Heat stress poses a significant threat to global food production and security by disrupting plant physiological and biochemical processes. Global simulation models predict a 4 to 5°C increase in atmospheric temperatures by the year 2100, as well as increases in the frequency of extreme heat events. This has necessitated preemptive measures to enhance the resiliency of horticultural crop production. Biostimulants, a class of agricultural products, show great promise in mitigating heat stress effects by enhancing physiological and biochemical stress tolerance. However, limited research exists on the efficacy of commercial biostimulant products in improving heat stress tolerance in horticultural crops. This study evaluated the impacts of three exogenously applied commercial biostimulants representing a range of active ingredients and included FRUIT ARMORTM, Optysil®, and KelpXpressTM [active ingredients glycine betaine, silicone, and kelp (Ascophyllum nodosum)extract, respectively], plus a water control, on physiological, biochemical, and growth parameters in different raspberry genotypes exposed to continuous heat stress (Tmax ≥ 35°C/day) in a glasshouse. Over a 28-day period, the biostimulants and the water control were applied weekly to three raspberry genotypes (‘Meeker’, WSU 2188, and ORUS 4715-2). The results indicated that ‘Meeker’ consistently maintained high chlorophyll fluorescence (Fv/Fm) and photosynthesis under control and biostimulant treatments. In contrast, WSU 2188 and ORUS 4715-2 exhibited increased Fv/Fm and photosynthesis when treated with FRUIT ARMORTM. Additionally, KelpXpressTM application improved Fv/Fm in WSU 2188. ‘Meeker’ and WSU 2188 treated with FRUIT ARMORTM and KelpXpressTM accumulated more anthocyanins and had greater shoot and total biomass compared to ORUS 4715-2. These findings underscore genotype-specific and biostimulant-dependent responses to heat stress mitigation. The superior physiological performance by ‘Meeker’ under both control conditions and biostimulant treatments indicates heat tolerance in the genotype. Furthermore, higher anthocyanins accumulation, improved Fv/Fm, enhanced gas exchange, and greater total biomass of WSU 2188, as well as improved Fv/Fm of ORUS 4715-2, suggests that application of the biostimulants contributed to enhanced repair and maintenance of photosystem II (PSII) structural integrity, improved photosynthetic performance, and increased antioxidative capabilities, which may have contributed to higher total biomass of raspberry treated with FRUIT ARMORTM relative to the untreated control. In summary, the positive impacts on physiological, biochemical, and growth parameters support the potential role of biostimulants in enhancing thermotolerance in raspberries and other horticultural crops exposed to heat stress.
