Heat Stress Affects Plant Morphology, Leaf Chlorophyll Content, and Photosynthetic Characteristics in Pelargonium

Pelargonium is a globally cultivated ornamental plant; however, its ornamental and commercial values are severely threatened by heat stress. To identify heat-tolerant Pelargonium , we evaluated plant morphological traits, leaf color, chlorophyll content, and photosynthetic characteristics of six Pelargonium wild species and three commercial cultivars under heat stress. The results showed that heat stress significantly increased leaf color hue ( h °), chlorophyll concentration indices (CCIs), and chlorophyll a content in P. sidoides and Pelargonium ‘Chocolate Mint’. In addition, the net assimilation rate ( P n ) was remarkably improved in P. sidoides under heat stress. Conversely, heat stress dramatically reduced whole plant dry weight and CCIs of new leaves, induced leaf chlorosis, and enhanced the heat damage index of P. stenopetalum and Pelargonium ‘Madame Thibault’. Moreover, heat stress caused significant reductions in P n , chlorophyll a, and soluble sugar contents in P. stenopetalum . In conclusion, among nine Pelargonium genotypes evaluated, P. sidoides and Pelargonium ‘Chocolate Mint’ exhibited heat tolerance, whereas P. stenopetalum and Pelargonium ‘Madame Thibault’ were identified as heat susceptible. Thus, these heat-tolerant genotypes offer important genetic materials and contribute to breeding efforts aimed at developing new heat-tolerant Pelargonium cultivars.