Effect difference of potassium fertilizer on leaf photosynthetic characteristics and storage root yield of sweet potato under drought stress and normal water condition

【Objectives】 The experiment was carried out to investigate the effect of different potassium application rates on the photosynthetic characteristic and storage root yield in order to provide theoretical basis for high yield cultivation of sweet potato under drought cordition. 【Methods】 Sweet potato cultivar (Taizhong No.6) was used as tested crop and potassium sulphate (K2SO4) as K fertilizer, the water treatments included normal water supply (W1, soil with water content of 60%-70%) and drought stress (W0, soil with water content of 30%-40%). The K fertilizer application treatments included K2O of 0 (K0), 12.0 g/m2(K1), 24.0 g/m2(K2) and 36.0 g/m2(K3). The relative water content, chlorophyll fluorescence and photosynthetic characteristics of the sweet potato leaves and storage root yield were analyzed. 【Results】 Under both W0 and W1 treatments, application of potassium increased leaf area, chlorophyll contents and photosynthetic rates (Pn), enhanced photosynthate production and accumulation, and increased storage root yield and harvest index. The highest storage root yields were all appeared in K2 treatments in both water conditions, the differences between K2 and K3 was significant but not under well-watered conditions. The effects of potassium application on photosynthetic parameters were significantly different in W0 and W1 treatments. Under drought stress, potassium application increased water use efficiency (WUE), reduced stomatal conductance (Gs) which increased stomatal resistance, and decreased transpiration rate (Tr) and intercellular CO2 concentration (Ci); under well-watered condition, however, the results were opposite. Application of potassium increased relative water content (RWC), actual photochemical efficiency (ΦPSII) and maximal photochemical efficiency (Fv/Fm) of leaves under both water conditions, but the increase levels under drought stress was greater than under well-water condition.【Conclusions】 There is a significant difference in the regulation effect of leaf photosynthetic parameters after potassium application under both conditions. Under drought stress, application of potassium is beneficial to improve the drought resistance of sweet potato and increase storage root yield, but excess application of potassium will decrease significantly the storage root yield. The yield was not differed in well-water condition. There are three main mechanisms in which potassium fertilizer alleviates the damage to sweet potato caused by drought stress: the first is the increased leaf area, chlorophyll content and photosynthetic performance; the second is the closing of leaf stomata, enlarging limiting factors like stomatal resistance to reduce the loss of water transpiration, increasing the relative water content of the leaf and increasing the water use efficiency and net photosynthetic rate; the third is improved PSII conversion efficiency for primary light energy and the actual photochemical efficiency of the leaf, reduced the destructive effects of the excess excitation energy on the photosynthetic apparatus and increasing the photosynthetic capacity of the leaves. The regulating effect of potassium fertilizer was stronger under dought stress than well-watered condition.