The impact of deficit and excess of soil water on plant growth, morphological plant features, N and P plant nutrition, soil properties, Rhizobium nodulation and the symbiosis between arbuscular mycorrhizal (AM) fungi and Lotus tenuis Waldst. & Kit. were studied in a saline-sodic soil. Water excess treatment decreased root growth by 36% and increased shoot growth by 13% whereas water deficit treatment decreased both root and shoot growth (26 and 32%, respectively). Differences between stress conditions on shoot growth were due to the ability of L. tenuis to tolerate low oxygen concentration in the soil and the sufficiency of nutrients in soil to sustain shoot growth demands. Water excess treatment decreased pH, and increased available P and labile C in soil. Water deficit treatment decreased available P and also increased labile C. In general, N and P acquisition were affected more by water excess than water deficit. The number of nodules per gram of fresh roots only increased in water excess roots (97%). Under both stress conditions there was a significant proportion of roots colonized by AM fungi. Compared to control treatment, arbuscule formation decreased by 55 and 14% under water excess and water deficit, respectively. Vesicle formation increased 256% in water excess treatment and did not change under water deficit treatment. L. tenuis plants subjected to water deficit or excess treatments could grow, nodulated and maintained a symbiotic association with AM fungi by different strategies. Under water excess, L. tenuis plants decreased root growth and increased shoot growth to facilitate water elimination by transpiration. Under water deficit, L. tenuis plants decreased root growth but also shoot growth which in turn significant decreased the shoot/ root ratio. In the present study, under water excess conditions AM fungi reduced nutrient transfer structures (arbuscules), the number of entry points and spore, and hyphal densities in soil, but increased resistance structures (vesicles). At water deficit, however, AM fungi reduced external hyphae and arbuscules to some extent, investing more in maintaining a similar proportion of vesicles in roots and spores in soil compared to control treatment.

The effect of phosphorus nutrition on the development of wheat plants, was studied. Phosphorus deficiency increased the duration of the phyllochron when the concentration in shoots was less than 0.8 %. Phosphorus concentration in shoots appeared to be a good indicator of the effect of plant phosphorus status on wheat development.

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The effect of phosphorus (P) nutrition and soil water availability (W) on the growth of wheat (Triticum aestivum L.) plants was studied in two pot experiments. Several levels of P supply were applied once before sowing. Before seedling establishment, the pots were kept near 100% of field capacity (FC). Afterwards, half of the pots were maintained between 60-70% FC. Control pots were kept at 85-95% FC by weighing and watering every two to three days. Several harvest of shoots were done before anthesis. At each harvest, dry matter and total P accumulation were measured in shoots. The main differences between both experiments were the way the drought stress was imposed, the levels of P supply, and the developmental stage of the plants at each harvest. In Experiment 1, no additional P resulted in a reduction of the shoot dry matter of 24 and 48% for well watered and drought-stressed plants, respectively. In Experiment 2, these reductions were of 33 and 65% for well-watered and drought-stressed plants, respectively. In both experiments, the effect of the drought-stress treatment was different at different levels of P supply. Interactions between P and W treatments were attributed to both, a less intense drought stress in P0 plants, and to the enhancement of drought-stress tolerance in P100 plants (Experiment 1), and P60 plants (Experiment 2). 30 RODRIGUEZ ET AL. INTRODUCTIONThe amount of plant material at the end of a growing season is the result of a large number of interacting processes in the soil-crop system. Particularly in rainfed agriculture, not one but many factors may limit this interaction. Water stress has often been directly or indirectly related to plant mineral nutrition. When little water is available, ions become less mobile in the soil, making the pathway from the soil to the root surface less direct (1). Because the rate of ion diffusion to the root surface is usually the rate-limiting step in nutrient uptake, reductions in water availability could affect plant growth not only directly but also through a lower availability of nutrients. Day (2) found that tissue concentrations of growth-limiting nutrients often decline during water stress, whereas one would expect them to increase if water directly restricted growth more strongly than nutrient uptake (3). This would suggest that the effects of low soil water content on nutrient availability may be nearly as important as the direct effects of water stress on plant growth. It has also been argued that a moderate nutrient stress would reduce the susceptibility of plants to other stresses. Chapin (4) mentioned that plants that are growing rapidly because of high nutrient availability are generally more susceptible to frost and water stress. However, Saneoka (5) found that despite that the stomatal resistance increasing with decreasing leaf water potential, at both high and low levels of P addition, the increase was higher at lower than at higher levels of P nutrition. For intermittent drought stress, a higher stomatal sensitivity under P stress conditio...