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 CO 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.