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 t rifoliate of Phaseolus vulgaris for 2 weeks following inoculation with Xanthomonas campestris pv. phaseoli. The control was the leaflet oppo site 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 inoculatio n, photosynthesis and export were 25 and 50% lower, respectively, in t he infected leaflets, even though only 7-10% of the total area measure d was necrotic. The reduced synthesis of sucrose was sufficient to acc ount for the reduced export from the infected leaflets. Starch synthes is was reduced to a greater extent in infected leaflets than was sucro se synthesis. In leaf disks infiltrated with mannose or xanthan, an ex tracelluar 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 c onsistent with an effect of xanthan at least in vitro on the triose ph osphate/phosphate exchange at the chloroplast envelope.