Effect of bacterial suspensions on vascular occlusion in stems of cut rose flowers

A suspension of Pseudomans aeruginosa at 5 × 10⁹ cfu/ml was either left untreated, pasteurized (15 min, 70°C) or autoclaved (15 min, 121°C). Stems of cut rose flowers ( Rosa hybrida L. cv. Sonia) placed in these solutions for 0·5–4·0 h showed decreased water uptake and a reduction of hydraulic conductance of the basal 5·0 cm stem segment. No difference was found between the treatments. When the pasteurized or autoclaved solutions were left at 4°C for 7 days, bacterial cells had autolyzed, and stems placed in these suspensions showed lower water uptake and hydraulic conductance than stems in freshly prepared solutions. The results show that living and non-living bacteria have the same effect on vascular occlusion, and indicate that hydraulic conductance was more reduced when the average size of the particles was smaller than that of bacterial cells. Vascular blockage occurred within 30 min after the start of treatment, apparently due to a physical effect rather than a physiological response of the stem tissue.