Effects of Different Nitrogen Application Rates on Starch Accumulation,Starch Synthase Gene Expression and Enzyme Activity in Two Distinctive Potato Cultivars

Nitrogen (N) is the most important nutrient for potato growth. N fertilizer has an important effect on the tuber yield and starch content in potatoes. In this study, taking the high-starch cultivar Kexin 22 and the low-starch cultivar Kexin 19 as experimental materials, three N fertilizer application rates—0, 150 and 300 kg/ha—were used to investigate the effects of different N application rates on starch accumulation and the expression of starch synthase genes in potato tubers with different starch contents. In the cultivar Kexin 22, the accumulations of amylose, amylopectin and total starch showed a ranking of N150 > N300 > N0 for the entire growth period. The cultivar Kexin 19 showed an accumulation pattern of N0 > N150 > N300 in the early growth period, N150 > N300 > N0 in the middle growth period and N300 > N150 > N0 in the late growth period. Compared with those in Kexin 19, the expressions of the glucose-1-phosphate adenyltransferase (AGPP-L), granule-bound starch synthase (GBSSI), starch-branching enzyme I (SBEI) and soluble starch synthase III (SSIII) genes in Kexin 22 were upregulated, whereas no obvious difference existed in the expression of the alkylglycerone phosphate synthase (AGPP-S) and soluble starch synthase II (SSII) genes between the two cultivars. In Kexin 22, the different N application rates had a significant effect on the peak expression levels of AGPP-L, GBSSI, SBEI and SSIII but had a small effect on the peak expression time for these genes. Among these genes, most showed a ranking of N150 > N300 > N0 in the early and middle growth periods, and all showed a ranking of N300 > N150 > N0 in the late growth period; most showed a peak expression time on the 65th day after emergence (DAE 65). In Kexin 19, the different N application rates had a significant effect on the peak expression levels and peak expression times of the AGPP-L, GBSSI, SBEI and SSIII genes. Among these genes, all showed a ranking of N0 > N150 > N300 in the early growth period, whereas most showed a ranking of N150 > N300 > N0 in the middle growth period, and all showed a ranking of N300 > N150 > N0 in the late growth period. In Kexin 19, the peak gene expression was shifted to an earlier date under the low N levels, and it was delayed under the high N levels. The effects of the N application rate on the activities of starch synthases AGPP, GBSS, SSS and SBE showed largely the same trends as those in the expression levels of the related genes. Therefore, to obtain a high harvest of starch yield, different N application rates should be recommended for different cultivars.