Photosynthesis and export during steady-state photosynthesis in bean leaves infested with the bacterium Xanthomonas campestris pv phaseoli

Export and distribution of C-14 assimilates during a period of steady-state photosynthesis were studied in attached leaflets of the second trifoliate of Phaseolus vulgaris for 2 weeks following inoculation with Xanthomonas campestris pv. phaseoli. The control was the leaflet opposite the inoculated leaflet at the same node. On either a chlorophyll or leaf area basis, photosynthesis and export declined over time after inoculation compared with control tissue. By 14 days after inoculation, photosynthesis and export were 25 and 50% lower, respectively, in the infected leaflets, even though only 7-10% of the total area measured was necrotic. The reduced synthesis of sucrose was sufficient to account for the reduced export from the infected leaflets. Starch synthesis was reduced to a greater extent in infected leaflets than was sucrose synthesis. In leaf disks infiltrated with mannose or xanthan, an extracelluar bacterial polysaccharide containing mannose, photosynthesis was reduced by 20% and the partitioning of C-14 photoassimilates into sucrose was reduced but was slightly increased into starch. This is consistent with an effect of xanthan at least in vitro on the triose phosphate/phosphate exchange at the chloroplast envelope.