Effect of nitrogen on the size, specific gravity, crisp colour and reducing sugar concentration of potato tubers (Solanum tuberosum L.) cv. Kennebec

Abstract

Four field experiments were conducted over 2 years on coarse grain siliceous sands in the Mt Lofty Ranges (3 sites) and Lower South East (1 site) to examine the effect of different rates of applied nitrogen (up to 320 kg N/ha) on the size, specific gravity, crisp colour and reducing sugar concentration of potato tubers of the cv. Kennebec. As the rate of applied nitrogen (N) was increased, there was a significant (P<0.05) increase in the yield of 80-350 and >350 g tubers. Rate of applied N did not significantly affect the yield of <80 g tubers. Highest yields of >350 g (large) tubers occurred at N rates in the range 160-320 kg N/ha. In contrast, the yield of 80-350 g tubers (preferred size for crisping), for the 80-120 kg N/ha sates, was not significantly different from the maximum yield of 80-350 g tubers for each site. At all sites, total tuber yields for the 80-120 kg N/ha rates were not significantly different from maximum yields. The effect of applied N on specific gravity (SG) was significant; the magnitude of the effect varied between sites. For the pooled data from all sites, correlation between SG and nitrate-N concentration in the petioles of youngest fully expanded leaves (P-YFEL) were not significant, however, there were significant correlations between the reduction in SG and petiole nitrate-N concentration. When tlie lengths of the largest tubers were <2 mm. 25 mm and 50 mm, reductions in SG of less than 0.0025 occurred when the P-YFEL nitrate-N concentrations were 1.70-2.50%, 0.70-2.00% and 0.25-1.25%, respectively. Tuber SG increased slightly after storage for 2 and 4 months but the changes were not related to the rate of applied N. There were no consistent trends between sites in vascular ring colour index (CI) and medulla CI, determined after 0, 2 and 4 months storage, as the rate of applied N increased. However, significant (P<0.05) changes in vascular ring and medulla CI did occur during storage, with significant reductions in medulla CI for 3 of the 4 sites. For all sites, vascular ring CI was highest after 2 months storage. Reducing sugar concentration of tubers was significantly affected by the rate of applied N at 2 sites. Tubers from all sites also showed significant changes in seducing sugar concentration during storage. Maximum reducing sugar concentrations (up to 0.23% fresh weight) occurred after 2 months storage. Coefficients of determination (r2 ) for the relationships between reducing sugar concentration and vascular ring and medulla CI were in the range 0.001-0.37. Based on data for 3 of the 4 sites, mean weight loss for tubers after 4 months storage was 6.6% and weight loss was not affected by the rate of applied N. Based on our data for the cv. Kennebec grown on siliceous sands, we suggest growers should use N fertiliser rates which are just sufficient to ensure that N is not limiting yield. This practice should result in acceptable tuber size, SG and crisp colour, even after the tubers have been stored for up to 4 months.