Analysis of the virus-induced inhibition of photosynthesis in malmsey grapevines

Virus infections decrease photosynthesis in plants, but the mechanistic basis is poorly understood. This was analysed in Banyalbufar malmsey, a grapevine (Vitis vinifera) variety of Mallorca (Spain). The aim of this study was to analyse the mechanisms by which virus infection affect photosynthesis. Gas exchange (limitation analysis), chlorophyll fluorescence and Rubisco activity were compared in potted virus-infected and virus-free potted plants, and in field-grown young lowly infected and older highly infected plants. Virus infection resulted in decreased photosynthesis (c. 50%). Stomatal limitation was unaffected in virus-infected plants, demonstrating that stomatal closure was not causing photosynthesis decreases. Chlorophyll fluorescence and limitation analysis suggested that the inhibition of primary light reactions was only a minor effect of virus infection. By contrast, mesophyll conductance to CO2 and Rubisco activity substantially decreased in virus-infected plants, corresponding to increases in the limitations to photosynthesis imposed by mesophyll conductance and carboxylation. It is concluded that decreases in carboxylation and, possibly, in mesophyll conductance are the primary mechanisms by which virus infection impairs photosynthesis in Banyalbufar malmsey.