Carbohydrate metabolism in leaves and assimilate partitioning in fruits of tomato ( Lycopersicon esculentum L.) as affected by salinity

Abstract

Salinity improve tomato fruit quality by enhancing hexose accumulation. To study the involvement of the carbohydrate influx into fruits under saline conditions, we have studied carbohydrate metabolism in leaves and the partitioning of photosynthetic 14C in fruits of tomato plants exposed to 0, 50, or 100 mM NaCl. Carbohydrate content and the activity of ADP-Glc-PPase (EC 2.7.7.27) in the fruits were also determined at different stages of fruit development. Photosynthesis rates per unit leaf area were only slightly affected by moderate salinity (50 mM NaCl) while causing a considerable inhibition of leaf area expansion. Sucrose concentration was higher in leaves of plants exposed to NaCl than in non-saline controls, correlating with the enhanced activity of sucrose phosphate synthase (EC 2.3.1.14) and with the low activity of acid invertase (EC 3.2.1.26). Salinity enhanced the transport of 14C-assimilates from the pulse leaf to adjacent fruits and the diversion of 14C label to the starch fraction of the fruit. It also prolonged the period of starch accumulation in developing fruits. The immature fruit from salinity-treated plants showed significant higher activity of ADP-Glc-PPase with approximately twofold higher starch content than in controls. It is concluded that, under saline conditions, both a higher concentration of sucrose in the leaves and a faster rate of starch synthesis in the immature fruit may constitute part of a mechanism responsible for a higher sugar content in the mature fruit.