiological variables did not show clear intra-trial patterns against δ18O, the only tissues that correlated with GY in the linear regression approach were that of silks, giving an insight of evapotranspirative demand during female flowering and thus of potential maize lines that are better adapted to drought. This finding will eventually help to interpret δ18O as a genotype selection and ecophysiological tool for the adaption of maize and other crops to drought, offering insight into source-sink relationships and transpiration processes.