Brown seaweed extracts, particularly Ascophyllum nodosum (AN), have shown beneficial effects on improving plant physiology, flower development and abiotic stress tolerance in various crops. However, limited research has been conducted on other seaweed species, such as Ecklonia maxima (EM) and Macrocystis pyrifera (MP). This study aimed to evaluate the thermotolerance of tomato (Solanum lycopersicum L.) seedlings in response to the application of three seaweed extracts, AN, EM and MP (1% v/v). ‘Big Beef’ tomato seedlings were root-drenched with seaweed extracts at transplanting and grown for 35 days with weekly foliar application in growth chambers set at 26/19°C (day/night, 16/8 h) for control and 33/26°C for mild heat stress treatments. Under heat stress, AN and EM treatments significantly increased shoot fresh weight by 12.5% and 10.8%, respectively, compared to the control, while MP treatments showed no significant differences. Also, seedlings treated with MP showed a numerical increase in chlorophyll fluorescence (Fv’/Fm’) by 12.8% and a reduction in leaf electrolyte leakage by 19.8% compared to the control under heat stress. However, no significant thermotolerance effects of seaweed extracts were observed in SPAD, net photosynthetic rate (Pn), pollen count and viability. In conclusion, the application of seaweed extracts provided differential response to thermotolerance benefits of tomato seedlings under mild heat stress conditions, with AN and EM enhancing shoot biomass and MP showing potential in mitigating physiological heat stress damage.
