Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

David J. Boethel


A series of greenhouse experiments was conducted to investigate the effects of insect-induced defoliation on growth and N$\sb2$-fixing ability of soybean. This research had three major objectives: (1) to examine the effects of different levels of defoliation, (2) to determine the impact of progressive defoliation, and (3) to study the ability of soybeans to recover following defoliation. All experiments were conducted in a similar manner. Larvae of the soybean looper, Pseudoplusia includens (Walker), were used to defoliate soybeans in the vegetative stage of development and the acetylene-ethylene reduction assay was used to provide an indirect measure of N$\sb2$-fixing ability of defoliated plants relative to control plants. When soybeans were subjected to varying levels of defoliation (0 to 85%), N$\sb2$-fixing ability and total nodule dry weight decreased linearly as defoliation increased. Nodule specific activity (NSA), a measure of N$\sb2$-fixing ability per gram of dry nodule weight, was not affected by levels of defoliation less than 47%, but decreased sharply as defoliation exceeded ca. 47%. Stem and root dry weight also decreased linearly as defoliation increased. In soybeans that were sampled during progressive defoliation, NSA was significantly reduced as early as 10 days post-infestation (DPI) at which time defoliation had reached 37%. This decrease in NSA resulted in a significant decrease in N$\sb2$-fixing ability. By 14 and 16 DPI (62 & 73% defoliation), there was a significant decrease in total nodule dry weight which exacerbated the continued decrease in NSA, causing even greater loss of N$\sb2$-fixing ability. Soybeans that were defoliated ca. 70% were able to recover their N$\sb2$-fixing ability as early as three weeks post-defoliation (WPD). Recovery of N$\sb2$-fixing ability was due to full recovery of NSA and partial recovery of total nodule dry weight. Leaf area, and stem and root dry weight of defoliated plants were significantly less than those of controls for as long as 5 WPD, at which time the experiment was terminated. Therefore, defoliated plants recovered their N$\sb2$-fixing ability despite prolonged effects of defoliation on plant growth.