Manipulation of the chilling requirement of sweet cherry trees
Commercial production of sweet cherries has recently increased in South Africa, with more than 400 ha planted by 2006. Cherry, a high chilling fruit variety, is however not suited for the mild winter climate of South Africa. This was recognizable through common observed symptoms of delayed foliation and poor fruit set. In addition, cherry is exposed to long and hot summers in the postharvest period. The objective of this study was to evaluate cherry cultural practices that can manipulate (reduce) the trees chilling requirement under South African conditions. Cultural practices where aimed at increasing reserves (nitrogen, cytokinin and carbohydrates) in the tree. In addition, bud dormancy progression of cherry buds was quantified to determine the bud dormancy progression pattern under mild winter conditions. This was achieved through sampling of cherry shoots from different cherry production areas which was then forced in the growth cabinets. A model was developed to identify possible factors and groupings that can explain the cherry bud dormancy pattern. A model, comprising two joined straight lines, was fitted in order to characterize bud dormancy behaviour for sweet cherry cultivars under mild winter conditions. All cherry cultivars followed the expected pattern of entrance and exit from dormancy. Factor analysis showed that factors related to the entrance into dormancy primarily characterize bud dormancy behaviour. Bud dormancy patterns were also a function of environmental conditions within a year as shown by cluster analysis. In addition, buds entered dormancy in mid-summer and remained dormant until chilling accumulation commenced. Bud dormancy release was generally extended over a three to five-month period for all cultivars. Prior to spring budburst exit of both lateral and terminal buds occurred rapidly. Data indicate that there is no ecodormant phase for cherry under the prevalent climatic conditions in South Africa. Further experimentation was aimed at increasing reserves within the trees through cultural practices. In the nitrogen trials, fertilization in the postharvest period had no significant effect on field budburst or bud dormancy progression in one-year-old shoots. Time of flowering was advanced in N treatments during 2007 only. Yield was not significantly increased. Therefore, in this trial, N fertilization in the postharvest period did not significantly reduce the chilling requirement of mature sweet cherry trees under mild winter conditions. Application of particle films (Surround® and Raynox®) or ethylene inhibitors (Retain®) in the summer did not reduce the heat stress the trees experienced. Treatments had no significant effect on carbon assimilation, stomatal conductance, leaf surface temperature, fluorescence, bud dormancy, budburst, flowering and fruit set. Cytokinins sprays (benzyladenine) in autumn did not affect bud dormancy progression, spring budburst or flowering. Hydrogen cyanamide application in spring significantly advanced budburst, time to full bloom and increased yield. Promalin® and Retain®, however, had no significant effect on budburst, flowering or yield. It is therefore evident that cherry, due to its unexpected bud dormancy behaviour and its inability to be significantly influenced by several cultural practices, adapts poorly to South African climatic conditions through not reducing its chilling requirement significantly.