Date of Award:

5-1964

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Horticulture

Committee Chair(s)

David R. Walker

Committee

David R. Walker

Committee

Leonard H. Pollard

Committee

R. L. Smith

Abstract

Many deciduous trees enter a stage each year when their visible growth ceases. This is not always associated with cold weather or lack of water, and may occur with many species in mid-to-late summer. Trees entering this phase are said to be in rest. Rest is caused when internal factors are unfavorable for growth, while dormancy is defined as external factors being adverse for growth.

Rest in woody plants was first thought, about 1910, to be caused by cold temperatures. However, Coville (1920) states that deciduous trees enter rest regardless of cold temperature, with a certain period of effective chilling being required in order to break the rest. Plants in rest "harden" rapidly and are protected greatly against winter injury.

Investigations on different aspects of rest have been performed for more than half a century. Studies concerning the chilling requirements, optimum effective temperatures in breaking rest, environmental factors affecting rest, effects of chemical treatments on rest, chemical changes during rest and correlations among these factors with initiation and termination of rest have been reported.

Many published papers on chilling requirements of some fruit trees have been of value to the horticultural industry. Growers use this information as part of the basis when selecting desirable varieties for their specific areas. Contradictory results however, caused by methods of chemical analyses, lack of facilities or improper procedures, especially in the area of biochemical studies of rest have been reported. Hence, much additional information is needed in order to understand the mechanism of rest.

Among climatic factors, suitable temperatures during the year play an important role in regard to the success of a fruit orchard. Extremely cold temperatures in the winter kill the flower buds which are potential fruit and, in severe cases, the trees themselves. Orchards may also suffer from mild winters. Prolonged rest as a result of relatively warm winters has been reported in fruit trees (Samish, 1948; Overcash and Campbell, 1955; Chandler, 1957; and Weinberger, 1956). Delayed foliation causes reduced growth of the trees; yields may be reduced and ripening delayed. Conversely, in those areas where the chilling period is adequate but a fluctuating temperature is present during the early spring, trees bloom as soon as weather is favorable, and late frosts often kill a high percentage of the flowers. In both cases, growers suffer from extensive losses. In order to curtail temperature problems, proper selection of varieties and the use of special cultural treatments are necessary.

Many chemicals for delaying or breaking the rest period have been reported, among which is dinitrocresol. This has been used commercially in Israel and other places for breaking rest (Samish, 1954). Gibberellic acid (GA) has been effective in breaking rest of non after-ripened or non-chilled peach seeds (Donoho and Walker, 1957), and also non-chilled peach seedlings (Walker and Donoho, 1959).

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