Substitution of winter chilling by spring forcing for flowering using sweet cherry as model crop

Many horticultural crops such as apple, pear, plum, cherry, strawberry and Asparagus require a cold period in winter (chilling) with a subsequent warm period (forcing) for flowering. The objective of the present work was to investigate the effects of more forcing due to projected diminishing available chill as a result of climate change and elaborate the possibility of substitution of chilling by forcing, using cherry as the most affected crop. Therefore, 160 potted sweet cherry trees were exposed to different chilling in four consecutive winters at Klein Altendorf, near Bonn (50 degrees N), Germany. Three cherry cultivars with a wide range of chilling requirement (3-fold) were employed in eight scenarios per variety per year, ranging from - 50% less/insufficient chill for warm temperature zone winters to + 50% more or excess chill for cold winter fruit growing regions: 1. The minimum chill fulfilment of the cherry trees ranged from 400 CH (Chilling Hours) in low chill, 550 CH in medium chill and 750 CH in the high chill variety associated with maximum forcing of ca. 11.000 GDH (Growing Degree Hours) for low, ca. 12.000 GDH for medium and ca. 13.000 GDH for high chill varieties for sufficient flowering. 2. With optimum chill, the optimum forcing was ca. 8.000 GDH ( > 12 degrees C), irrespective of variety, allowing upscaling of the results to possibly other varieties. Trees exposed to excess chilling (150%) required less forcing (ca. 4000 GDH) to reach full bloom. Hence, chilling can compensate for up to half of the required forcing, i.e. ca 4.000 GDH. 3. Ratios of forcing to chilling were computed for future comparisons, which ensure flowering in the orchard. 4. Slightly negative temperatures ( - 5 degrees C to 0 degrees C), which are presently exempt in the common chilling models but common in the fruit growing regions, contributed to chilling accumulation of the fruit trees. 5. A novel scheme was developed to visualise these regulatory mechanisms in tree physiology. 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.