Impact of Phakopsora pachyrhizi Infection on Soybean Leaf Photosynthesis and Radiation Absorption

Yield loss in soybean [Glycine max (L.) Merr] due to soybean rust (SBR) (caused by Phakospora pachyrhizi Syd. & P. Syd.) has been associated with reduced radiation interception and reduced radiation use efficiency by green leaf area. 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 (i) absorption of photosynthetic photon flux density (PPFD) and (ii) carbon exchange rate (CER) per unit absorbed PPFD of soybean leaves. A controlled-environment study was conducted using a split-plot design with three replications over time, with two disease levels as main plots and two soybean cultivars as subplots. The two disease levels were a disease-free control and inoculation with P. pachyrhizi. 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 68% 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 11 (PSII) photochemistry and damage to PSII reaction centers. Therefore, visual indicators such as light absorptance and disease severity do not reflect the degree of impact of SBR on leaf photosynthesis.