Effect of Trichoderma longibrachiatum and Chitosan Spraying on Morphophysiological Characteristics and Yield of Sweet Basil (Ocimum basilicum L.) under Deficit Irrigation Conditions

 Drought is one of the most important environmental stressors that adversely affects agricultural products, especially in arid and semi-arid regions. Using Trichoderma fungus along with biopolymers such as chitosan is one of the ways to reduce drought stress. Trichoderma fungus as plant growth-promoting fungus is the most common fungal and soil-modifying species that are able to directly with plant roots in the rhizosphere and improve growth as well as biological control of living stresses such as pathogenic fungi and non-living stresses such as drought, salinity and heavy metals. On the other hand, one of the effective ways to protect the plant in conditions of low irrigation is the use of anti-transpirants, including the biostimulant of chitosan, which markedly limits transpiration from the plant surface. The anti-transipirants action of chitosan can be attributed to the involvement of chitosan in the abscisic acid pathways, which closes the stomata and thus reduces transpiration. Chitosan is readily soluble in water and organic acids. Therefore, it can be used in various methods such as mixing with soil, foliar spraying and impregnation with seeds in agriculture.

 This research was conducted in a split factorial arrangement based on randomized complete block design. The main plot factor was irrigation interval in three levels (two days as normal irrigation and three and four days as deficit irrigation conditions) and sub-plots were inoculated with T. longibrachiatum at two levels (inoculation and uninoculated control) and chitosan at three levels (0, 0.2 and 0.4 g/L). Each experimental plot consisted of three planting lines two and a half meters long and one meter wide .T. longibrachiatum was obtained from Tabarestan Agricultural Genetics and Biotechnology Research Institute. The first irrigation was done simultaneously with planting basil. Up to one month after sowing the seeds (six to eight leaf stage of plants), the plots were irrigated evenly with tubes and from this stage onwards, irrigation treatments were applied. Pesticides and herbicides were not used during the experiment and weed control was done manually. Chitosan was prepared from Sarina Teb store and prepared in three levels of zero, 0.2 and 0.4 g/l and sprayed in three stages: vegetative, before flowering and 50% flowering.

 The results showed that with increasing the irrigation period from two to four days, the morphological traits of basil, such as root length and stem length, leaf dry weight, root, stem and dry matter yield decreased. Also, physiological traits of basil such as carotenoids and chlorophyll meter increased while chlorophyll a, b and total chlorophyll decreased. Application of 0.2 g/L of chitosan inoculated plants increased chlorophyll b content by 68%. The highest percentage and yield of essential oil in both normal and irrigation deficit conditions were obtained when plants inoculated with Trichoderma and foliary sprayed by chitosan. The highest percentage and yield of essential oil were observed with an average of 0.88 and 42.87% in normal irrigation conditions, application of Trichoderma and zero level of chitosan, respectively. According to the results, increasing the irrigation cycle along with chitosan application and fungal inoculation increased the percentage and yield of essential oil. However, by increasing the irrigation cycle, chitosan alone decreased the percentage and yield of essential oil and only in the three-day irrigation cycle, it increased the percentage of essential oil compared to the control.

 Overall, the findings showed the positive effect of concomitant use of Trichoderma fungus and chitosan on improving the growth of sweet basil and increasing drought resistance.