Impact of Phakopsora Pachyrhizi Infection On Soybean Leaf Photosynthesis and Radiation Absorption.

Yield loss in soybean due to soybean rust (SBR) has been associated with reduced radiation interception and reduced radiation use efficiency by green leaf area. The effects of the disease on radiation absorption by the leaf were not measured simultaneously with photosynthetic rates to distinguish between the reduction in radiation absorption and the reduction in photochemistry on the photosynthetic efficiency of diseased leaves. The objective of this study was to determine the mechanisms involved in the SBR-induced reductions in radiation use efficiency by quantifying the effect of SBR on absorption of photosynthetic photon flux density (PPFD), carbon exchange rate (CER) per unit absorbed PPFD, and light and dark-adapted chlorophyll fluorescence in healthy and SBR-infected leaves. A controlled-environment study was conducted using a split-plot design with three replications over time, with main plots two inoculum rate (high versus zero) and subplots two commercial soybean cultivars. Measurements included leaf CER, PPFD absorptance, and chlorophyll fluorescence. As disease severity increased, there was a small linear decline in leaf absorptance and a negative exponential decline in leaf CER. At 50% disease severity, absorptance was reduced by 10% and CER was reduced by 65% compared with the disease-free control. The relative decline in CER was similar at low and saturating PPFD levels. Soybean rust–induced reductions in CER were mainly associated with lower efficiency of photosystem II (PSII) photochemistry and damage to PSII reaction centers. Therefore, visual indicators such as light absorptance and disease severity don’t reflect the degree of impact of SBR on leaf photosynthesis.