Genetic variability of the phloem sap composition of maize (Zea mays.l) during the grain filling period

Maize is a major food and energy crop which is widely cultivated worldwide. Nevertheless, our knowledge on the genetic variability of the phloem sap composition of maize and relationship with grain yield is still limited. In this work by using a metabolomic approach, we have quantified the metabolite composition of the phloem sap exudate of the leaf located below the ear during the kernel-filling period of seventeen European and American lines of maize classified into five main groups on the basis of molecular markers polymorphism. We observed that, in addition to sucrose, glutamate and aspartate, which are abundant in the phloem sap of many plant species, large quantities of aconitate and alanine were also found in the phloem exudates of maize. Genetic variability of the phloem sap composition was shown in the different lines. We did not find any obvious relationship between the phloem sap composition and the five groups of maize lines. However, there was a clear relationship between two main groups of lines defined on the basis of the phloem exudates composition and the earliness of the flowering time. A comparison between the metabolite content of the ear leaf and the phloem sap exudates of each genotype revealed that for most of the carbon- and nitrogen-containing metabolites identified, there was a high proportionality in their relative content. Correlation studies performed between the phloem sap exudates metabolite content and yield-related traits also revealed that for a few carbohydrates there was either a negative or positive correlation with kernel yield and kernel weight. We suggest that these metabolites could be good predictive physiological markers for maize kernel yield, irrespective of the genetic background.