Mild Water Stress Makes Apple Buds More Likely to Flower and More Responsive to Artificial Forcing— Impacts of an Unusually Warm and Dry Summer in Germany

Abstract

Climate change may result in increasingly frequent extreme events, such as the unusually dry conditions that occurred in Germany during the apple growing season of 2018. To assess the effects of this phenomenon on dormancy release and flowering in apples, we compared irrigated and non-irrigated orchard blocks at Campus Klein-Altendorf. We evaluated bud development, dormancy release and flowering in the following season under orchard and controlled forcing conditions. Results showed that irrigated trees presented longer (39.2%) and thinner shoots compared to non-irrigated trees. In both treatments, apical buds developed a similar number of flower primordia per cyme (4-5), presenting comparable development and starch dynamics during dormancy. Interestingly, buds on non-irrigated shoots exposed to low chill levels responded earlier to forcing conditions than those on irrigated shoots. However, chill requirements (~50 Chill Portions) and bud phenology under field conditions did not differ between treatments. In spring, buds on non-irrigated trees presented a higher bloom probability (0.42) than buds on irrigated trees (0.30). Our findings show that mild water stress during summer influenced vegetative growth during the same season, as well as the response of buds to forcing temperatures and flowering of the following season. The differences between irrigation levels in the phenological responses of shoots under low-chill conditions point to a so-far understudied impact of water supply on chilling requirements, as well as subsequent bud behavior. Accounting for the effects of both the water status during summer and the temperature during the dormant season may be required for accurately predicting future tree phenology in a changing climate.to 1986-2005 and using the scenario RCP4.5, total precipitation during April-September is predicted to change between about −20% (25th percentile of multi-model distribution) and +10% (75th percentile) in central Europe by 2046-2065 [3]. These changes may result in more frequent extreme events, similar to the extraordinary heat and dry weather observed during the apple growing season in June-September of 2018. Such seasons do not only have immediate impacts, but they may also affect orchard performance in the following growth period. In fact, tree development in a given season depends on processes that occurred during the previous year, including the accumulation of reserves and dormancy induction, as well as flower initiation and differentiation [4,5].Apple trees are cultivated in a wide range of climates, including temperate and tropical regions [4]. Accordingly, growing techniques have been adapted to a range of temperature and water regimes. Under drought conditions, where water scarcity limits apple production, regulated deficit irrigation (RDI) allows maintenance of productive orchards [5,6], although this practice may have undesirable implications for fruit maturity, quality, and shelf life [7]. On the other hand, RDI has been proposed as a useful approach to control excessi...

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