Impacts of nitrogen fertilization and planting date on the physiology and yield of purple sweet potato at the extreme Northern edge of cultivation

Global warming causes plant stress and reduces crop productivity. Cultivation of the warmer region crop sweet potato (Ipomoea batatas (L.) Lam) in Northern regions can be an opportunity to benefit from climate warming, but there is little information of how growing season length interacts with agricultural practices such as nitrogen (N) fertilization. We studied the photosynthetic characteristics, biomass accumulation, carbon (C) and N contents of plant organs of the cultivar 'Purple Bud' in relation to the planting date (the 2nd of May, 10th of May, 20th of May, 30th of May and 10th of June) and N fertilization (kg ha-1; N0, N50, N100 and N150). Nitrogen content of leaves (NL) and tubers (NT) increased with N application dose and was moderately affected by planting time. Despite the fertilization-dependent increase of leaf N content, photosynthesis rate (A) was unaffected or somewhat reduced by N fertilization. This reflected reductions in stomatal conductance (gs) and ratio of intercellular CO2 to ambient CO2 (Ci/Ca), suggesting that enhanced N availability and concomitant increase in whole plant area resulted in reduced plant water availability. The highest values of leaf C/N ratio, tuber to root mass ratio and dry weight content of roots (DWR) were found in N0 plants and the ones planted on the 10th of May and 20th of May. Our results collectively demonstrate that the growth and productivity of sweet potato is strongly dependent on the length of the growing season, and can be further constrained by utilization efficiency of N. We conclude that future research should focus on optimum sweet potato cultivation technologies at Northern latitudes.