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

Abstract

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.

Tolerância da soja ao desfolhamento afetada pela redução do espaçamento entre fileiras

“…Isto resulta em menor fonte de fotoassimilados, limitando o desenvolvimento das estruturas reprodutivas (demanda). Conforme relatado por INGRAM et al (1981), o efeito do desfolhamento sobre o rendimento de grãos demonstrou ter sido causado pela redução da interceptação da luz (IL) e fotossíntese do dossel. Outro fator que pode ter contribuído para a queda do rendimento foi a insuficiência de reservas que as plantas possuíam nos caules, ramos e pecíolos, para suprir a demanda das estruturas reprodutivas, mesmo permanecendo parte da área foliar como aconteceu no nível de 67% de desfolhamento.…”

The effect of elevated CO2 and grazing by Gastrophysa viridula on the physiology and regrowth of Rumex obtusifolius

“…Responses of this kind have been observed in grasses, Oesterheld (1992) showing that allocation towards the restoration of basic allometric ratios was a major priority regardless of the intensity of defoliation, Herbivory caused a significant decrease in SLA, Impacts of herbivory on SLA have not been covered to any degree in the literature. Both increases and decreases in response to insect feeding have been reported, but only two studies have examined the efTect of defoliators and there was no consensus (Bassman & Dickmann, 1982;Schmid et al, 1988), A decrease in SLA might occur as a result of selective feeding (Ingram et al, 1981), Gastrophysa preferentially eat the area of the leaf between the ribs, and this might cause a decrease in SLA, This does not, however, preclude the possibility that physiological responses to herbivory might also affect SLA. In an examination of the responses of Solidago altissima to herbivory caused by insects from three different feeding guilds, Meyer (1993) found that both xylem-feeding spittlebugs and leaf-chewing beetles reduced SLA, but the greater efTect was caused by the spittlebug which unlike the beetle did not cause a direct loss of leaf area.…”

Pontas de pulverização e horários de aplicação no controle químico de ferrugem asiática da soja

“…É possível que a ocorrência tardia da doença e o baixo potencial de inóculo tenham contribuído para tal fato. Ingram et al (1981) relatam que o efeito da desfolha sobre a peso de grãos é causado pela redução da interceptação da luz e da fotossíntese. A tolerância da soja á desfolha é dependente de vários fatores como o nível de desfolha, o estádio fenológico na época da desfolha e fatores ambientais (PEDIGO; HUTCHINS; HIGLEY, 1986;PARCIANELLO et al, 2004).…”