Sucrolytic activities in relation to sink strength and carbohydrate composition in tomato fruit growing under salinity

The effects of low (control), moderate, and high salinity on sucrose metabolism, in relation to fruit growth, were assayed in a commercial F-1 tomato hybrid (Lycopersicon esculentum L. Mill) Radja (GC-793). High salinity reduced both fruit growth rate and the mature fruit weight by 44%, while moderate salinity did not affect them significantly. The highest sink strength in control fruits was found between 20 and 30 days after anthesis (DAA). During this critical growing period, the import rate was reduced by more than 30% and 50% under moderate and high salinity, respectively, although the highest sink strength in salinized fruits was registered between 40 and 50 DAA. Starch was accumulated up to 40 DAA and to a greater extent in the salt-treated fruits, with a negative correlation between starch accumulation and the sink strength during the critical growing period (20-30 DAA). The hexose accumulation in mature fruits (60 DAA) was about 2 and 1.5 times greater in moderately salinized fruits than in control and highly salinized ones, respectively. The sucrose content, which declines in control fruits with time, was accumulated in the salinized ones during the rapid growing period. The acid invertase (E.C. 3.2.1.25) was the main sucrolytic activity in control fruits between 10 and 40 DAA, with a positive correlation between this activity and the sink strength during this period. However, the cytoplasmic sucrolytic activities, neutral invertase (E.C. 3.2.1.26) and sucrose synthase (E.C. 2.4.1.13) were more important in salinized fruits. Sucrose synthase activity increased in relation to the intensity of stress between 30 and 40 DAA coinciding with the highest starch accumulation and the depletion in the sucrose content, and preceding the highest sink strength in the salinized fruits (50 DAA). The implication of sucrolytic activities in the import of assimilates and fruit growth under these adverse conditions is discussed.