Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Plant, Soil and Environmental Sciences

Major Professor

Fred L. Allen

Committee Members

David Coffey, John Reynolds, Otto Schwarz


The plant breeder uses many tools to produce higher quantity and quality crops suitable for various environments. Plant growth regulators are one possible tool. This study was undertaken to evaluate the potential of some promising plant growth regulators in modifying soybean (Glycine max L. Merrill) plant size and shape, selectively promoting flower and pod abortion and enhancing yields.

Two determinate varieties, Forrest and Essex, and one indeterminate line, TN77-31, were planted in early to mid-May and early to mid-June of 1983-1985. The treatments gibberellic acid, 6- benzyladenine, daminozide, chlorflurenol, mepiquat chloride, ancymidol, ethephon, and brassinosteroid 1105 were applied at various growth stages. Measurements taken included yield, frequency of one-, two- and three-seed pods on mainstem and branches, hundred-seed weight, protein, oil, linolenic acid and linoleic acid percentages, plant height, number of nodes on mainstem, internode length, number of branches and nodes on branches.

Results indicated growth regulator treatments did not produce consistently significant results. Response was also dependent on planting date. Gibberellic acid treatments mainly produced positive effects on the number of pods on branches of Forrest and Essex and also increased their height, branch number and nodes per branch; for TN77-31, only increased branch node number occurred. Plant growth regulator treatments following delayed planting produced similar results for Forrest, but for Essex, results for the morphological characteristics changed--node number decreased, and internode length and nodes per branch increased; TN77-31 plants were shortened, nodes per branch were increased and yields were lowered.

Ethephon decreased yield and hundred-seed weight, mainstem two seed pods of Essex, three-seed pods of Forrest and hundred-seed weight of TN77-31 and shortened plants of all varieties in the May plantings. Following ethephon treatments in delayed plantings, Essex seed yield and hundred-seed weight decreased, but branch number, nodes per branch and leaf area increased; only TN77-31's hundred-seed weight decreased, but Forrest branch two-seed pods and hundred-seed weight increased, and nodes per branch decreased.

All chlorflurenol-treated plants were dark green and senescence was delayed. In the earlier plantings, chlorflurenol applications significantly shortened Essex and Forrest heights, decreased Forrest yield, and linoleic and linolenic acid percentages, and increased hundred-seed weight, mainstem three-seed pods and oil. For TN77-31, hundred-seed weight was significantly affected, percentage of protein and number of mainstem three-seed pods were increased; whereas oil and linolenic acid percentages were decreased. Delayed planting combined with chlorflurenol applications reduced seed yields and shortened internode lengths for TN77-31 as well as increased hundred-seed weight and linolenic acid percentages but caused a decrease in percentage oil for Forrest.

Although significant responses to the growth regulators occurred erratically and no positive effects on yield occurred, plant size was modified and seed quality changed from treatments imposed in this study.

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