Effects of irrigation water salinity levels and Zn applicaton rates on wheat (Triticum aestivum L.) yield under field conditions of yazd province

Introduction As a micronutrient, zinc is essential for protein and carbohydrate metabolism, auxin synthesis, membrance integrity and reproduction. Plant growth under calcareous and saline soils, charactetrized by high soil pH, is usually negatively affected due to zinc deficiency as a result of low micronutrient avalilability. It is reported that 56 percent of agricultural lands suffer from zinc defficiency (Shahbazi and Besharati, 2013). It is well documented that zinc fertilizers improves plants quality as well as human health (Malakouti et al., 2010). Saline soils usually result in osmotic pressur and nutrient disorders in plants and leads to plants growth decline. As a worldwide abiotic stress responsible for reduced crop production, it is estimated that annual losses of yield due to salt induced land degradation is US$ 27.3 billion globally (Qadir et al., 2014). Social and economic dimentions of salinity stress can be employment losses as well as environmental degradation (Butcher et al., 2016). While application of chemical fertilizers at arid and semiarid soils usually improves plant performance, the efficiacy of fertilizers application may be affected by salinity stress. While there is little evidence of yield benefits due to application of fertilizers in salinized fields at rates beyond optimal in non-saline conditions, there is enough evidence indicating that soil salinity does not affect or decrease plant fertilizer needs (Hanson, 2006). It is reported that zinc fertilizer improves salinity tolerance (Saedinejad et al., 2016; Ahmadi et al., 2005) and alleviates the negative effectd of salinity stress and this is due to the Na concentration reduction in wheat plants (Ahmadi et al., 2005). However, contradictory results have been reported. These contradictory results can be attributed to the types of experiments (field, greenhouse or laboratory), composition of the saline substrate, studies conducted over the short term vs. the long term and many other differences in experimental conditions (Grattan and Grieve, 1999). Thus zinc fertilizer management under arid and semiarid conditions of Yazd peovince with wide range of irrigation water qualities may need to be modified. Accordingly, the objectives of this field study were to (a) elucidate the interactions between zinc nutrition and the salinity of irrigation water and their effects on wheat growth and (b) test the possibility of wheat improvement at saline conditions by applying higher levels of zinc fertilizer. Materials and methods A field experiment was conducted on wheat at Sadooq Salinity Research Station, Ashkezar, Yazd, Iran. The soil at the experimental site was calcareous with 30.92% total neutralizing value, sandy loam texture, pH 8.06 and 0.22 % organic carbon. Mean annual temperatue is 18°C and precipitation is 70 mm. The treatments, four zinc sulphate application rates (0, 20, 40 and 80 kg ha-1) and three irrigation water qualities (1.88, 7.22 , 14.16 dS/m), arranged in a compelet randomized block, split plot design with three repelications. Consisting 12 rows of wheat, each field plot was 3*5 m. All plots received common agricultural practices including tillage and fertilizer application. Rgarding typical recommendations and guidelines for this region and soil type (Balali et al., 2000: Moshiri et al., 2015), all fertilizers, except urea that applied in 4 splits, were soil-applied before plnating and included 100 kg ha-1 triple superphosphate, 40 kg ha-1 FeSO4,, 40 kg ha-1 MnSO4 and 20 kg ha-1 CuSO4. For modeling the relationship between plant properties and irrigation water salinity, the data were subjected to different regression models at the probability level of 0.01 and 0.05 with the help of the Sigmaplot software. The analysis of variance for different parameters was done following ANOVA technique. When F was significant at p ≤ 0.05 level, treatment means were separated using DMRT. Results and discussion The maximum grain yield for plants irrigated with both irrigatin water salinity of 1.88 and 16.14dS/m was found at zinc sulphate application rate of 25kgha-1. In addition, wheat grain yield response to salinity stress at zinc sulphate rates of 0, 20 and 40kgha-1 were similar and followed linear regression model. Indicating the increasing salinity tolerance, the decline per unit slope decreased with increasing Zn application rates. Interestingly, wheat grain yield response to salinity stress at highest Zn application rate of 80kgha-1 followed the quadratic regression model. This observasion, also, proves that wheat response to salinity stress depends on soil fertility level. The results of this experiment showed that wheat response to salinity stress depends on soil fertility level (Zn application rate). In additions, the results showed that with increasing irrigation water salinity from 1.88 to 16.14dS/m the grain yield decreased from 6.5 to 3.5 tonnes/ha but ZnSO4 requirement was not changed. Conclusion It was concluded that wheat response to salinity stress depends on soil fertility (zinc sulphate application rate) and salt tolerance increased by increasing zinc application rate. While salinity stress decreased wheat yiled from 6.5 to 3.5 ton ha-1, application of zinc sulphate at a rate of 20 kg ha-1 is needed for all salinity levels.