Effects of Rootstock, Pre-Grafting Light Intensity and Healing Relative Humidity on Photosynthetic Performance of Grafted Tomato Seedlings

The production of high-quality grafted tomato seedlings depends on both the physiological condition of seedlings before grafting and the healing environment after grafting. The present study evaluated the combined effect of rootstocks, pre-grafting light intensity and healing relative humidity on graft success, growth, gas exchange, and chlorophyll fluorescence responses of grafted tomato seedlings. The tomato scion cultivar ‘Jalila F1’ was grafted onto four rootstocks: ‘Auroch’, ‘Ficus’, ‘Goldrake’, and ‘TOR23901’. Rootstock seedlings were grown under three pre-grafting light intensities (150, 250, and 350 µmol m−2·s−1) and, after grafting, seedlings were healed under three relative humidity levels (60%, 75% and 90% RH). Three-way ANOVA revealed significant effects of rootstock, light intensity, RH, and their interactions on most growth, photosynthetic, and chlorophyll fluorescence traits. Graft success rate varied among treatments, with the highest values observed at 250 µmol m−2·s−1 + 75% RH and 350 µmol m−2·s−1 + 60–75% RH. In contrast, 90% healing RH resulted in more variable graft success, particularly in ‘Ficus’. Rootstocks strongly influenced post-grafting growth, with ‘Auroch’ showing greater dry matter accumulation, whereas ‘Ficus’ produced thicker stems and larger leaf area. Gas-exchange responses indicated that 250 µmol m−2·s−1 pre-grafting light combined with 75% RH enhanced photosynthetic performance after healing. Chlorophyll fluorescence responses further showed treatment-dependent variation in the effective quantum yield of PSII [Y(II)], non-photochemical quenching (NPQ), and electron transport rate (ETR). Among the tested levels of pre-grafting PPFD and healing RH, 250 µmol m−2·s−1 combined with 75% healing relative humidity provided the most consistent overall performance in terms of graft success, seedling growth, and photosynthetic performance., although the magnitude of the response varied among rootstock genotypes.