Developmental changes of sucrose synthase activity and abundance in root and leaf tissues of Zostera marina

The activity of sucrose synthase (EC 2.4.1.13) is correlated with tissue carbon (C) demand in vascular plants. The patterns of SS activity and abundance, and soluble protein abundance are described along age gradients within leaf bundles and in roots along the subterranean rhizome of the marine angiosperm Zostera marina L. (eelgrass). Soluble protein content decreased exponentially with increasing age in root tissue (concentrations in oldest roots were 20% of those of the youngest roots), but decreased in a linear fashion and overall to a lesser extent in leaves (to 62% of values in youngest shoots). Variation of SS activity with tissue age depended on the tissue and parameter to which measurements were normalized. Biomass-specific root SS activity decreased exponentially from 90–40 μmol g −1 fresh weight h −1 in root bundles numbers 1 (youngest) through 9, while protein-specific activity remained constant in the same tissue (approximately 16 μmol mg −1 protein h −1). Roots 10–12 had elevated, but variable SS activities. In leaves numbers 1–5, the decreases in both biomass- (200 to 50 μmol g −1 FW h −1) and protein-specific SS activity (23–12 μmol mg −1 protein h −1) were approximately linear. The abundance of SS protein in roots and leaves (determined by western analysis) also decreased with the increasing age, though not to the same extent (leaves), or with similar pattern (roots) as for SS activity. The age-dependent changes in SS activity are consistent with other indices of C metabolism (e.g., respiratory rates, carbohydrate content), but not with an index of N metabolism (glutamine synthetase activity) during development of Z marina tissues.