Malus Species with Diverse Bloom Times Exhibit Variable Rates of Floral Development

In response to challenges caused by climate change, apple (Malus ×domestica) breeding programs must quickly develop more resilient cultivars. One strategy is to breed for various bloom times. Members of the genus Malus, including domesticated apple, wild species, and hybrids, exhibit striking variations in the bloom date. Although bloom time is strongly influenced by chilling requirements, other aspects of floral development in Malus and their contributions to bloom time are less known. The purpose of this study was to investigate potential connections between predormancy flower development and final bloom time in Malus species. We performed a phenological analysis of flower development in wild and domesticated apple with extreme differences in bloom time over the course of one developmental season. We tracked histological changes in the floral apex of representatives of three early-blooming Malus genotypes (M. ×domestica ‘Anna’ PI 280400, M. orthocarpa PI 589392, M. sylvestris PI 633824) and three late-blooming genotypes (M. angustifolia PI 589763, M. angustifolia PI 613880, M. ×domestica ‘Koningszuur’ PI 188517). Our study documented their floral meristem progression and organ development and expanded on current staging systems for apple flower development to describe the changes observed. The developmental trajectories of each genotype did not group according to bloom category, and we observed variations in the floral development stage at the time of dormancy onset.