The effects of irrigation regimes on the water use, dry matter production and physiological responses of paddy rice

In most cases, rice production is associated with flooding irrigation and t he efficiency of irrigated water use (WUEi) is generally lower for producti on of rice than for other crops. We have examined the effects of various ir rigation regimes on water consumption in a well-puddled paddy field, as wel l as on dry matter production, grain yield and physiological responses of t he plants. Four sets of conditions were studied, with two replications, in the well-puddled paddy field: Continuous flooding irrigation treatment (CSF ); three intermittent irrigation treatments, designated II-0, II-1 and II-2 , in which plants were re-irrigated when the water potential of the soil fe ll below 0, -10, and -20 kPa at a depth of 5 cm, respectively. Water consum ption was lower during II-0 than during CSF because the percolation rate wa s reduced by the reduction in the hydraulic head of the ponded water. Inter mittent irrigation led to the repeated shrinking and swelling of soil durin g II-1 and II-2 and, therefore, soil cracks developed rapidly. Since they b ecame the major routes of water percolation, these soil cracks increased wa ter consumption during II-1 and II-2 above that during CSF and II-0. There were no significant differences in dry matter production and grain yield be tween CSF and II-0, but both were significantly greater than in the case of II-1 and II-2. Therefore, WUEi increased in the following order: II-0, CSF , II-2, II-1, although the difference was very small between II-1 and II-2. A lower crop growth rate (CGR) resulted from a decrease in the net assimil ation rate (NAR) during II-1 and II-2, and there was also a reduction in th e leaf area index (LAI) during II-2. Early senescence with ripening and wat er stress around midday decreased the rate of photosynthesis in leaves, cau sing the lower NAR. These physiological responses of the plants were respon sible for the reduction on the dry matter production and grain yield in the intermittent irrigation.