Early Planting Didn't Increase Rice Productivity in Cool Climates Under Global Warming.

Under global warming, rice productivity in cool climates, restricted by low temperature, is expected to increase with early planting with increasing canopy radiation capture and biomass throughout growth season. We conducted a 2-year field experiment at two planting time (control (transplanted at mid-May) and early planting (transplanted at earlier ca. 8~10 days than control)) in northern part of Japan with using two cultivars, Hitomebore (dominant cultivar) and Koshihikari (later maturity cultivar). The growth period, from transplanting to heading and maturity, was prolonged in both cultivars by early planting than control by ca. 10 days, that is expected to increase biomass and yield.  Koshihikari had ca. 10-d longer growth period than Hitomebore. However, early planting did not significantly increase final biomass and grain yield, although the magnitude depended on cultivars, later cultivar was larger in early planting. No significant effects of cultivars on biomass and yield were observed. The responses of biomass and grain yield to early planting the differences between cultivars were explained from leaf expansion during early growth stages. On the other hand, the early planting improved the grain quality, and lodging resistance. In conclusion, the early planting prolonged the total growth period, but did not increase biomass and grain yield. An improvement of leaf expansion during early growth stages under low temperature would be a key for adapting rice productivity under future climates.