Factors affecting the vase life of Rosa cultivar "Sonia" : microbiological and scanning electron microscopic investigations = Factoren die van invloed zijn op het vaasleven van Rosa cultivar "Sonia" : microbiologisch en raster electronenmicroscopisch onderzoek

The papers compiled in this thesis comprise a series of successively executed investigations into the role of micro-organisms in xylem plugging, and disturbance of the water relations and the vase life of cut flowers. For this purpose Rosahybrida cultivar 'Sonia' (the hybrid tea-rose Rosa cultivar 'Sweet Promise') was selected.

Chapter 1 comprises an introduction into the economic importance of cut flowers and the capability of cut flowers for post harvest life. In addition, a brief review is given of literature on physiological and microbiological factors which can influence the vase life of cut flowers. Gaps in our knowledge of the mechanisms leading to vascular blockage of cut flowers are discussed. An outline is given of: (1) the selection of a greenhouse Rosa cultivar as plant material to study, (2) the experimental methods applied, (3) the results of preliminary investigations, and (4) the microbiological factors investigated.

Chapter 2 shows that stems of freshly cut flowers contain a wide variety and low numbers of microbial species. The initial microbial load on stems of Rosa flowers was found to be much lower than those on Chrysanthemum and Gerbera stems. Their distribution on the flower stem is not homogeneous.

The stem flora, predominantly Enterobacter , Bacillus and fungal species, lost its dominance in the vase fluid. The vase water showed an initial predominance of Pseudomonas species which do not require organic growth factors. In the course of the vase life Enterobacter and Bacillus species became dominant. These bacterial genera require organic nutrients and special growth factors for their multiplication.

Fungal growth was shown at a later stage, mainly in vase fluids of Chrysanthemum and Gerbera flowers, which have a much longer vase life and a higher initial number of fungi on their stems than Rosa flower stems.

Chapters 3, 4 and 5 demonstrate that the extent of infiltration of viable microbial cells into the xylem vessel system of Rosa and Gerbera cut flowers depends upon the number of microbial cells per ml initially added to the vase fluid, the shape and the size of the individual microbial cells and the width of the xylem vessels.

Scanning Electron Microscopic (SEM) observations as well as the assessment of the number of bacteria Infiltrated into Rosa xylem vessels showed in addition that: (1) the number of bacteria which infiltrated into xylem vessels increased with time and, (2) this number increased with increasing numbers of bacteria initially added to the vase fluid, (3) this number decreased with increasing distance between cutting point and sampling point, (4) only a minor part of the bacterial and fungal cells suspended in the vase fluid was able to infiltrate into the xylem vessels of the flowers, a major part of the microbial cells remained attached to the cut surface, (5) even low numbers of infiltrated microbial cells caused a significantly decreased conductivity of stem segments, (6) a similar water conductivity decreasing phenomenon of stem segments was observed when low numbers of heat- inactivated microbial cells or low concentrations of microbial EPS (exopolysaccharides) were added to the vase fluid, as shown in Chapters 5 and 6.

Chapter 7 does show that purified microbial pectic enzymes added to the vase fluid of Rosa flowers cause a rapidly decreasing uptake of vase water, promoting desiccation of the flower petals and leaves. This phenomenon may have been due to enzymatic degradation of the pectin-linked structures of the xylem vessels as observed in SEM preparations. The SEM figures show: (1) degradation of the xylem vessel wall structure, (2) loose particles in the vessels, (3) released spiral vessels, and (4) injury of vessel pits.

The consequences for the ornamental value of the roses of vessel plugging and flower desiccation were easy to assess by means of macroscopic observations. Since pectic enzymes in extremely low concentrations exert a dramatic effect on cut flowers, it is likely that some of the products of the enzymatic activity elicit specific responses in the flowers, further affecting their vase life.

Chapter 8 covers the results of an electron microscopic study, using cryoSEM techniques and X-ray microanalysis to demonstrate the effects of aluminium sulphate added to the vase fluid of Rosa flowers on the ability of bacteria e.g. Bacillus subtilis cells to infiltrate into the xylem vessels of the Rosa flowers. The biocidal activity of aluminium sulphate added to the vase fluid was negligible. Aluminium sulphate probably acts as a flocculating agent, in which bacterial cells are embedded. Indeed, Al ³⁺ions react easily with inorganic and organic anionic substances e.g. Bacillus subtilis cell wall peptides, substances of wounded stem cells and stem cell wall compounds of cut Rosa flowers. The Bacillus /aluminium complexes contribute to the formation of flocky deposits which adhere onto the cut surface and the adjacent open xylem vessel wall of the roses. The flocculated material may form a filter bed at the cut surface of the flower stem, hardly decreasing the water uptake, but strongly obstructing the embedded Bacillus calls to enter the vascular system of the roses, as clearly shown by cryoSEM figures.