The infiltration ability of micro‐organisms Bacillus, Fusarium, Kluyveromyces and Pseudomonas spp. into xylem vessels of Gerbera cv. ‘Fleur’and Rosa cv. ‘Sonia’cut flowers: a scanning electron microscope study

Abstract

Scanning electron microscope (SEM) observations have been made of transverse and longitudinal sections of xylem vessels of cut flowers of Gerbera cv. ‘Fleur’and cut Rosa cv. ‘Sonia’after a maximum of 24 h vase life. Vase waters used were: sterile tap water; sterile tap water plus suspensions of single pure cultures of Bacillus polymyxa, B. subtilis, Fusarium oxysporum microspores, segments of young mycelium of F. oxysporum, Kluyveromyces marxianus, or Pseudomonas putida, up to a maximum of 106 and 5×107 cells/ml of vase water.The results of the SEM observations showed that only a small fraction of the microbial cells entered into the vascular system with the normal intake of vase water; most microbial cells remained attached to the submerged cut surface while a small fraction of the initially attached microbes were sometimes liberated into the surrounding vase water. The SEM figures give the impression that adhesion on to the cut stem surfaces of the bacterial cells (Bacillus and Pseudomonas spp.) by their capsular materials may have occurred, although the cut surface possibly acted as a filter, trapping cells because of the upward pressure of the transpiration stream into the vessels. Few attached yeast cells were found, but Fusarium microconidia formed cell clusters on the cut surface, partially blocking the xylem vessel entrance; Fusarium mycelium covered the cut‐surfaces as another filter layer through which only small mycelial particles could enter the vascular system.The extent of infiltration of microbial cells into the xylem vessel system depended upon the number of microbial cells/ml of vase water (up to a certain maximal number) and on the shape and size of the individual cells and the width of the xylem vessels. The end of the vase life of Gerbera and Rosa flowers was marked by an extensive water stress in the xylem vessels so that the water intake necessary for a normal flower development was insufficient and wilting ensued. The water deficit may have been enhanced by secondary biochemical and physiological interactions between the microbes and the plant tissues, which may also have affected the infiltration of micro‐organisms.