A pot experiment was conducted in the greenhouse to determine and compare the responses of rice (Oryza sativa L. var. IR36), maize (Zea mays L. var. DMR-2), and soybean (Glycine max [L.] Merr. var. Clark 63) to soil water stress. Leaf elongation, dawn leaf water potential, transpiration rate, and nutrient uptake in stressed rice declined earlier than in maize and soybean. Maize and soybean, compared with rice, maintained high dawn leaf water potential for a longer period of water stress before leaf elongation significantly declined. Rice transpired more than soybean and maize at the same dawn leaf water potential. Nutrient uptake under water stress conditions was influenced more by the capacity of the roots to absorb nutrients than by transpiration. Transport of nutrients to the shoots may occur even at reduced transpiration rate It is concluded that the ability of maize and soybean to grow better than rice under water stress conditions may be due to their ability to maintain turgor as a result of the slow decline in leaf water potential brought about by low transpiration rate and continued uptake of nutrient, especially K, which must have allowed osmotic adjustment to occur.

Several plant processes are affected by loss of turgor during drought (3, 9). Osmotic adjustment, the lowering of osmotic potential by the net increase of intracellular solutes, is now recognized as an adaptive mechanism of many crops to water stress (14,27,28). It allows the maintenance of turgor at lower water status, which in turn enables plants to maintain processes such as cell enlarement and stomatal opening (10, 28). Osmotic adjustment has now been observed in several crop species (28). Greenhouse studies on four cultivars ofupland rice (Oryza sativa L.) have shown that osmotic adjustment of0.5 to 0.8 MPa occurs in this species (5, 6) and that osmotic adjustment caused leaf elongation to decrease and stop at lower predawn leaf water potentials than in nonosmotically adjusted plants (7).Leaf rolling and leaf or tiller death also occur as a result of water deficits. Both symptoms are used as criteria in visual scoring ofrice varieties being screened for drought reistance (11, 16). Visual drought resistance scores, in particular leaf rolling, are correlated with leafwater potential (20) In the second greenhouse experiment, pregerminated rice was sown in 10 containers, 0.75 m in diameter and 1.0 m deep, containing a well-fertilized Maahas clay loam soil. On 19 June, 90 seedlings/m of row were planted in 3 rows, 0.2 m apart. Water was withheld from half of the containers 27 d after planting, whereas the remaining containers were watered daily. Predawn and midday leaf water potentials and turgid osmotic potentials were measured 27, 28, and 29 d after the strss treatment was imposed. Samples were also obtained to establish the relationship between leafdeath and leaf water potential.In the first field experiment, the experimental plots of Puckridge and O'Toole (23) were utilized. The plants were sown on December 19 in plots 12 m long and 3 m wide. The soil at the site, a Maahas clay loam, was fertilized at planting and again 54 d from planting (23). The plots were watered uniformly for the first 23 d after planting; thereafter, a line source sprinkler system (8) maintained a continuously decreasing water application rate over the 12-m plot length (4). Midday leaf water potentials and turgid osmotic potentials were measured 57 and 64 d after the differential irrigation treatment was imposed in plots located 0 to 1.5 mi, 4.5 to 6.0 m, 7.5 to 9 m. and 10.5 to 12 m from the point of highest water application.In the second field experiment, the experimental plots ofAngus et al. (1)

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The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty‐one‐day‐old seedlings in high (28.6 × 10 −4M) and low (7.14 × 10 4M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi‐permeable membrane. Nutrient solution water potential was −0.6 × 105 Pa and was lowered stepwise to −1 × 105, −2 × 105, −4 × 105 and −6 × 105 Pa at 2‐day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 104M N had more root dry matter than the corresponding plants in 28.6 × 104M N. Dawn leaf water potential of stressed plants was 1 × 105 to 5.5 × 105 Pa lower than nutrient solution water potential. Nitrogen‐deficient water‐stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water‐stressed plants. It is suggested that this was due to higher root‐to‐shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 105 Pa higher in 7.14 × 10−4M than in 28.6 × 10−4M N and osmotic adjustment of 2.6 × 105 and 4.3 × 105 Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water‐stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.

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