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The growth-promoting and heat-mitigating effects of a commercially available protein-hydrolysate-based biostimulant, Kaishi, during the early vegetative stage was investigated by applying it as a foliar spray on soil-grown maize plants or in the nutrient solution of hydroponically grown plants. At 10⁻³ dilution, the biostimulant inhibited germination and delayed the growth progress, while at 10⁻⁶–10⁻¹² dilutions, it promoted shoot and root growth. Heat stress caused biomass reduction, decreased leaf pigment content and the chlorophyll <i>a</i>/chlorophyll <i>b</i> (chl <i>a</i>/<i>b</i>) ratio, caused starch depletion, and increased lipid peroxidation. Kaishi priming resulted in the substantial mitigation of negative stress effects, maintaining growth, stabilizing pigment content and the chl <i>a</i>/<i>b</i> ratio, restoring the leaf starch content, lowering the malondialdehyde (MDA) level, and significantly increasing the free proline content. The expression profiles of a set of genes coding for heat shock proteins (HSPs), dehydrins (DHNs), and proteases were analysed using qRT-PCR after heat stress exposure. The biostimulant-treated plants had higher transcript levels of certain <i>HSPs</i>, <i>DHNs</i>, and protease-coding genes, which remained stable or increased after the applied stress. The results demonstrate that very low concentrations of the biostimulant exerted stress-mitigating effects that could be linked to organ-specific changes in the gene expression of certain stress-inducible proteins.