ROOT-GROWTH, WATER POTENTIAL, AND YIELD OF IRRIGATED RICE

Root length density (Lv), leaf water potential (Psi leaf) and yield of rice were studied in 1983 and 1984 on a Phool bagh clay loam (Typic H aplaquoll) and on a Beni silty clay loam (Aquic Hapludoll) in the Tara i region of Uttar Pradesh under naturally fluctuating shallow (0.07-0. 92 m) and medium-depth (0.13-1.26 m) water table conditions with six w ater regimes ranging from continuous submergence under 0.05 m +/- 0.02 m (Ic) to completely rainfed (Io). In irrigation treatments, Id, Ic3, Ic5, and Ic7, 0.07 m irrigation was applied on days 1, 3, 5, and 7 re spectively, after the disappearance of ponded water. Maximum rooting d epth (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 w as more strongly influenced by fluctuations in water table depth than by the water regime. For wet regimes (Ic1-Ic5), roots were concentrate d at and above the water table interface and had greater horizontal de velopment, whereas in dry regimes, (Ic7 and Io) they were concentrated in lower horizons and had a more vertical distribution. Like Lv, Psi leaf was not significantly affected by water regime up to 90-95 days a fter rice transplanting but was significantly affected thereafter, exc ept for Lv beneath 0.2 m-0.25 m. Grain yields with irrigation treatmen ts Icl and Ic3 under shallow and Icl under medium-depth water table co nditions were not significantly different from those under continuous submergence, but there was a (nonsignificant) trend to lower yield wit h less water. However, differences among the wet regimes (Ic, Id, 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 t hat 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 day s under medium-depth water table conditions, in place of continuous su bmergence.