Quantitative Assessment of Morphological Traits Related to Grafting in Tomato Seedlings under High Temperature

Tomato seedling production for grafting faces significant challenges in growth management during summer because of high air temperatures (HTs) in greenhouses. This study evaluated the effects of daytime HTs on key grafting-related morphological traits of tomato scions and rootstocks and interpreted their responses using multiple quality indices and thermal time-based modeling. Growth chambers simulated midsummer greenhouse conditions by maintaining temperatures at 35/20 °C (photo-/dark periods) as HTs and 25/20 °C as normal air temperature (NTs). Light conditions were set at a photosynthetic photon flux density of 250 μmol·m −2 ·s −1 with a photoperiod of 12 h·d −1 under warm-white light-emitting diodes. Seedlings with expanded cotyledons were exposed to HTs for 0, 2, 4, 8, or 10 days and then transferred to NTs for the remainder of 10 days. The HT exposure promoted stem elongation, while stem thickening and leaf expansion showed no significant differences among treatments, consistent with thermomorphogenetic responses. Prolonged HT exposure induced excessive elongation and unbalanced biomass distribution, thus reducing seedling quality, whereas short-term HT exposure at early stages promoted the balance between leaf and stem growth and biomass, resulting in improved seedling quality. Key grafting parameters obtained via noninvasive imaging, including epicotyl length and diameter, hypocotyl length and diameter, and leaf area index, followed a sigmoidal pattern with thermal time. In addition, leaf pigment composition changed and intumescence incidence showed cultivar-specific responses in scions, both of which are crucial for post-grafting performance. Integrating thermal time-based modeling with noninvasive imaging could improve environmental control in greenhouses during summer, support decision-making, and facilitate the timely production of seedlings with grafting-appropriate morphologies.