Impact of Seed Inoculation with <i>Trichoderma afroharzianum</i> Strains on Plant Growth, Root Morphology, and Leaf Phenolic Content in Hemp (<i>Cannabis sativa</i> L.) at Early Growth Stages

Industrial hemp (<i>Cannabis sativa</i> L.) is receiving increasing attention for its multiple end-uses; therefore, an improvement in its production is needed to meet the increased demand. In the present study, the effect of seed inoculation with two <i>Trichoderma afroharzianum</i> strains, T-AA and T-22, on plant growth and root morphology of hemp plants at sixth-leaf (S6) and tenth-leaf (S10) stages was assessed for two consecutive years (2020 and 2021). In addition, the ability of the two strains to enhance the accumulation of phenolic compounds in hemp leaves was also evaluated. The results obtained revealed the ability of T-22 to improve the growth and root morphology of hemp plants both in 2020 and 2021, although with different impact, probably ascribable to the different weather conditions in the two years. In 2020, the positive effects of T-22 were detected at S10 stage with significant increases in the shoot and root length (38% and 17%, respectively) and dried biomass (35% and 30%, respectively) compared to untreated plants. The total root surface area and the number of tips, forks, and crossings also increased significantly (24–36%) at this stage. In 2021, significant increases in the shoot length and dried biomass (40% and 30%, respectively) were observed at S6 stage, whereas root length and dried biomass increased significantly at S6 (55% and 47%, respectively) and S10 stage (121% and 40%, respectively). Significant increases in the total surface area and volume, as well as in the number of tips, forks, and crossings were also observed at both S6 and S10 stage (50–63% and 105–187%, respectively). Interestingly, in both years and at both stages, the two strains induced significant increases in the leaf accumulation of phenolic compounds and the antioxidant activity, which were greater in T-22- compared to T-AA-treated plants (18–102% and 13–34%, respectively). The results are discussed in light of the potential practical applications of T-22 as a biostimulant of hemp plant growth under favorable and unfavorable environmental conditions, and of both strains as promising tools for the improvement of the leaves’ economic value as a source of health-promoting compounds.