Evaluation of the effect of endophyte and melatonin on the improvement of stevia root growth as affected by salinity under hydroponic conditions

Soil and water salinity are considered as the main factors limiting plant growth. The effect of salinity will affect root spread before it appears in the over ground parts. In saline conditions, the plant tries to increase the absorption potential of key elements, especially potassium, by increasing the number of roots as well as the root length. These changes are accompanied by shrinkage of the aerial parts. Therefore, the stronger the plants appear in this category, the more powerful they will be in relation to salinity.The increasing trend of soil salinity increases due to the imbalance of precipitation and annual evaporation on saline soil surfaces. Therefore, increasing the area under salinity-resistant plants or using conservation techniques will be very important.Stevia, despite its importance, is sensitive to soil and water salinity. Today, the role of biotic and abiotic stimulants in the adaptation of plants, especially medicinal plants to adverse environmental conditions is very important. In the present study, the effect of endophytic fungi and melatonin on stevia root changes in treatment with saline under hydroponic conditions will be investigated.

The present study was performed as a factorial study in a completely randomized design with six replications under greenhouse and hydroponic cultivation. This experiment was repeated twice to obtain more reliable results. Treatments used included endophytic fungi isolated from yew (in three levels of fungus-free control, strain TB20, strain TB2-3), melatonin spraying (in three levels of melatonin-free control, 0.5 μM pure melatonin and 0.5 μM Thymus vulgaris extract) And three salinity levels of NaCl (no salinity, medium salinity of 80 mM and high salinity of 150 mM irrigation water). In both experiments, the first foliar application was performed seven days and the first irrigation with salinity levels was performed 10 days after planting. Experimental plants were examined in terms of root traits. The over ground biomass yield was also examined.

In both experiments, similar results were obtained and showed the positive effects of melatonin and endophytic on the improvement of root growth characteristics under salinity stress. The best treatments used included thyme extract and TB20 endophytic fungus under salinity conditions. Most of the measured traits such as root volume, root length, fresh and dry weight of roots increased 2.5, 1.5-2 and 1.2-2, respectively, compared to the control using fungal and melatonin treatments. Although salinity increased the root traits to a moderate level, but with increasing salinity, the amount of these traits decreased sharply and significantly. A similar trend was observed in over ground biomass yield.

Since the use of melatonin and endophytes, especially TB20, has reduced the growth limitations of stevia in salinity conditions, it is suggested that in these salinity conditions, plant extracts containing melatonin be further studied.