Genotypic Variation in Foliar Heat Tolerance Among 35 Malus Genotypes: Implications for Urban Tree Selection Under Climate Change

The frequency and intensity of heatwaves are increasing annually worldwide due to climate change. Combined with the urban heat island effect, elevated heat stress episodes threaten the survival and performance of urban trees, in turn reducing their ecosystem benefits. For this reason, the foliar heat tolerance of 35 Malus genotypes (two species, 32 cultivars, one variety, one hybrid) was evaluated under controlled laboratory assays. Heat injury to foliar tissue was quantified using chlorophyll fluorescence (Fv/Fm) to assess photosystem II (PSII) damage and an electrolyte leakage index (ELI) to evaluate cellular membrane integrity. A preliminary dose–response experiment using six genotypes exposed to a temperature gradient (40–50 °C) was conducted to establish thermal response curves and derive LT50 values (temperature at 50% decline in Fv/Fm). These analyses confirmed substantial genotypic variation in thermal tolerance and identified 45 °C as an optimal discriminatory temperature for large-scale screening. This temperature was subsequently applied to assess heat injury across all 35 genotypes. Measurements were conducted in May (spring foliage) and August (summer foliage) to evaluate ontogenetic influences. In some instances, only one genotype was available for experimental purposes. Consequently, conclusions regarding genotypic differences in heat tolerance are based on replicated datasets, whereas genotypes represented by single-tree sampling are presented for descriptive purposes only. Heat stress significantly affected Fv/Fm and ELI, with strong genotype and seasonal effects recorded. In most genotypes, foliar damage was greater in spring than in summer. Good correlations between Fv/Fm and ELI confirmed their value as complementary physiological measures of heat tolerance in plants. Of the 35 genotypes evaluated, Malus sargentii, M. ‘Prairifire’, M. baccata ‘Jackii’, M. ‘Royal Fountain Huber’ and M. Donald Wyman were the most heat tolerant. The substantial variation in foliar heat tolerance detected across the 35 genotypes tested demonstrates potential for selecting Malus genotypes with superior foliar heat tolerance and highlights opportunities for identifying heat resilient candidates among other under-utilized urban tree taxa.