Sucrose Accumulation and Related Metabolizing Enzyme Activities in Seeded and Induced Parthenocarpic Muskmelons

To clarify the cause of low sucrose accumulation in seedless `Crest Earl's' netted muskmelon [ Cucumis melo L. (Reticulatus Group)] fruit induced by CPPU, the activity level of sucrose metabolizing enzymes was compared between seeded and seedless fruit. CPPU promoted growth of the ovary in both pollinated and nonpollinated flowers until 10 days after anthesis (DAA), and thereafter the growth rate of nonpollinated fruit was lower than in the controls. Sucrose accumulation of seedless fruit remained lower than in seeded fruit, but there was no difference in fructose and glucose content between seeded and seedless fruit. Acid invertase activity declined sharply 20 DAA in seeded and seedless fruit, and was hardly detectable at 35 DAA, when sucrose accumulation began. Neutral invertase (NI) activity in both seeded and seedless fruit decreased from 20 DAA until 35 DAA; thereafter, NI activity in seeded fruit remained relatively constant, with a small but insignificant increase in maturity. Sucrose synthase (SS-c: sucrose cleavage direction) activity in seeded fruit decreased from 20 to 30 DAA, and then increased as fruit matured, while SS-c activity in seedless fruit did not change during development. Sucrose phosphate synthase (SPS) activity in seeded fruit increased from 25 to 30 DAA and remained relatively constant until harvest. SPS activity in seedless fruit declined gradually from 30 to 45 DAA, then remained at a low level. Sucrose synthase (SS-s: sucrose synthesis direction) activity in seeded fruit increased rapidly after 30 DAA, concomitant with sucrose accumulation. In contrast, SS-s activity in seedless fruit increased only slightly after 30 DAA indicating levels of SS-s activity are closely related to sucrose accumulation in parthenocarpic seedless muskmelons. Chemical name used: [1-(2-chloro-4-pyridyl)-3-phenylurea] (CPPU).