Nitrogen availability and water stress interaction on rice growth and yield

Water stress and insufficient N supply are important factors reducing grain yield in rainfed rice. An upland experiment was conducted to investigate the interaction of N application and water stress during the vegetative stage (35-66 days after planting) on growth and grain yield using two rice cultivars Rikuto-Norin 12 and Todoroki-Wase. Nitrogen application treatments were: (i) 0 kg ha(-1), (ii) 60 kg ha(-1) at planting, (iii) 60 kg ha(-1) at 28 days after planting, and (iv) 180 kg ha(-1) split evenly among planting, 28 and 77 days after planting. Predawn osmotic adjustment was small throughout the stress period whereas osmotic adjustment during the day was large immediately after the stress commencement but could not be sustained when leaf water potential persisted at a low level. During the water-stress period, the cultivar with a small canopy (Todoroki-Wase) maintained higher plant water status than did Rikuto-Norin 12 which produced a large canopy, but it could not recover fully from the stress due to the small leaf area at the end of the stress period. Leaf water potential in Rikuto-Norin 12 decreased rapidly during the water-stress period but the plants recovered well after stress. Yield of Rikuto-Norin 12 in the stress trial was not significantly different from that under irrigated condition whereas yield of Todoroki-Wase was reduced by 34%. Nitrogen application had a small effect on growth during the stress period, despite a large effect under irrigation. There was, however, a positive effect of N on growth after recovery in the stress trial. Under irrigated conditions, applied N promoted yield in Todoroki-Wase but caused lodging in Rikuto-Norin 12. Uptake of N was high at first harvest but there was no further uptake under irrigated conditions whereas it increased after stress was relieved in the stress trial, resulting in similar N content at maturity in both trials. It is concluded that growth recovery is important for yield determination in rice when water stress develops during late vegetative to early panicle development.