Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Plants, Soils, and Climate

Committee Chair(s)

J. LaMar Anderson


J. LaMar Anderson


Two areas of research are reported: an experiment on the effects of warm temperature prestratification treatments on seed dormancy and a new chill unit model.
Crabapple seeds (Malus sargentii Rehd.) were allowed to imbibe water and were given warm pretreatments at temperatures of 16, 20, 24 and 28C for periods of 3, 10, 20 or 30 days before cold stratification at 4C for 20, 40, 60, 80 or 100 days. Pretreatments resulted in increased chilling requirements for seedling emergence. A short chilling period (20 days) also altered the leaf area, shoot length, internodal length and root/shoot ratio of the resulting seedlings.
The new chill unit model was developed from data from seed experiments and tested with records of 11 years. A three-dimensional model for the transition through apple tree dormancy is proposed. The new model evaluates the effectiveness of different temperatures for the transition between dormancy induction and dormancy release according to physiological time. The standard of measurement for this model is the chill unit (CU), which is defined as the equivalent of one-hour exposure to the optimal temperature during the optimal physiological time for dormancy development. The general pattern of temperature activity for dormancy development is sigmoidal; and temperature effectiveness through the process varies according to length of exposure, temperature cycling and time. The new model permits a more accurate prediction of dormancy development under subtropical conditions than previous models and will predict the amount of leafing that will occur in spring. The prediction efficiency of leafing under subtropical conditions was improved from an r2 of 0.66 for the Utah Chill Unit Model to an r2 of 0.74 for the new model when compared under Mexican conditions.