Heat stress mitigation by zinc oxide nanoparticles in pepper and watermelon

High temperature limits growth in pepper (Capsicum annuum L.) and watermelon (Citrullus lanatus L.). We tested whether foliar zinc oxide nanoparticles (ZnO NPs; 100 mg L⁻¹) mitigate heat stress (35 °C) in a side-by-side, multi-genotype design spanning both species under identical conditions. By evaluating multiple genotypes of two species under matched regimes, we enable direct cross-species comparison; multivariate analyses (PCA, PLS-VIP) summarize trait importance. Heat significantly reduced SPAD, growth, and biomass (p < 0.05). ZnO NPs partly alleviated injury in a genotype-dependent manner: in pepper P2, SPAD rose by ~ 35% and dry weight by ~ 33% versus heat controls; in watermelon W1, shoot length increased from 21.4 to 30.0 cm and fresh weight ~ 3.3-fold. Conversely, W2 showed limited or negative responses, indicating Zn sensitivity at the tested dose. PCA separated heat from control groups and shifted several ZnO-treated profiles toward controls. PLS-VIP prioritized SPAD and biomass traits; given the small dataset and negative LOOCV R², multivariate outputs are interpreted as exploratory. ZnO NPs can mitigate heat injury, but effects are species- and genotype-dependent. These results motivate targeted, genotype-informed optimization before agronomic deployment.