A study on dormancy and chilling requirement of peaches and nectarines

[摘要] ENGLISH ABSTRACT:Most South African peach and nectarine production areas lack in adequatewinter chilling. To address these issues, a conventional breeding programmewas started, to develop new and improved stone fruit cultivars with specialemphasis on climatic adaptation and pomological attributes. This study wasconducted to test the accuracy of the scientific method called the phenologicalmethod of classification, currently used by the breeding programme to classifythe selections in the second phase of evaluation according to chillingrequirement.During April 2000 and May 2001 and continuing until the next spring, 20 oneyear-old shoots of 30 peach and nectarine selections were harvestedfortnightly from an evaluation orchard on Bien Donné Experiment Farm,Simondium, Western Cape (34° S). All selections were previouslycategorised as high (>800 Utah chilling units [CU]), medium (400-800 CU) or_low «400 CU) chilling requirement based on phenologic observations.According to the preliminary classification of the selections included in thisstudy, six selections were classified as high, three as medium and 21 as lowchilling requirement. Two replicate bundles of shoots of each selection wereprepared and forced at 25° C with continuous illumination until no furtherchanges in bud burst occurred for a period of five days after which the shootswere then discarded. CU in the orchard were calculated according to theUtah and Infruitec models. The hours below 12° C and 7° C were alsocalculated. For each selection the number of days until 20% vegetative andreproductive bud break was plotted over day of year, Utah CU, Infruitec CU,hours below 12° C, and hours below 7° C, and expressed as a parabolicfunction. Similarly, the inverse of the number of days until 20% bud break orthe rate of bud growth was also plotted against all the above variables. Thearea under these parabolas was statistically analysed using the CANDISCprocedure of SAS Release 8.1. The groupings of the CANDISC procedurewere more or less consistent with the preliminary groupings obtained with thephenoligical classification method.On 16 May 2000 and 15 May 2001, 100 one-year-old shoots of the samepeach and nectarine selections were harvested from the evaluation orchardon Bien Donné Experiment Farm, covered in wet paper towelling and blackplastic bags and placed in a cold room kept at a temperature range between4° C and 7° C. Two replicate bundles of 10 shoots of each selection wereprepared fortnightly and forced at 25° C with continuous illumination until nofurther changes in bud burst occurred for a period of five days after which theshoots were then discarded. CU accumulated in the cold room at eachtransferral date was calculated according to the Utah model. For eachselection the number of days until 20% vegetative bud break was plotted overUtah CU, and expressed as a parabolic function. Similarly, the inverse of thenumber of days until 20% bud break or the rate of bud growth was alsoplotted against the above variables. The area under these parabolas wasstatistically analysed using the CANDISC procedure of SAS Release 8.1.Once again, the groupings of the CANDISC procedure were more or lessconsistent with the preliminary groupings obtained with the phenoligicalclassification method.Due to the nature of the scientific method used in this study, there is room fora certain margin of experimental error to occur, which could account for themisclassifications by the CANDISC procedure, when performed on the 2001season's data. It can be concluded that the phenological method ofclassifying the selections, as currently used in the breeding programme, isconsistent with the results of the scientific method described here. Therefore,it is recommended that the phenological method be used in future to classifythe selections according to chilling requirement (CR), as this method is lesstime consuming and less costly to perform.Finally the outcome of the analysis of one season's data was used ascalibration data against which the other season's data was tested and theconsistency of the results, using one set of discriminant functions, was tested.It can be concluded that a unique set of discriminant functions is necessaryfor each winter season to accurately classify selections according to CR withthe CANDISC procedure.

[发布机构] Stellenbosch University