Problems in standardizing methods for evaluating the chilling requirements for the breaking of dormancy in buds of woody plants

Abstract

Modeling can be useful in predicting budbreak, especially in the subtropics, where special treatments may be required to supplement natural chilling. It has limited practical use in the cooler temperate regions, as chilling is always adequate. The Utah Model appears to be reliable in the cooler regions, whereas the dynamic model may be more useful in warmer zones. Regardless of the model used, attention must be paid to species and cultivar, and appropriate adjustments made. To standardize methods for determining the stage of rest, researchers should first decide upon the portion of the plant to be used as the experimental unit (e.g., single-node cutting, entire cutting, rooted cutting, whole plant). This will vary, of course, with the purpose of the research. Single-node cuttings are of little use for determining response of whole trees, as bud development in the latter will be affected by correlative inhibition, including apical dominance. Cuttings bearing many buds will be the better choice for researchers wishing to predict the field response. The larger the cutting, the better the expected response. The source of cuttings is also important, especially for theoretical studies; previous year's shoots are normally used, but their vigor could affect response. Therefore, selected shoots should be similar in length and taken from similar positions on the plant. The environmental conditions to be used, especially temperature, must be established for each species and cultivar used. A temperature of 15-20°C should be adequate. Light should be supplied unless it can be shown to have no effect. The criteria to be used for evaluation of response, including observation time and stage of bud development, must also be agreed upon. These conditions will undoubtedly differ among species, but some agreement should be possible within species. Greening of the bud scales alone appears to be too limited a response to predict behavior under orchard conditions. On the other hand, full bloom may be difficult to obtain with small cuttings. Recording time to reach a given stage of development ("speed of budbreak") appears to be superior to recording the percentage budbreak during a specific time interval. The problem with the method is selecting a stage that will be reached by partially chilled buds so that comparisons can be made throughout the period of dormancy. Otherwise, response can be quantified only during the later stages of dormancy. Despite all precautions, differences in results will occur; e.g., contrast the results of several workers in using single-node cuttings of apple (see above). Such differences may reflect differences in choice of plant material, or in environmental conditions during the previous growing season. Some of these problems might be solved by using a cooperative approach. For example, experimental protocols could be established and tested by researchers at several locations, using the same cultivars, and/or cuttings could be exchanged, so that the responses of cuttings from the same trees could be compared at different locations.