Carbon metabolism and partitioning in citrus leaves is determined by hybrid, cultivar and leaf type

The partitioning and metabolism of carbohydrates and lignin in leaves are essential for numerous physiological functions, growth and development of plants. This study was aimed to characterize these processes in four leaf types (i.e., autumn-, summer-, spring- and current-year spring shoots) of two citrus hybrids (loose-skin mandarin cultivars OP (i.e., cultivars 'Orah' (OR) Citrus reticulata Blanco and 'Ponkan' (PO) Citrus reticulata Blanco and the sweet orange cultivars NT 'Newhall navel orange' (NO) Citrus sinensis (L.) Osbeck and 'Tarocco' (TA) Citrus sinensis (L.) Osbeck) differing in fruit maturation under field conditions. For this purpose, we analyzed the levels of foliar structural, non-structural carbohydrates and lignin and the expression of related genes. Our results showed that the contents of structural, non-structural carbohydrates and lignin measured in the two hybrids and its partitioning were mostly determined by differences in gene expression recorded in hybrids, cultivars and leaf type. Particularly, differences between leaf types were largely attributed to up- and down-regulation of the expression of genes of cellulose synthesis, lignin precursor synthesis, the Calvin cycle, glycolysis, the tricarbonic acid and starch synthesis and degradation pathways. These differences between leaf types required more complex transcriptional regulation than differences between hybrids and cultivars. The present results indicated that the two citrus hybrids studied differed in the expression of structural, non-structural carbohydrates and lignin-related genes. Future studies have to show if the differences observed in foliar partitioning and metabolism of carbohydrates and lignin are translated into partitioning and metabolism of carbohydrates and lignin in the roots.