EFFECT OF RYEGRASS MOSAIC-VIRUS ON CARBON ASSIMILATION AND GROWTH OF RYEGRASSES

The growth of simulated swards of Italian [Lolium multiflorum] and perennial [L. perenne] ryegrass, artificially infected with ryegrass mosaic virus (RMV) or RMV-free, was investigated. During the build up of RMV infection, dry matter yield was usually decreased in infected swards, and the effect was confirmed when single vegetative regrowths of swards were investigated in detail. The primary cause of lower yields in RMV-infected swards apparently is a decrease in net canopy photosynthesis (maximum decrease after 50%) and as associated increase in dark respiration (maximum increase about 50%). The decrease in net canopy photosynthesis is due primarily to a lowered rate of net photosynthesis of constituent leaves within the canopy. A secondary effect of RMV infection is to decrease tillering (maximum decrease about 30%), which results in a change in canopy structure and, in particular a lower leaf area index. Lower light utilization by the swards is less important than the decrease in leaf photosynthetic efficiency in lowering yield.