Analysing variations in flowering time based on the dynamics of chill and heat accumulation during the fulfilment of cultivar-specific chill requirements in apricot

The fulfilment of cultivar-specific chill requirements (CR) and heat requirements (HR) is crucial for breaking dormancy and achieving optimal blooming and fruit set in temperate fruit trees. The reduction in winter chill availability over the coming decades due to global warming is expected to compromise the fulfilment of CR in many production areas, leading to production losses. The interplay between the accumulation of CR and HR remains unclear. In this study, we used three-year-old potted trees of the apricot cultivar ‘Deseo’ (60 chill portions; CP) to examine how the dynamics of chill and heat accumulation affect the timing of full bloom. We supplied the trees with their specific chill needs by moving them between three distinct environments (constant temperature cold store, ambient conditions and heated glasshouse) before transferring them to a heated glasshouse for flowering. Despite trees being exposed to similar chilling, the variation in temperature dynamics during the dormant period affected flowering dates. The findings suggest that chill and heat accumulation phases can overlap, and once a specific percentage the cultivar specific CR is fulfilled, the simultaneous accumulation of chill and heat appears to advance flowering. A higher heat accumulation (measured in Growing Degree Hours; GDH) during the fulfilment of CR was associated with lower heat accumulation needed for blooming in a heated glasshouse where the accumulation of chill had ceased. The higher correlation was observed when the 75 % of the CR had been fulfilled suggesting that simultaneous accumulation of chill and heat in the last stages appears to be more effective in triggering earlier blooming. This experimental set up could also help to develop new modelling approaches for temperate fruit trees, considering changes in typical patterns of chill accumulation as well as abnormal high winter temperatures caused by climate change.