![]() A microscopical investigation, however, did not show evidence for any material, be it tyloses, amorphous plugs or hydrophobic substances, in the xylem lumen.Īfter cutting air absorption occurs at the cut surface, due to the receding water columns in the xylem conduits opened by cutting. The blockage that is not microbial in origin could be due to processes in the stem, for instance as a response to the low water potential. However, in Cara mia, Madelon, and Sonia roses stomatal response, water loss, and water potential during dry storage, showed the same time course. The last cultivar apparently conserves water as its stomates often closed more rapidly, and its rate of cuticular transpiration was lower. In Cara mia roses, for example, it occurred within 3-4 h (at 20☌), in Madelon within 9-14 h, in Sonia after 24-36 h, and in Frisco after about 48 h of dry storage. Among rose cultivars great differences were found in the time-course of this blockage. When such stems are held dry an occlusion also develops. When the stems are not temporarily placed in water after harvest, however, they do not become contaminated with bacteria. The effects of dry storage can also be partially microbial in origin, as the number of bacteria associated with the cut surface and the xylem interior increase during dry storage. The blockage is probably mainly due to the obstruction of the passage of water at the pit membranes between the xylem conduits. It is concluded, therefore, that the occlusion is due to a purely physical effect of living bacteria with their extracellular polysaccharides, as well as dead bacteria and their degradation products. Isolated bacterial polysaccharides and proteins such as cellulase and the inert ovalbumin, both with a molecular mass of about 50 kilodaltons, also resulted in rapid blockage. Isolated living or dead bacteria both resulted in vascular blockage, when given either at room temperature or at 1☌ This indicates that physiological activity, either from the part of the bacteria or from the part of the stem, is not a prerequisite in the bacterial blockage. It was concluded, therefore, that the blockage occurring in the stems that are placed in water directly after harvest is caused by bacteria. When bacterial growth was excluded the blockage was absent. Fungi were also observed at the cut surface, but only after the occlusion had already occurred, and yeasts were not observed. The bacteria were always accompanied by extracellular polysaccharides. During vase-life a population of bacteria developed on the cut surface and inside the opened xylem conduits. These membranes contain small pores, through which the water flows from one conduit to the next. Slime on the colonies of bacteria isolated from the stems also stained with ruthenium red, indicating that the material found in some conduits might be bacterial polysaccharide that was able to pass the pit membranes between the conduits. This material stained with ruthenium red, a dye on polysaccharides. Light- and electron microscopy revealed no tyloses in the lumen of the xylem conduits (vessels and tracheids) and in only a few conduits a deposit of gummy material occurred in the absence of bacteria. Inhibition of the production or action of ethylene had no effect on the blockage. Alternatively, it could be due to microbial growth. as a result of a wound-reaction, in which ethylene is generally involved. ![]() When the stems are placed in water directly after harvest the blockage could be due to processes inherent in the stem, e.g. This study distinguishes between the occlusion which occurs when the stems are placed in water immediately after harvest, and the occlusion which occurs as a result of dry storage. Experiments in which a razor blade was introduced into the stem showed that more than 66% of all xylem conduits must become non-functional before a reduction in the rate of water uptake becomes apparent. These symptoms are due to an inability to take up adequate amounts of water from the vase solution, which in turn is due to an occlusion in the lower part of the stem. During vase-life premature signs of water stress occur, such as slow growth of the bud which often results in poor flower opening, wilting of both the flowers and the leaves, and bending of the stem just underneath the flower. The quality of cut rose flowers, a major horticultural crop in the Netherlands, is often unsatisfactory.
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