Root growth, water potential, and yield of irrigated rice

Root length density (Lv), leaf water potential and yield of rice were studied in 1983 and 1984 on a Phool bagh clay loam (Typic Haplaquoll) and on a Beni silty clay loam (Aquic Hapludoll) in the Tarai region of Uttar Pradesh under naturally fluctuating shallow (0.07-0.92 m) and medium-depth (0.13-1.26 m) watertable conditions with six water regimes ranging from continuous submergence under 0.05 m +/- 0.02 m (Ic) to completely rainfed (Io). In irrigation treatments, Ic1, Ic3, Ic5, and Ic7, 0.07 m irrigation was applied on days 1, 3, 5, and 7 respectively, after the disappearance of ponded water. Maximum rooting depth (0.55 m in the shallow and 0.65 m in the medium-depth water table) was attained at the dough stage (125 days after transplanting) and was more strongly influenced by fluctuations in water table depth than by the water regime. For wet regimes (Ic1-Ic5), roots were concentrated at and above the water table interface and had greater horizontal development, whereas in dry regimes, (Ic7 and Io) they were concentrated in lower horizons and had a more vertical distribution. Like Lv, water potential leaf was not significantly affected by water regime up to 90-95 days after rice transplanting but was significantly affected thereafter, except for Lv beneath 0.2 m-0.25 m. Grain yields with irrigation treatments Ic1 and Ic3 under shallow and Ic1 under medium-depth water table conditions were not significantly different from those under continuous submergence, but there was a (nonsignificant) trend to lower yield with less water. However, differences among the wet regimes (Ic, Ic1, and Ic3) were small (141-490 kg ha-1) under shallow and 413-727 kg ha-1 under medium-depth water table conditions. The results demonstrate that optimum yield (5500-6000 kg ha-1) could be obtained under Tarai conditions by adopting an intermittent irrigation schedule of 3-5 days after the disappearance of ponded water under shallow, and of 1-3 days under medium-depth water table conditions, in place of continuous submergence.