Drought is the most important abiotic constraint in rainfed rice systems. In these systems, Amaranthus spinosus and Leptochloa chinensis are the dominant weed species, which may reduce the available water to rice by competition and cause water stress in the crop. Two studies were conducted in a greenhouse to evaluate the growth response of A. spinosus and rice and L. chinensis and rice to water stress. The water stress treatments were 12.5%, 25%, 50%, 75%, and 100% of field capacity and the plants were grown until weed maturity (i.e., 63 days from seeding). Rice plants did not survive at 12.5% and 25% of field capacity, but both weed species survived in all the treatments. Both weed species produced a significant number of tillers/branches and leaves even at the lowest soil water content. The maximum amount of total shoot biomass produced by weed plants was 2.5 to 3 times more than that of rice plants. The soil water content to achieve 50% of the maximum aboveground biomass was 47%-50% of field capacity for rice, whereas it was 39% and 31% of field capacity for A. spinosus and L. chinensis, respectively. A. spinosus plants responded to increasing water stress with increased leaf weight ratio, whereas the leaf weight ratio of L. chinensis decreased with increases in water stress. The ability of A. spinosus and L. chinensis to produce shoot biomass under water-stressed conditions suggests that weed management strategies are needed that can minimize weed survival in water-limited environments. These strategies may include the use of weed-competitive and drought-tolerant cultivars, high seeding rates, and optimum timing of fertilizers.
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