Doctoral Dissertations

Date of Award

8-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major Professor

David L. Coffey

Committee Members

Dennis Deyton, Carl Sams, John H. Reynolds, Otto J. Schwarz

Abstract

Paclobutrazol ((2RS, 3RS)-1-(4-chlorophenyl)-4, 4-dimethyl-2-1, 2, (4-triazol-1-yl) pentane-3-ol) was applied in single or multiple foliar applications to ‘Golden Delicious’ apple trees in 1985. The growth regulator had little effect on shoot growth in that growing season. The highest rate of paclobutrazol (2.67 kg•ha-1) applied as a single or multiple application inhibited overall shoot growth by 89% the following growing season. No differences in shoot growth were observed between single and multiple applications in either 1985 or 1986. A significant negative linear relationship was found between paclobutrazol concentration and shoot growth for both single and multiple applications resulting in increased light penetration into the canopy as measured by Fisheye photography. No significant differences in growth reduction were found between horizontal and vertical shoots. Leaf area was decreased by up to 48% in a significant linear relationship with increasing application rates. Fruit length and the length:diameter ratio were reduced by as much as 15% and 8%, respectively, by paclobutrazol.

Rates of 0, 300, 600, 1200 or 2400 ppm of paclobutrazol were applied as a foliar spray to potted ‘Golden Delicious’ apple and ‘Redhaven’ peach trees in 1985. Paclobutrazol had no effect on stomatal conductance of either young or old leaves of apple or peach trees in the year of application. During the year following application, the 2400 ppm paclobutrazol treatment resulted in higher stomatal conductance levels of both old and young apple leaves. No significant differences in the photosynthetic rates of peach leaves were found in the year of application. In the year following application the 2400 ppm application tended to result in higher photosynthetic rates than the control for both young and old apple leaves.

As paclobutrazol concentrations increased, leader growth of apple trees was significantly reduced by 31 to 47% compared to the untreated controls in 1985. Paclobutrazol decreased total shoot growth by 31 to 48% for apples in 1985 over all application rates. Leader growth, total branch length, and total growth of peach trees were reduced by 5 to 16%, 18 to 55%, and 13 to 35%, respectively, in 1985. In the year following application, paclobutrazol caused a significant linear reduction in total shoot growth ranging from 2 to 28% for ‘Golden Delicious’ apple trees. A significant reduction of 10 to 38% in total branch length was also found for apple. Reductions in total branch length, total shoot growth, and leader length of peach trees by 6, 10, and 11%, respectively, were found with the highest application rate for the year following treatment.

Paclobutrazol rates of 0, 300, 600, 1200 and 2400 ppm were applied as a foliar spray to potted ‘Golden Delicious’ apple trees in 1986. Net photosynthesis (Pn) and stomatal conductance readings began with the initial soil moisture at field capacity. Trees treated with the 1200 ppm application rate maintained a significantly higher Pn rate than the control trees under adequate soil moisture conditions. Tbe 2400 ppm application rate maintained significantly higher net photosynthesis levels for apple leaves at soil moisture values between 2.3% (295 MPa) and 5.8% by volume (2.5 MPa). At 5.8% soil moisture by volume, trees treated with 2400 ppm rate had a net photosynthesis rate 10% above the control. This rate maintained net photosynthesis at 95% above the control at 2.3% soil moisture by volume level.

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