Effects of leaf-sprayed salicylic acid on sucrose to starch conversion in underground tuber of yam (Dioscorea alata L.).

【Objective 】Effects of leaf-sprayed salicylic acid(SA) on the conversion of sucrose to starch in yam underground tuber was elucidated in order to provide references on knowing the mechanism of sucrose transformation and enhancing starch formation of yam. 【Method】Four treatments of SA with 0.0(i.e. the control), 0.5, 1.0, and 5.0 mmol/L were respectively sprayed thoroughly on leaves of Dioscorea alata L. cv. Baibian both at early and later of tuber formation stages. Underground tuber was periodically sampled to determine the contents of predominant carbohydrates and the activities of enzymes related to sucrose cleavage and starch biosynthesis. 【Result 】With the exception of the sharp increases of total soluble sugar(TSS) content at later stage for 5.0 mmol/L SA treatment, the contents of TSS, sucrose,glucose, fructose, starch, amylose(AM), amylopectin(AP), and cellulose among the other treatments had the same profiles of general change during the sampled period. Compared with the control, 0.5 and 5.0 mmol/L SA presented opposed effects on underground tuber development, the former not only significantly increased activities of sucrose synthase(SuSase), soluble acid invertase(SAInv), cell wall-bound acid invertase(CWBAInv), and neutral invertase(NInv),but also significantly enhanced activities of adenosine diphosphate pyrophosphorylase(AGPase), soluble starch synthase(SSS), grain-bound starch synthase(GBSS), and starch branching enzyme(SBE). But 1.0 mmol/L SA treatment had slightly positive effects on these measured enzymes only at middle stage. In all treatments, although sucrose content showed insignificantly positive correlations with activities of both SuSase and related starch-synthesized enzymes, the ratio of sucrose to total soluble sugar was significantly and positively correlated with activities of SuSase, SAInv, CWBAInv, NInv,AGPase, SSS, GBSS, SBE and starch content. The contents of glucose, fructose and their ratios in total soluble sugar significantly and negatively correlated to these measured enzyme activities, respectively. Glucose-fructose ratio had insignificant correlation with the aforementioned enzyme activities, but the ratios of sucrose-glucose and sucrose-fructose with activities of AGPase, SSS, and GBSS were significantly positive correlations. 【Conclusion 】Leaf-sprayed SA influenced both the transport and the fate of sucrose throughout yam underground tuber development through affecting the activities of enzymes related to both sucrose degradation and starch synthesis. SAs at 1.0 mmol/L or less promoted the transport of sucrose into tuber and the transition of sucrose to starch through increase of enzyme activities, and other contents of SAs had converse effects.