Sucrolytic activities during fruit development of Lycopersicon genotypes differing in tolerance to salinity

The different growth responses under control and moderate salinity (70 mM NaCl) in relation to the carbon partitioning and sucrose metabolism in developing tomato fruits [20 days after anthesis (DAA), start of ripening and ripe stages] were studied in the cultivated tomato Lycopersicon esculentum Mill (cv. H-324-1), in the wild relative species L. cheesmanii (ac. LA-530) (hexose-accumulators), L. chmielewskii (ac. LA-1028) (sucrose-accumulator) and in two interspecific F-1 hybrids (hexose-accumulators) (F1-530: H-324-1 x A-530, F1-1028: H-324-1 x A-1028). The higher salt-tolerance of the wild species and hybrids with respect to the domestic tomatoes was also observed at the fruit level because these genotypes were less affected in the assimilation of dry weight (DW) under salinity. With the exception of the wild tomatoes, the sink strength, evaluated as the dry matter accumulation rate (mg DW day(-1) ) and the sink activity, evaluated as a relative growth rate (mg DW mg(-1) day(-1) ), were reduced during the early fruit growing period (20 DAA-start ripening). However, a total recovery of growth was registered in the salinized hybrid fruits during the late growing period (start of ripening-ripe fruits). The early reduction in sink activity in the hybrid and domestic fruits was related to a sucrose accumulation and a decrease in the total sucrolytic activity at 20 DAA, especially the cytoplasmic sucrolytic activities sucrose synthase (EC 2.4.1.13) and neutral invertase (EC 3.2.1.26). The further recovery in sink strength of the hybrid fruits was related to the maintenance of the insoluble acid invertase (EC 3.2.1.25) and the induction of the cytoplasmic sucrolytic activities, namely at the start of ripening stage, demonstrating the existence of an inverse relationship between these activities, which suggests a regulatory mechanism in order to maintain the sink capacity. The roles of different enzymes in the control of assimilate import under salinity in relation to the sucrose transport and possible regulatory mechanisms are discussed.