IntroductionMedicinal plants have long had a special place in the traditional agricultural system of Iran and the use of these plants as medicine to prevent and treat diseases has been considered by traditional medicine experts since ancient times. Medicinal plants with rich sources of secondary metabolites provide the basic active ingredients of many medicines. Although the biosynthesis of secondary metabolites is genetically controlled, but their construction is strongly influenced by environmental factors. One of the important climatic factors that affect the distribution of plants around the world and can cause morphological, physiological and biochemical changes in the plant is the lack of available water. Basil (Ocimum basilicum L.) seems to show little resistance to water stress. For this reason, there is a need for protective mechanisms for the basil plant against stress due to water shortage. Plants are able to reduce or eliminate the effects of water shortage stress by coexisting with a number of soil microorganisms. Coexistence relationship plant with T. harzianum fungi is one of the ways to reduce dehydration in plants. These mushrooms by altering some of the root properties and absorbing nutrients in the host plants, they reduce the destructive effects of water shortage stress. Observing the positive effect of these fungi in increasing the absorption of nutrients from the soil, improving plant water relations, increasing water use efficiency in plants and finally increasing plant resistance to water deficit stresses on the one hand and on the other hand. The existence of water crises in different countries has prompted researchers to further study this aspect of the symbiotic relationship between the host plant and Trichoderma harzianum. Materials and MethodsPresent study was performed to investigate the effect of T. harzianum on the antioxidant content of Ocimum basilicum under water deficit stress. It is also intended to evaluate the effects of an endophytic fungi namely T. harzianum on the shoot yield, photosynthetic pigments, content and yield of essential oil from O. basilicum under water deficit stress. All experiments were performed as the factorial based on completely randomized design blocks with three replications in greenhouse condition. The experimental factors were the different irrigation regimes including 100, 75, 50 and 25% of field capacity and two concentrations of the T. harzianum (106 and 109 CFU/ml). At the beginning of flowering, water deficit stress was applied at four levels of 25, 50, 75 and the control treatment (100 percent) of field capacity until three weeks later. The application of water stress was such that the pots were weighed daily and the moisture deficiency in each treatment was removed by watering the pots until reaching the desired treatment level. After applying the stress, sampling and measuring traits were done at the full flower stage. After the plants reach the full flowering period, various traits including plant height, number of leaves, fresh and dry weight yield of the plant (as the economic yield of basil) and fresh and dry weight yield of roots per square meter, chlorophyll a, b, total (T), carotenoid, colonization percentage, yield and essential oil content were measured in all the plants in the pots. Results and DiscussionIncreasing the level of water stress reduced the economic performance of basil (O. basilicum). Coexistence with Trichoderma reduced the destructive effects of dehydration on the plant. With the application of dehydration stress, the essential oil content increased in mild and moderate stresses, but the yield of essential oil did not differ significantly from the control and plant height, number of leaves and plant pigments decreased under dehydration stress. According to the experimental results, it can be said that Trichoderma with a population density of 106 (spores per ml of inoculum) had a better effect on most growth indices, while the effect Trichoderma with a population density of 109 (spores per milliliter of inoculum) on important traits such as shoot fresh weight yield (212.2 g), percentage (0.7%) and essential oil yield It was higher. According to the obtained results, it can be stated that inoculation of basil with Trichoderma increased the percentage and yield of essential oil in both water stress and non-stress conditions, and considering that in cultivation of plants the goal is to increase the effective substance. It is in these plants. ConclusionFinally, the results indicated that the use of Trichoderma mushroom in comparison with the control (without inoculation with the fungus) under water stress conditions has the ability to improve plant growth and leads to an increase in plant efficiency under water stress conditions.