Abstracts Biological Sciences

A study of the physiology of dormancy in the genus Pyrus

by Stanley Douglas Strausz

This investigation was undertaken to observe possible changes in growth substances in Pyrus throughout the dormant period. The primary objective was to determine if a correlation existed between growth substance content and the inherent chilling requirement of seeds and buds. Samples of whole buds were taken at intervals from the onset of rest in September through full-bloom in March from three Pyrus types differing in chilling requirements. Single samples were also taken during rest from several additional Pyrus species of varying lengths of chilling requirements. Growth substances were removed with methanol and ether extractions. Seed and fruit samples were taken from several varieties of P. communis before and after subjection to periods of chilling temperatures. Individual seed samples were divided into three fractions – surface materials, seedcoats and embryos—-prior to water and ether extractions. Separation of growth substances was done with paper chromatography and active materials detected with an oat coleoptile straight-growth test. A single growth inhibitor was found in all samples of Pyrus buds, seeds and fruit tissue. Based on both paper and thin-layer chromatographic separations and UV spectra, this material is tentatively identified as abscisic acid. The inhibitor content in bud extracts of the three Pyrus types, while fluctuating from sample to sample, did not correlate with the amount of chilling that had accumulated at each sampling time. Inhibitor concentrations in whole buds remained relatively stable throughout rest. The inhibitor did not disappear from the buds when rest was broken or provide evidence of a consistent reduction with chilling. Pyrus species with longer inherent chilling requirements consistently showed higher inhibitor concentrations during rest than did species requiring short chilling periods to break rest. No growth promoting materials were detected until full-bloom when a strong, unidentified promoter was found in samples of each of the three Pyrus types. Significantly higher concentrations of the material were present in the samples of the long chilling types than in the short chilling species. In the fruit and seed samples, significantly higher concentrations of the growth inhibitor were present in fruit tissue, on the seed surface and in the seedcoats than in the embryo. After subjection to chilling temperatures the inhibitor content decreased significantly in the embryo, but little change took place in the extracts of the fruit and seedcoats. No consistent growth promoters were found in fruit or seed tissues. The presence of the growth inhibitor in each sample of whole buds throughout the dormant period indicates that rest may result in part from growth inhibitors. Growth inhibitor content appears to be related primarily to chilling requirement rather than to the amount of chilling the buds have received at any given time. Inhibitors may increase to functional levels during the period of rest induction, remaining at these…