Global temperatures have already risen by an average of 0.5°C-0.7°C since the late 1800s and that trend is expected to accelerate in the next 100 years, as the release of greenhouse gases continues. Forecasts suggest that temperatures could rise by 0.2°C every 10 years over the next century and that this could have severe implications for fruit growers.
For fruit trees, it is essential for temperatures to drop in late autumn and winter (‘chilling’) to promote the dormancy phase in the annual growth cycle. Outwardly buds appear dormant, despite the cold, however, processes in the bud continue, very slowly, in the cycle of flower development as a prerequisite for flowering in spring. From a production viewpoint, the delay in spring bud break and flowering brought about by chilling is vital for most perennial fruit crops. Despite this, the extent to which chilling is important, depends on the genetic traits of the cultivar, for example, later flowering types requiring a great chill accumulation prior to bud break.
This accumulation phase, ‘the chilling degrees’, by the buds occurs when temperatures are generally between 2°C and 7°C and it enables flowers to develop which have the capacity to set and retain fruit. It also provides a defence mechanism which has evolved to avoid flowering occurring at time which climatic damage (e.g. freezing temperatures) has a high probability of occurring; it also synchronizes flowering within the tree and a cultivar and between cultivars.
Without chilling, floral bud development can be sporadic, or early, occurring at a time when the climate is less suitable for pollinators and the processes of flower fertilization. It may also be that the flowers are produced so early they miss the emergence of the bulk of pollinator population altogether, again hampering fertilization and potential fruit growth. Flowering synchronization is also highly important in commercial fruit production, influencing maturity, harvest date and storage.