Effect of Repeated Foliar Applications of Urea on Photosynthetic Parameters of Pistachio Tress cv. ”Kalleh-ghuchi” in Different Stages of Fruit Growth

Pistachio (Pistacia vera), as one of the most important horticultural products of Iran, has been embraced as one of the main commercial products. Rafsanjan is an important area of pistachio production in the world. High production is primarily dependent on the ability of the trees to produce large amounts of photoassimilates during the growing season. Nitrogen is the primary nutrient element needed in the greatest quantities for plant growth and physiology. The greatest impact of nitrogen is in relation to photosynthesis. Therefore, the rate of CO2 assimilation has been closely related to leaf nitrogen status. Nitrogen deficiency leads to the reduction in the activities of enzymes involved in photosynthesis and consequently a decrease in the rate of CO2 assimilation. Regarding the key role of nitrogen in metabolic activities, especially photosynthesis, this research was conducted to investigate the effects of repeated foliar applications of urea on photosynthesis parameters of “Kalleh-ghuchi” pistachio cultivar.

This research was conducted to assess the effect of repeated foliar applications of urea on photosynthetic parameters of pistachio trees in different stages of fruit growth, as a factorial experiment based on a randomized complete block design with five replications. In this experiment, urea was applied at the rate of 0.5% at three different stages of fruit growth including initiation of embryo growth, rapid growth of embryo and during flower bud abscission with two weeks interval. Measurement of photosynthesis parameters (including photosynthesis (A), transpiration rate (E) and stomatal conductance (gs) and intercellular CO2 concentration (Ci)) were done by used a portable photosynthesis system, after measuring photosynthesis parameters, chlorophyll fluorescence parameters (Fv/Fm and PII) were recorded by using a Chlorophyll Fluorometer. Leaf nitrogen content was determined by the kjeldahl method. Leaf chlorophyll and carotenoids content were measured according to porra (2002). Photosynthetic nitrogen use efficiency (PNUE) was calculated as the ratio of photosynthesis to foliar nitrogen content. All above parameters was performed in two times, one day after the last spraying and 10 days after the first stage of measurement. The experimental data was analyzed by SAS software and the significant differences among the treatment were tested by Duncan test.

The results showed that photosynthesis and transpiration rate and stomatal conductance were increased in urea treated trees compared to control in both sampling dates, while intercellular CO2 concentration was decreased by urea application. Three times applications of urea caused a 20% increase in photosynthesis rate compared to control trees. Leaf nitrogen content was also increased by repeated urea application in two sampling dates, as three times application of urea resulted in a 65% increase in leaf nitrogen concentration compared to the control , while photosynthetic nitrogen use efficiency decreased by urea application. Fv/Fm and PII were improved by repeated urea applications in two sampling dates. The results also indicated that chlorophyll pigments were increased by urea application. Generally, results showed that the maximum of photosynthesis and related parameters were obtained with three applications of urea at second date of sampling.

Any increase in photosynthesis with urea application indicates that more nitrogen maybe allocated to the enzymes of the carbon assimilation. Thus, more CO2 could be used by the plant, therefore increases in CO2 assimilation was accompanied by a decrease in the Ci due to nitrogen application. In the present study, nitrogen application increased stomatal conductance of pistachio trees. Application of nitrogen can influence stomatal opening, and thus transpiration rate. Increased chlorophyll content with applied nitrogen indicates paramount role of nitrogen in determining of chlorophyll synthesis. Increasing photosynthesis rate is always accompany with chlorophyll fluorescence reduction that is in agree with our results. Differences in PNUE are mainly brought about by differences in photosynthetic capacity or foliar nitrogen allocation either within the photosynthetic apparatus or to non-photosynthetic pools (e.g. cell walls, nitrate).

The results of this study showed that urea application can improve photosynthesis capacity by increasing nitrogen content of pistachio leaves. In this experiment, repeated foliar applications of urea in different stages of fruit growth had positive effects on photosynthesis and other dependent parameters which may be effective in reduction of alternate bearing in pistachio trees. On the other hand, the effect of nitrogen application on photosynthesis was continued for more than a month after application, compared to control.