..Effects of Humic Acid and Amino Acid on some Growth Traits on Super Daminus Cucumber (Cucumis sativus L.) under Drought Stress

Introduction: The deficiency of organic matter in the soil is one of the factors limiting cultivating and growing of plants. While the organic matter is the main factor of soil fertility and the ideal organic matter content of agricultural soils is about 4-6% according to the experiments performed, this amount is very low in arid and desert areas such as those  in Iran. The significant capability of humic fertilizers in the rapid improvement of soil fertilization level is quite recognizable. Humic acid has a direct role in determining the producing potential of the soil. Humic substances can have a positive effect on plant growth in different ways. Humic acid results in an increasing nitrogen uptake by plants and stimulates and enhances the uptake of Ca, Mg, P, and K. By enhancing the resistance to environmental stresses, increasing chlorophyll concentration and thus affecting photosynthesis, amino acids are effective in plant growth and yield. Given the conditions that agriculture faces, drought is the main limiting factor of agricultural production in the present and future. Research has reported that the improvement of plant growth under stress conditions by application of humic substances and amino acid. The use of these factors that increase plant growth in stressful conditions and improve yield is a useful and effective method. The aim of this study was to investigate the effect of organic fertilizers on growth traits of cucumber plants under drought stress.Materials and Methods: In order to explore the effect of drought stress on cucumber plant, an experiment was carried out in the research field of agriculture faculty of Ferdowsi University of Mashhad in 2015-2016. This study was performed as a factorial design in a completely randomized block design with three replications. The first factor was fertilization at four levels (control, humic acid application (200 ppm), glycine amino acid and glutamic acid application (6 liters per hectare), and combined application of humic acid (200 ppm), glycine amino acid and glutamic acid (6 liters per hectare), and the second factor was irrigation at three levels of 3, 5 and 7 days. Furrows with 2 m apart, 40 cm deep, 50 cm wide and 20 m long were created. Following the soil irrigation, the cucumber seeds were planted on the edge of the experimental hills (4-5 seeds per hill) that were spaced 50 cm apart. The treatments were applied via irrigation at specified concentration. At the end of the experiment Morphophysiological traits were measured.Results and Discussion: The results showed that drought stress reduced fresh and dry weights of plant, a number of branches, root length, stem length, and internode length. The amount of available water to plants affects their growth traits. Increasing irrigation interval decreased vegetative growth parameters in this plant, which is undoubtedly due to the lack of water available to the plant. Drought stress leads to stomata closure, plant wilt, swelling reduction, and reduced water content and total potential of the leaf, resulting in decreased cell division, especially in shoots and leaves. As a result, the first effects of dehydration in plants can be seen in their smaller leaf size and lower height than normal conditions. The highest level of drought stress (7 days) decreases all the traits. Unfortunately, the use of humic acid and acid amine had no effect on the highest levels of stress (7 days). The application of humic acid alone could improve the effects of drought stress on the plant. Humic acid enhances plant growth by affecting plant cell metabolism as well as chelating power, increasing nutrient uptake, and increasing nitrogen content. Experimental studies have shown that glycine betaine strengthens enzymatic structures and activities and protein components, and cell wall stability against the damaging effects of environmental stresses is considered to be one of its activities. It seems that organic matter and humic compounds including humic acid through positive physiological effects including effects on plant cells metabolism, increased leaf chlorophyll concentration, increased metabolism within cells, and increased durability of photosynthetic tissues (Increasing leaf durability) leads to increase of production performance and biomass production in plants.Conclusion: The use of humic acid alone led to improved leaf area, root length, and other traits. These factors caused an improvement in the photosynthetic rate in the plant, resulting in improved yield and growth under drought stress conditions (increased irrigation intervals). The protective and moderating role of the humic acid application under drought stress is one of the reasons for growth improvement by these compounds. In general, the combined application of humic acid and amino acid in the form of irrigation fertilizer in cucumber under drought stress improved growth traits and prevented yield reduction.