Studying the effect of application time of humic acid foliar spraying on qualitative and quantitative characteristics of cowpea (Vigna unguiculata L.) under different irrigation regimes

Background and Objectives The cowpea (vigna unguiculata L.) including grains, grown in tropical and sub-tropical countries, especially in the countries of Asia, Africa and South America, and it is considered an important source of nutrition. Among legums, in terms of cultivars and economic value, the first place belongs to beans. Good climate (warm to moderate summer) and adequate water in the areas under beans, high quality soils and high depth of clay are the main reasons for increasing its cultivation area. Water is a key factor in the production of crops. The yield of crops in many areas is limited by living or non-living environmental stresses, and therefore, there is a significant difference between actual yield and yield potential of crops. Low moisture content in each of the different stages of growth reduces water absorption, nutrients, decreases the transfer of elements within the plant, and ultimately reduces the yield of the grain or the final product. Optimal use of water is of great importance, especially in areas where dry climatic conditions and Semi-dry is dominant. Therefore, the present study aimed to investigate the effects of humic acid spraying on the quantitative and qualitative characteristics of bean-eyed beans under irrigation regimes in Hamidieh region. Materials and methods This research was carried out in 2016 in a farm located in Hamidieh with a longitude of 48 degrees and 10 minutes east and 31 degrees and 33 degrees north latitude and 13 meters above sea level. This study was carried out as split split plot in a randomized complete block design with three replications. Treatments included irrigation regimes at three levels (60, 90 and 120) mm evaporation from class A evaporation pan in the main plots and different times of humic acid spraying at three levels (no spraying, spraying at vegetative stage, Spraying at flowering stage) in sub plots. Results The results showed that irrigation regimes had a significant effect on grain yield, number of pods per plant, number of seeds per pod and protein yield. The highest grain yield (205.42 g/m2) was obtained from 60 mm evaporation from the evaporation pan and the lowest with an average of 160.2 g/m2, with 120 mm evaporation from evaporation pan. Also, the application of humic acid spraying dates significantly affected seed yield and protein function. Humic acid increases the amount of nutrients (seed storage compositions) by increasing the activity of the rosicose enzymes and increasing the photosynthesis activity of the plant, possibly by improving the production of sugar, protein and vitamin in the plant, and the positive effect on the various aspects of photosynthesis.The highest grain yield (200.77 g/m2) was obtained from spray application during vegetative time and the lowest (152.52 g/m2) was obtained from non-spray application. The interaction of irrigation regimes and spraying periods of humic acid had a significant effect on grain yield. The highest grain yield was assigned to 60 mm evaporation from evaporation pan and humic acid spraying treatment at a vegetative stage of 213.88 g/m2.   Conclusion Therefore, it can be concluded that the use of humic acid due to different physiological effects, in addition to increasing yield, can play a positive role in decreasing the use of chemical fertilizers as well as environmental pollution. Therefore, as a natural source material, To sustain and increase the production of crops. Drought stress increased protein percentage, but irrigation at flowering and podding stage reduced the protein percentage. Despite the high percentage of protein in 120 mm evaporation from the pan, the protein function in this treatment was less than the other two treatments due to low grain yield. Therefore, in order to achieve maximum grain yield, 60-millimeter water irrigation with evapotranspiration from the pan is recommended by application of humic acid spraying at the vegetative stage.