Abstract—In this study, the effect of extracts of TBi isolated strain of Trichoderma harzianum fungal species on increased or decreased amount of chlorophyll a and b as well as on the nitrate accumulation on lettuce in greenhouse conditions and in the soilless culture system was studied. The experiment was performed in a complete block randomized design with factorial arrangement of (2×4) in 6 replication. To conduct the research, four concentrations of 0%, 5%, 10% and 15% of the extract per each source, and also 2 varieties of Siahoo and Grade Lake were used. The amounts of chlorophyll "a" and "b" were measured by Lichtenthaler and Wellburn method; nitrate concentrations were measured using Spectrophotometer (Diazo) method. The test results showed that different levels of this fungus extract have had different effects on the parameters listed on lettuce plant. Among different levels used, the concentration of 5% included the highest concentration of nitrate and showed a significant difference at the 5% probability level with other levels. This level also had higher chlorophyll content than other levels, but the difference was not significant. The Siahoo variety showed higher nitrate accumulation as well as larger amounts of chlorophyll "a" and "b" compared to the Grade Lake variety, and the difference was significant at the 1% probability level.

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