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.