Determination of water-salinity-dry matter yield production function in vegetative growth phase of date palm

Accurate planning is very important for optimized use of water resources in agricultural sector due to climate condition of Iran. According to the research results, drought and salinity are two factors limiting agricultural production in arid and semiarid. Since changing drought or salinity adjust or increase other effect on plant, it is necessary evaluating combined effects of drought and salinity on plants such as date palm. In this reseach, the effect of water stress and salinity were studied in vegetative growth stage of Barhee date.
This research was carried out in factorial method based on randomized complete design with three replications on Barhee juvenile date palms. The treatments were three irrigation depths of 100%, 85% and 70% of plant water requirement and three irrigation water salinities of 2.5, 8 and 12 dS/m. The irrigation depth was calculated by measuring soil moisture. The production functions of water-salinity-dry matter yield were determined in linear, Cobb-Douglas, quadratic and transcendental equations. Five statistical indices of adjusted coefficient of determination (R2adj), modeling efficiency (EF), maximum error (ME), normalized root mean square error (nRMSE) and coefficient of residual mass (CRM) were used in evaluation of models.
The results showed that irrigation depth, water salinity and interaction of irrigation depth and water salinity had significant effect on all vegetative characters of plant except shoot relative water content. Decreasing irrigation depth from 100% to 85% of plant water requirement showed non-significant decrease in mean of shoot wet and dry matter that were only 5.0 and 5.6 percent, respectively. Amounts of these characters decreased significantly 26.3 and 24.4 percent, respectively, in irrigation depth equal 70% of plant water requirement. While, increasing water salinity from 2.5 to 8 dS/m was caused significant decrease in mean of shoot wet and dry matter that were 44.0 and 42.0 percent, respectively. Amounts of these characters decreased 54.1 and 52.0 percent, respectively, in irrigation with water 12 dS/m. The most amounts of plant vegetative characteristics obtained from water salinity of 2.5 dS/m and irrigation depth equal 100% of plant water requirement. This treatment had significantly different with other treatments expect water salinity of 2.5 dS/m and irrigation depth equal 85% of plant water requirement.
The Barhee juvenile date palms can be irrigated with 85% of water requirement when water salinity is 2.5 dS/m. The comparison of production functions of water-salinity-dry matter yield showed that quadratic equation had more accuracy than other equations in estimation of dry matter yield in vegetative growth phase of Barhee date palm. All equations estimated shoot dry matter less than actual amount expect quadratic equation.