Influence of Exogenous Uniconazole Application on Grafted Tomato Transplant Production

Limited research information is available on the practical application of plant growth regulators in vegetable grafting. Uniconazole, a gibberellin biosynthesis inhibitor, was reported to competitively inhibit abscisic acid (ABA) 8’-hydroxylase, a key enzyme regulating ABA catabolism in Arabidopsis thaliana. Exogenous application of uniconazole may result in accumulation of endogenous ABA, leading to stomatal closure. The objective of this study was to determine the impacts of uniconazole application on the efficiency of grafted tomato transplant production, especially without facilitation of a healing chamber. In the first fall study (Fall 1), 4-week-old tomato scion (‘Florida 47’) plants were sprayed with uniconazole at 0, 0.5, 1.0, and 1.5 mg/L at least 24 h prior to grafting, after which stomatal conductance was measured before grafting onto tomato rootstock, ‘Maxifort’. Newly grafted plants with water treated scions were placed inside a healing chamber, while grafted plants with scions treated with uniconazole were healed outside the chamber in the greenhouse. Plants were either floated or misted daily during the healing period. Number of non-wilted plants was counted daily until 10 days after grafting, after which scion length, chlorophyll content, scion fresh and dry weight, and leaf area were measured on day 11. Plants were transplanted into 0.4 L plastic pots, and the growth parameters were determined after 3 weeks. In the second fall study (Fall 2), uniconazole concentrations of 0, 0.7, 1.4, and 2.1 mg/L were utilized. Uniconazole application reduced stomatal conductance prior to grafting, and delayed the onset of wilting in plants healed outside of the healing chamber. However, recovery from wilting and graft survival were not improved with uniconazole application. Scion length was reduced when 2.1 mg/L uniconazole was sprayed, which may be beneficial in helping grafted seedlings withstand wind damage following field transplanting. In Fall 1, application of 1.5 mg/L uniconazole resulted in higher plant chlorophyll content compared to the 1.0 mg/L uniconazole treatment. However, uniconazole application did not significantly affect chlorophyll content in the second fall study. In Fall 2, plants kept outside of the healing chamber with uniconazole application at 1.4 mg/L did not differ significantly from plants kept inside the chamber in terms of scion fresh and dry weight and leaf area. Post-transplant growth of grafted tomato seedlings was not significantly affected by uniconazole application. Application of uniconazole in combination with other plant growth regulators may yield synergistic effects that further improve grafted tomato transplant production efficiency.