Determination of Non-Limiting Water Range for Seedling Growth of Pistachio at Two Levels of Soil Compaction

    Continuous availability of soil water is an important factor for plant growth. Both lack of adequate access to water in soils and adequate oxygen in saturated soils, may reduce or even stop plant growth. In order to study the behavior of pistachio seedling (Pistachio vera L.) at various moisture levels and to determine non limiting water range for its growth, a greenhouse experiment with two levels of compaction at six moisture levels with three replications was conducted. The air dried soil was passed through soil 4.75 mm sieve, and transferred into 36 PVC cylinders (diameter 15.24 and height 50 cm), the soils of cylinders were compacted in order to prepare two levels of soil bulk density (1.35 and 1.65 g cm^-3). After transferring the pistachio seedlings into soil cylinders and their establishment, six different volumetric water contents, from saturation to permanent wilting point, for each compaction level were applied. Stomatal conductance in the third mature leaf was measured in the course of exerting moisture treatments between 1:0 to 2:0 pm, during the experiment. Number of leaves and height of seedlings were recorded and leaf relative water content (RWC) and proline content were determined. The effect of moisture levels on all growth parameters were significant (p≤0.01). With bulk density of 1.35 g cm^-3, the seedlings faced with vegetative and physiological limitations at the range of 7 to 14 percent volumetric water content. Thus non-limiting water range for pistachio seedling at this bulk density level ranged from 14 to 49 percent volumetric moisture. In the compacted treatment (bulk density of 1.65 g cm^-3) both vegetative growth and physiological activities were considered within the range of 9-22 percent water content due to moisture deficit and at the range of 33-38, because of soil poor aeration led to growth retardation. Also non-limiting water range at this compaction level ranged from 22-33 percent water content. The least limiting water range predicted from Dasilva et al. model at the two D_b of 1.35 and 1.65 g cm^-3 were 10- 24.5 and 17- 28 percent, respectively. The differences between the predicted range and the range obtained based on the seedling response implies the need for the calibration of the predicting model for each particular soil and plant.