Effect of different potassium application time on the vigor of photosynthate transportations of edible sweet potato (Ipomoea Batata L.)

【Objectives】 In order to evaluate the effect of potassium application time on vigor of photosynthates transportation of edible sweet potato, a field experiment was carried out with different potassium application times. 【Methods】 Field experiments were conducted in 2011 and 2012 at the Agricultural Experiment Station of Shandong Agricultural University using a typical sweet potato cultivar (Beijing 553) as tested crop and potassium sulphate (K2SO4) as K fertilizer. The four treatments included no K application (CK), all potassium applied as basal fertilizer (JS), all K top dressed when the field was completely covered by crop (FS, about 50 days after planting) and all K top dressed at peak growth time (GS, about 110 days after planting). The photosynthates transport abilities of functional leaves, agronomic characters of stems, permeation power of photosynthates transportation between source and sink (concentration gradient of carbohydrate, amino acid and K+ from stem top to base) and photosynthates accumulation characteristics of root tubers were analyzed. 【Results】 K application could increase fresh yield, dry weight and bulking rate of root tubers of sweet potato significantly, and the treatments of JS and FS increased considerably. Treatment JS decreased carbohydrate content and the ratio of sucrose to starch content significantly at early growth stage (50 d after planting), and improved sucrose content and the ratio of sucrose to starch content of functional leaves notably during the middle and late growth stages (110 d and 170 d after planting), and the increases were 10.01% and 27.14% (2011), 16.16% and 61.57% (2012), which could improve output ability of photosynthates of functional leaves at this time. Treatment JS also increased the diameter of stem base at early growth stage, and the increase was over 20%, which increased cross sectional area of transport corridor of photosynthates. During middle and late growth stages, treatment JS and FS improved the number of branches, and the average increases of treatment JS were 36.53% and 48.44%, while treatment FS were 19.60% and 46.17%; they also decreased the mean distance between source and sink but increased the percent of effective distance (51-200 cm). The gradient in content of free AA and K+ was observed; the highest content of AA and K+ occurred near the shoot apex, decreasing towards the base of the stem. Potassium application intensified the decrease, and treatment JS and FS reached the significant level, and the amino acid average increases of treatment JS were 87.58% and 39.56%, while K+ average increases were 272.81% and 75.58%. JS treatment also reduced sucrose and starch content of stem base considerably during early and middle growth stages. Furthermore, JS and FS reduced the sucrose content in both stem top and stem base significantly at late growth stage, meanwhile declined the starch content. Treatments with potassium application added the dry matter weight of root tubers, and improved the bulking rate of root tubers as well, in which treatments JS and FS had large increase. 【Conclusions】At early growth stage, potassium application reduced the synthesis of starch in functional leaves and guarantee the abundant supply of photosynthates for growth of root tubers. Meanwhile, potassium application enlarges the cross sectional area of photosynthates transport corridor and improves osmotic pressure of photosynthates transportation along the stem. All above accelerates photosynthates unloading from stem base and helps the formation of strong sink in time. During middle and late growth stages, potassium application will improve photoaynthates unloading by means of increasing the sucrose content and the ratio of transportable sucrose, adding the number of transport corridors, shortening the transport distances, raising the efficiency of transportation and improving osmotic pressure of photosynthates transportation along the stem. The beneficial functions of potassium application are in favor of photosynthates transportation from stem base to root tubers, forming more root tubers and high fresh yield as a result. Under above experimental conditions, all potassium fertilizer applied as basal fertilizer gives the best result.