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Effects of Defoliating Pests on Soybean Canopy CO₂ Exchange and Reproductive Growth¹

Defoliation during reproductive growth of soybean [Glycine max (L.) Merr.] may significantly reduce yields. The objective of this study was to elucidate the relationships among organism-induced defoliation, CO₂ exchange rates, and reproductive growth in fieldgrown soybean. Soybean (‘Bragg’) plots were sprayed a factorial design with diflubenzuron (0.035 kg a.i./ha) and a benomyl-maneb mixture (1.12 a.i./ha) on two dates, first at early flowering (5 August) and again at early pod set (19 August). Diflubenzuron application prevented defoliation by velvetbean caterpillar (Anticarsia gemmatalis Hubner). By 12 September, LAI was reduced from 5.5 to 2.8, leaf dry weight from 185 to 110 g/m², and midday light interception from 94 to 72% in defoliated as compared to non-defoliated plots. While canopy specific leaf weight (SLW) increased in all treatments during the experimental period, SLW was 1.0 mg/cm² higher in defoliated than non-defoliated plots. On 4 September defoliated plots were sprayed with diflubenzuron (0.035 kg a.i./ha) and carbaryl (0.56 kg a.i./ha) prevent further leaf loss.

Canopy CO₂ exchange was measured at various photosynthetic photon flux densities (PPFD) to develop photosynthesis light response curves. Net canopy photosynthesis at 1,500 µE·m^–2·sec^–1 PPFD (PN1500) declined steadily in all treatments during seed growth. PN1500 and plant dark respiration, respectively, were reduced by 6.8 and 3.3 mg CO₂·dm^–2·hour^–1 in defoliated compared to non-defoliated plots. Soil CO₂ efflux rates did not differ significantly (P < 0.05) between treatments. Differences in pod growth rates (7.6 and 9.21 g·m^–2·day^–1 and yields (434 and 502 g·m^–2) between defoliated and non-defoliated plots, respectively, were attributed to differences in canopy photosynthetic capacity. There was no apparent effect of defoliation on seed growth duration or seed abortion. Rather, yield reduction in defoliated plots was related primarily to slowing of the individual seed growth rate. Balance of carbon flux to seeds showed relationships between instantaneous canopy CO₂ exchange measurements and sequential harvest data.

Key Words: Carbon balance • Pest management • Canopy photosynthesis light response • Anticarsia gemmatalis • Glycine max (L.) Merr.

² Visiting assistant research scientist of agricultural engineering, IFAS Gainesville, FL 32611; associate professor of entomology, AREC, Quincy, FL 32351; associate professor of agronomy, IFAS, Gainesville, FL 32611; professor of agricultural engineering, IFAS, Gainesville, FL 32611; and assistant professor of entomology, IFAS, Gainesvile, FL 32611, respectively.

Received for publication January 22, 1981.

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