Effects of warmer winters due to climate change on chilling and dormancy release of sweet cherry

Abstract

The main objective was to contribute to dormancy research by one the one hand investigating the chilling and forcing requirements of sweet cherry trees in one of the major fruit growing regions in Germany and on the other hand to investigate the changes in carbohydrate levels and water relations of cherry buds to identify and distinguish the dormancy phases on a plant physiological base, and to determine transition points between dormancy stages. To quantify the requirements at an entire tree level under natural photoperiod rather than single buds or cut shoots in dark cold chambers, 160 potted sweet cherry trees of three varieties with a very wide range of chilling requirements (3-fold) were raised over two years to initiate uniform flower buds before applying 24 chilling regimes (8 regimes per variety) per year followed by forcing to determine the effect on flowering. This expense was needed to see real flower behaviour of the entire fruit tree. To assess future scenarios for fruit growing (e.g. global warming), potted trees where placed in an unheated greenhouse with an average of ca. 2°C temperature increase. To assess the carbohydrate fluxes ca. 10,000 flower buds from nine varieties and bi-weekly sampling over two years were sampled. The results can be summarized as follows:<br /> 1.When applying the three chilling models presented to calculate winter chilling in a cherry orchard at Klein-Altendorf, Meckenheim, Germany, to an unheated greenhouse (average temperature increase by ca. 2°C) to simulate global warming, in the unheated greenhouse, the available chilling increased by 12 % (Chill Units - CU), 15% (Chill Portions - CP) and 20% (Chilling Hours - CH), respectively. In climate change predictions for other locations, especially in the South of Europe or in the North of Africa, the warmer winter temperatures often induce lack of chilling. Options and limitations of countermeasures in terms of cultivation methods such as microclimate manipulation, rest breaking agents, change of orchard location to a higher altitude and breeding are options to counter possible lack of chilling. <br /> 2.The Meckenheim fruit growing region in Western Germany may be affected as a consequence of climate change and lack of chilling. The orchard temperatures in the warmer winter (6.0°C) exceeded those in the unheated greenhouse (4.7°C) in the cold winter. Maximum chill accumulation in very warm winters at this location is currently achieved, so that even warmer winters may reduce the available chill, but there will be still enough to grow high chilling varieties. Chilling computations for this region, with all three major chilling models, showed that cherry trees of low chill cultivar '6000CZ' required 22.3-26.6 (CP), 465-684 (CU) or 402-483 (CH), medium chill cv. 'Brooks' about 37.9-54.4 CP, 819-1,267 CU, or 779-941 CH and the high chill cv. 'Schneiders späte Knorpelkirsche' about 54.4-79.3 CP, 1,267-1,696 CU, 941-1,494 CH, respectively, for a natural flowering. Cherry trees of cv. '6000CZ' receiving <300 CH, cv. 'Brooks' <500 CH and cv. 'Schneiders späte Knorpelkirsche' <700 CH were unable to flower, equivalent of 50% of the assumed chilling optimum of the respective cultivar. The beginning of leaf drop was identified as a suitable initiation point for computing chill accumulation. <br /> 3.Four transition points were proposed to clearly distinguish dormancy phases by relating carbohydrate and relative water content (RWC) in reproductive buds to concomitant chilling fulfilment. Further, two groups of cherry varieties were defined based on their different initial sorbitol and starch level in the autumn. The first separation between para- and (deep) d-endo-dormancy is characterized as a transition from a decrease (variety group 1) or a constant level (variety group 2) to a sharp increase in hexoses (glucose and fructose), sorbitol and a drop of starch content. The second transition point (d-endo- to f-endo-dormancy) is characterized as the changes in both hexoses (increase) and starch (decrease) terminate, ca. 650 Chilling Hours (CH), i.e. insufficient chilling as measured in the concomitant forcing experiment. This third transition point (f-endo- to eco-dormancy) was characterized by ca. 1,000 CH, the minimum chilling requirement and restrained flowering (cut branches). The fourth transition point (forcing initiation) marked an increase in water content at ca. 1,550 CH, optimum chilling for cherry and coincided with natural flowering. A ratio of hexoses to starch content (<2:1) appeared to be a potential indicator of the beginning of chilling (para-dormancy); a ratio of 14-20:1 typical for endo-dormancy, whereas the release from dormancy was associated with a decline to less than 10:1 at the end of winter (eco-dormancy).<br /> 4.The effects of more forcing due to diminishing available chill as a result of climate change the possibility of substitution of chilling by forcing were investigated. In the scenarios applied, minimum chill fulfilment ranged from 400 CH (Chilling Hours) in low chill, 550 CH in medium chill and 750 CH in the high chill variety with maximum forcing of ca. 11,000 Growing Degree Hours (GDH) for low, ca. 12,000 GDH for medium and ca. 13,000 GDH for high chill varieties for sufficient flowering. With optimum chill, the optimum forcing was ca. 8,000 GDH (>12°C), irrespective of variety, allowing upscaling of the results to possibly other varieties. Trees exposed to excess chilling (150%) required less forcing (ca. 4,000 GDH) to reach full bloom. Hence, chilling can compensate for up to half of the required forcing, i.e. ca 4,000 GDH. Ratios of forcing to chilling were computed for future comparisons, which ensure flowering in the orchard. Slightly negative temperatures (-5°C to 0°C), which are presently exempt in the common chilling models, contributed to chilling accumulation of the fruit trees. Overall, the results have shown that diminishing chilling as a result of climate change can be compensated for, in part (up to 50%), by a larger amount of forcing to obtain natural flowering in the orchard.