Impacts of nitrogen fertilization and planting date on the physiology and yield of purple sweet potato at the extreme Northern edge of cultivation - Ivar Zekker; Astrid Kännaste; Viacheslav Eremeev; Kaia Kask; Pille Meinson; Helina Nassar; Erkki Mäeorg; Eve Runno-Paurson; Ülo Niinemets (PLoS ONE, 2025)