Improving Yield, Yield Components and the Absorption of Nutrients of Wheat by Growth Stimulants under Normal Irrigation and Drought Stress

Drought stress is one of the most important factors in reducing crop production in many arid and semi-arid regions of the world. In recent years, the use of growth stimulants to prevent the excessive use of chemical fertilizers and induce tolerance to environmental stresses has increased in order to achieve the goals of sustainable agriculture. Unlike chemical fertilizers, these compounds with the least adverse environmental effects are involved in increasing crop yields and conserving natural resources. The use of growth stimulants is one of the promising ways to overcome drought stress. Based on reports expressing the positive effects of growth stimulants drought tolerance, the present study was designed to investigate the moderating effect of growth stimulants at different irrigation intervals on improving yield, yield components and nutrient concentration in wheat.

The experiment was performed as a split plot based on randomized complete block design with three replications. This research was conducted during the growing seasons of 2017-2018 and 2018-2019 in Fars Agricultural and Natural Resources Research Center (Darab Agricultural Research station). The main factor includes different irrigation intervals at two levels (irrigation after 70 and 140 mm of cumulative evaporation from Class A evaporation pan) and the secondary factor includes the use of growth stimulants at seven levels (control, soil application of humic acid, foliar spray of amino acids, fulvic acids and seaweed extract, seed inoculation of Azotobacter and the combination of growth stimulants). The amount of irrigation water required in irrigation treatments was determined based on soil moisture supply at the depth of root development to reach the field capacity. Soil moisture was measured by weight method and through soil sampling in the middle of each plot to determine the evacuated moisture after reaching the desired cumulative evaporation. Foliar application of amino acids, fulvic acids and seaweed extract was performed at the concentration of 5 kg m-3of water in two stages of tillering and complete emergence of spike. Humic acid fertilizer at a rate of 5 kg.ha-1 was applied in the early stages of growth with the second irrigation. Seed Azotobacter inoculum was used at a rate of 1.5%. Finally, the concentration of nutrients in straw and grain, straw and grain dry weight, biological yield, harvest index and yield components were measured. Analysis of data variances was performed using SAS software version 9.1. Bartlett test was performed on all studied traits.

The results showed that the highest concentration of macro and micro nutrients in grain and straw were obtained in the combined treatment of growth stimulants. After this treatment, a significant increase in the concentration of nutrients was observed in the individual consumption of growth stimulants. Combination treatment of growth stimulants increased wheat biological yield by 18% compared to the control. Also, at a lower level, individual application of growth stimulants improved the biological yield compared to the control. The combination of growth stimulants and subsequently the individual application of these compounds improved the yield components of wheat. In the first and second year, irrigation after 140 mm of cumulative evaporation reduced the biological yield by 14 and 25% compared to 70 mm of cumulative evaporation, respectively. Also, irrigation after 140 mm of cumulative evaporation reduced the dry weight of straw, grain yield and harvest index compared to 70 mm of cumulative evaporation. In general, the results showed that the uptake of some nutrients was affected by the 140 mm cumulative evaporation treatment from the evaporation pan. However, in the second year of planting, due to the presence of frequent rains before flowering, the treatment of 140 mm of cumulative evaporation from the evaporation pan had less effect on nutrient uptake.

Overall, the use of a combination of growth stimulants was effective in increasing grain yield, biological yield and yield components of wheat. The results of this study showed that the growth stimulants to some extent caused drought tolerance by improving nutrients uptake. Therefore, the combination of growth stimulants in both normal irrigation and drought stress conditions can be used to improve nutrient uptake and wheat grain yield.