Temperature effects on apple (Malus x domestica Borkh.) reproductive development after budbreak

Temperature, as a primary driver of climate change, significantly impacts both natural and agricultural systems, particularly influencing critical stages like bud dormancy and flowering in temperate deciduous species. However, its specific effects on fruit yield development, such as in apples, remain poorly understood. This research project employed a combination of fieldwork and experimental studies to explore the relationship between temperature and reproductive development in apples. The fieldwork study examined the developmental patterns of three apple cultivars (‘Discovery’, ‘Egremont Russet’, and ‘Red Windsor’) in a Sheffield orchard over two growing seasons. The findings revealed distinct phenological patterns and thermal requirements for each cultivar. The study also proposed a modification to the apple BBCH scale aimed at addressing challenges encountered in monitoring fruit development and maturation macrostages. This innovation improves phenological data collection, orchard management, and harvest scheduling, ultimately contributing to better fruit quality and increased market value. On the other hand, the controlled environment studies quantitatively described the temperature sensitivities of phenology, organ growth, leaf physiology, and fruit yield of ‘Red Windsor’ apple. Temperature-induced phenological shifts not only altered the timing, progression, and growth of reproductive structures but also modified the developmental overlap of reproductive and vegetative organs. These alterations in reproductive structures were closely linked to changes in leaf physiology, as evidenced by changes in functional traits, photosynthetic pigments, and gas exchange parameters across fruit developmental stages. Specifically, manipulating the temperatures by ±4°C from the ambient levels did not significantly affect the final fruit yield because of source limitations. Significant treatment effects were evident on the fruit yield components, with the warmer temperatures resulting in a markedly earlier harvest with fewer but bigger fruits. The higher pigment levels and assimilation rates at warmer temperatures supported early fruit development but subsequent stimulation of vegetative growth intensified sink competition at the expense of fruit development. On the other hand, sub-ambient temperatures remarkably enhanced fruit set, but the inherently low assimilation rates under this treatment caused an equally high incidence of fruit drop, thus the comparable yield level. Overall, this research contributes to a better understanding of the physiological dynamics of temperature and its implications for apple fruit productivity. This information is essential for refining thermal models and developing other practical, resilient, and sustainable approaches to maximise apple productivity amidst changing climatic conditions.