Inhibition of photosynthesis and export in geranium grown at two CO2 levels and infected with Xanthomonas campestris pv. Pelargonii

The effects of CO2 enrichment on growth of Xanthomonas campestris pv. pelargonii and the impact of infection on the photosynthesis and export of attached, intact, 'source' leaves of geranium (Pelargonium x domesticum, 'Scarlet Orbit Improved') are reported. Two experiments were performed, one with plants without flower buds, and another with plants which were flowering. Measurements were made on healthy and diseased leaves at the CO2 levels (35 Pa or 90 Pa) at which the plants were grown. There were no losses of chlorophyll, or any signs of visible chlorosis or necrosis due to infection. Lower numbers of bacteria were found in leaves at high CO2, suggesting growth at elevated CO2 created a less favourable condition in the leaf for bacterial growth. Although high CO2 lowered the bacterial number in infected leaves, reductions in photosynthesis and export were greater than at ambient CO2. The capacity of infected source leaves to export photoassimilates at rates observed in the controls was reduced in both light and darkness. In summary, the severity of infection on source leaf function by the bacteria was increased, rather than reduced by CO2 enrichment, underscoring the need for further assessment of plant diseases and bacterial virulence in plants growing under varying CO2 levels.