Inhibition of sucrose and galactosyl-sucrose oligosaccharide metabolism in leaves and fruits of melon (Cucumis melo L.) under low light stress

Low light stress is a major limiting factor for melon to reach its full potential of fruit yield and quality in cold season production. This study investigated the effect of low light stress on sucrose synthesis and galactosyl-sucrose oligosaccharide loading in melon leaves, galactosyl-sucrose oligosaccharide unloading, and sugar accumulation in fruits under greenhouse conditions using the low light-tolerant cultivar Yujinxiang and the low light-sensitive cultivar Yuxue. The results showed that in melon leaves, sucrose content and activities of related synthase enzymes decreased significantly under low light stress. The reduction of sucrose content and the activity of sucrose phosphate synthase (SPS) and sucrose synthase in the synthesis direction (SS-s) in Yujinxiang (30.9%, 16.5%, 30.0%, respectively) were significantly less than those in Yuxue (60.6%, 31.6%, 40.5%, respectively). The raffinose and stachyose content as well as the activity of stachyose synthase also significantly decreased after shading. The reduction of stachyose synthase activity in Yujinxiang (23.8%) was significantly less than that in Yuxue (32.4%), indicating that sucrose synthesis and galactosyl-sucrose oligosaccharide loading in melon leaves were inhibited under low light stress and the low light-sensitive cultivar was more greatly influenced. In melon fruits, stachyose content and the activities of acid and alkaline α-galactosidases significantly decreased under low light conditions, but the raffinose content was higher from 10 to 20 days after anthesis, indicating that the imported assimilates reduced, and galactosyl-sucrose oligosaccharide unloading was inhibited after shading. During the earlier stage of fruit development, significant decreases in the activities of acid invertase (AI), neutral invertase (NI), and sucrose synthase in the cleavage direction (SS-c) were observed in the two cultivars after shading, sink strength reduced. In the later stage of fruit development, sucrose accumulation was inhibited by decreasing the activities of SPS and SS-s, but no significant difference was observed between the two cultivars suggesting that the difference of sugar content among the two cultivars might be due to the imported assimilates under low light conditions.