Effects of shading and light restoration on yield formation and sugar substances of different types of sweet potato

【Objective】In order to provide theoretical basis for improving quality and efficiency of sweet potato in low light radiation area of southwestern China，the effects of shading and light restoration on root tuber yield and sugar substances of different types of sweet potato were studied.【Method】High quality edible variety Yanshu 25 and multi-purpose variety Xuzishu 8 were used as test varieties with a planting density of 60000 plant/ha in field. Three light/shading treatments were set：（1）Natural light control during the entire planting period（CK）；（2）continuous 40% shading from 50 d post planting until harvest（S）；（3）40% shading from 50 d after planting，lasted 40 dand then restored light（R）. Sampling analysis was conducted 1 d before shading（T1 period），40 d after shading（T2 period），80 d after shading（T3 period），and 120 d after shading（T4 period）. The effects of shading and light restoration on yield，dry matter accumulation and distribution，root tuber sugar substances content and starch quality of sweet potato were compared.【Result】During the T4 harvest period，the fresh tuber root yield of Yanshu 25 and Xuzishu 8 treated with S treatment was significantly decreased by 22.34% and 10.78% compared with CK（P<0.05，the same below），and the fresh tuber root yield of R treatment was decreased by 13.28% and 2.25% compared with CK. Under S and R treatments，the total dry weight per plant of Yanshu 25 was significantly decreased by 21.60% and 18.92% compared with CK，and the root tuber dry weight per plant was significantly decreased by 27.80% and 18.77% respectively. There was no significant difference between the treatments of Xuzishu 8（P>0.05，the same below）. At T2 period，the leaf area per plant and SPAD values of Yanshu 25 and Xuzishu 8 were higher than those of CK. The specific leaf weight of Yanshu 25 was significantly decreased，but there was no significant difference between the treatments of Xuzishu 8. At T3 period，the leaf area and specific leaf weight of Yanshu 25 were not significantly different from CK under S treatment，and the SPAD value was significantly higher than CK. The leaf area per plant and SPAD value of Xuzishu 8 treated with S treatment were significantly higher than CK，and the specific leaf weight was significantly lower than CK. Under R treatment，the leaf area per plant and leaf SPAD value of Yanshu 25 were significantly decreased by 50.92% and 5.46% compared with CK respectively，and specific leaf weight was significantly increased by 13.03%. Compared with CK，the leaf area per plant of Xuzishu 8 treated with R treatment was significantly increased by 17.37%，and the specific leaf weight and SPAD value were not significantly different from CK. S treatment significantly increased the contents of soluble sugar，reducing sugar and sucrose in Yanshu 25，and significantly decreased the starch content and starch yield per plant. The contents of soluble sugar and starch in R treatment were not significantly different from those in CK. The contents of soluble sugar and sucrose in S treatment of Xuzishu 8 during harvesting period was significantly decreased than that in CK. The soluble sugar content of R treatment was significantly higher than that of S treatment，but there was no significant difference between R treatment and CK. There was no significant difference in starch content and starch yield per plant among different treatments. Shading treatment reduced the particle size of sweet potato starch. At T4 harvest period，the solubility and swelling power of starch of Yanshu 25 were as follows：S treatment > CK > R treatment，and there were significant differences among all treatments. There were no significant differences in solubility and swelling power of starch of Xuzishu 8 among all treatments. The results of correlation analysis showed that the root tuber dry weight per plant of Yanshu 25 was significantly or extremely significantly （P<0.01）positively correlated with the total dry weight per plant，and the correlation coefficients between root tuber dry weight per plant and plant material accumulation，sugar substance of Xuzizhu 8 was low.【Conclusion】Shading reduces the yield of Yanshu 25 but improves edible quality. Sufficient light in the later period can make up for the yield loss caused by the early shading under the premise of ensuring its quality. Xuzishu 8 has high adaptability to the light environment， and can maintain stable yield under long-term shading and light environment changes. It can be planted in areas with low light energy radiation resources.