Physio-biochemical responses of grafted tomatoes differing in thermotolerance to heat stress and recovery

The aim of this study was to evaluate the physiological and biochemical heat responses of grafted tomato (Solanum lycopersicum L.) seedlings and their changes during the post-stress recovery period. Heat-tolerant ‘Celebrity’ and heat-sensitive ‘Arkansas Traveler’ tomatoes were grafted onto a commercial ‘Maxifort’ rootstock, and transplants were grown in growth chambers for 14 days of heat treatment (38/30 °C, day/night) followed by 7 days of recovery (25/18 °C). Results showed significant differences between ‘Maxifort’-grafted and ungrafted seedlings in antioxidant enzyme activities with increased leaf ascorbate peroxidase (APX), root superoxide dismutase (SOD) and catalase (CAT), and decreased root guaiacol peroxidase (POD) in ‘Maxifort’-grafted plants when heat stress was imposed. ‘Maxifort’-grafted plants had lower chlorophyll:carotenoid ratios and higher leaf total soluble protein than ungrafted controls under heat stress. Heat stress increased leaf chlorophyll, carotenoid, electrolyte leakage (EL), malondialdehyde (MDA) and leaf and root proline content, and reduced leaf chlorophyll fluorescence (Fᵥ/Fₘ) during the stress period. Antioxidant enzymes were up-regulated by heat stress in leaves, while those in roots were selectively regulated with increased glutathione reductase (GR) and CAT, and decreased POD and APX activities in heat stressed plants. After the recovery period, chlorophyll, carotenoid, EL and proline contents showed partial or full recovery from the stress, whereas the heat responses of MDA and Fᵥ/Fₘ remained constant. Cultivar effects were detected in leaf antioxidant enzyme activities of heat stressed plants after recovery as ‘Arkansas Traveler’ had higher POD, GR, CAT and lower SOD activities than ‘Celebrity’. These results demonstrate the potential for using ‘Maxifort’ rootstocks to enhance the thermotolerance of tomato plants during the vegetative stage, although the beneficial traits did not lead to a significant alleviation of heat stress.