We have investigated photosynthetic changes of fully expanded pea leaves infected systemically by pea enation mosaic virus (PEMV) that often attacks legumes particularly in northern temperate regions. A typical compatible virus-host interaction was monitored during 40 post-inoculation days (dpi). An initial PEMV-induced decrease in photosynthetic CO(2) assimilation was detected at 15 dpi, when the virus appeared in the measured leaves. This decrease was not induced by stomata closure and corresponded with a decrease in the efficiency of photosystem II photochemistry (Φ(PSII)). Despite of a slight impairment of oxygen evolution at this stage, PSII function was not primarily responsible for the decrease in Φ(PSII). Chlorophyll fluorescence imaging revealed that Φ(PSII) started to decrease from the leaf tip to the base. More pronounced symptoms of PEMV disease appeared at later stages, when a typical mosaic and enations appeared in the infected leaves and oxidative damage of cell membranes was detected. From 30 dpi, a degradation of photosynthetic pigments accelerated, stomata were closing and corresponding pronounced decline in CO(2) assimilation was observed. A concomitant photoprotective responses, i.e. an increase in non-photochemical quenching and accumulation of de-epoxidized xanthophylls, were also detected. Interestingly, alternative electron sinks in chloroplasts were not stimulated by PEMV infection, which is in contradiction to earlier reports dealing with virus-induced plant stresses. The presented results show that the PEMV-induced alterations in mature pea leaves accelerated leaf senescence during which a decrease in Φ(PSII) took place in coordinated manner with an inhibition of CO(2) assimilation.
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