Grafting onto eggplant rootstock improves water status and fruit quality in tomato under deficit irrigation

Tomato (Solanum lycopersicum L.) is a vital vegetable crop in Taiwan. The small-fruit cultivar ‘Rosada’ dominates the market due to its thin skin and high sweetness; however, these traits cause a significant trade-off: high susceptibility to fruit cracking. This study evaluated the physiological and morphological responses of ‘Rosada’ grafted onto ‘EG203’ (T/E), self-grafted (T/T), and non-grafted (T) plants under varying water regimes, focusing on yield stability and fruit cracking. Results indicated that grafting onto eggplant rootstocks altered carbon partitioning. T/E plants showed significant sugar accumulation in shoots compared to controls, despite unchanged photosynthetic rates, suggesting restricted carbohydrate transport across the graft union. This “top-heavy” allocation suppressed the T/E root system and reduced leaf area, mimicking drought acclimation. Under water-limited conditions, this restricted root system paradoxically buffered fluctuations in Relative Water Content (RWC). In Frequency Halved (FHI) and Volume Halved Irrigation (VHI) treatments, T/E plants maintained more stable leaf water status than the controls. Although water deficit generally reduced yield, T/E plants sustained significantly higher fruit set under stress. Crucially, the sttudy linked physiological stability to fruit quality. Under full irrigation, the average fruit cracking rates across two experiments for T/E, T/T, and T were 10.3%, 12.1%, and 7.9%, respectively. The stable water-deficit treatment (VHI) significantly reduced cracking to 2.3%, 1.6%, and 5.8% for T/E, T/T, and T, respectively. While fluctuating water stress (FHI) increased fruit cracking rates by 7% in self-and non-grafted tomatoes, no such increase was observed in T/E tomatoes.