Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Nick E. Christians


Inconsistent efficacy has limited the application of turfgrass growth retardants to low-traffic, less-visible areas. More information is needed regarding the environmental conditions in which plants are more sensitive to growth regulation;A three-year field study was conducted to determine if the seasonal growth phases of Poa pratensis had any effect on plant receptivity to five growth retardants. Mefluidide and amidochlor were more effective in spring, flurprimidol in summer, while ethephon and paclobutrazol were equally effective across seasons. The greatest differences in growth retardation appeared between the spring reproductive and summer vegetative growth phases. It is concluded, therefore, that the transition of Poa pratensis from its reproductive to vegetative phases affects its degree of response to turfgrass growth retardants. Ethephon was the only chemical to act as a true growth regulator by altering plant growth habit. Leaf elongation was restricted while internodes were stimulated to elongate from a normally compact stack of nodes;Four laboratory experiments were conducted to determine conditions necessary for plant sensitivity to ethylene and to test the hypothesis that some component in ethephon degradation in addition to ethylene predisposes Poa pratensis to ethylene sensitivity. Ethephon was effective in most environments tested while ethylene was effective only after eight days of continuous application under full-sun conditions in the greenhouse. Under those conditions, ethephon and ethylene had similar effects on tiller intornode elongation, root weight, and stem/leaf dry weight ratio. Ethylene effects on leaf length, however, varied compared to those of ethephon. Ethylene treatment during ethephon degradation had an additive but not a synergistic effect on plant morphogenesis. Reduction in root growth was always associated with shortness in leaves. It is concluded that the reliability of ethephon effectiveness is due largely to its slow release of ethylene over a four to ten day period and that some threshold irradiance level is necessary for exogenouse ethylene at 1000 [mu]L/L to cause a morphogenic response in Poa pratensis. Ethephon did not appear to predispose Poa pratensis to ethylene sensitivity, but the possibility of a contribution in growth regulation from some other component of ethephon degradation cannot be ruled out.



Digital Repository @ Iowa State University,

Copyright Owner

Kenneth Lynn Diesburg



Proquest ID


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136 pages