Variation in leaf orientation and its related traits in sunflower (Helianthus annuus L.) breeding populations under high temperature

A sunflower genotype may differ in its ability to adjust canopy temperature by regulating leaf inclination in order to cope with heat stress. Therefore, this study was carried out to determine the temporal changes in leaf temperature, leaf inclination, and heat stress injury in elite sunflower germplasm with changes in ambient air temperature at two locations over a three-year period. In this study, two distinct sunflower genotypes were identified, with leaves that inclined upward or downward. The genotype with upward inclined leaves was superior to that with downward inclined leaves as it maintained lower post noon temperature and heat injury. Moreover, the upward inclined leaf type also avoided high light stress and maintained leaf gas exchange traits under heat stress. The upward inclined leaf type reduced post-noon leaf temperature more than pre-noon leaf temperature and showed negative leaf temperature adjustment over time (Δ) while the downward inclined leaf type showed a positive Δ. Δ was a useful trait for selecting heat-tolerant genotypes due to its relationship with heat stress injury and could be utilized for practical breeding of field crops under heat stress.