plant surface microbiology.pdf

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Lukas Schreiber, Ursula Krimm and Daniel Knoll
thomonas and Bacillus (Ercolani 1991; Morris et al. 1998). Cladosporium,
Alternaria and Aureobasidium have been described as being abundant filamentous
fungal species and Cryptococcus and Sporobolomyces were
described as abundant yeast species in the phyllosphere (Andrews and Harris
2000; Blakeman 1993).
However, it must be mentioned that the description of the epiphyllic
microflora up to now is exclusively based on an identification of the species
after cultivation on standard media. However, it is well known today from
environmental microbiology that many bacterial species cannot be cultivated
with standard techniques (Amann et al. 1995). PCR-based approaches
showed that the bacterial species composition of aquatic environments, but
also of soil and rhizosphere communities, is much more complex and
diverse as it was originally concluded from cultivation-based approaches
(Marilley et al. 1998; Tiedje et al. 1999; Ogram 2000). A similar approach has
rarely been carried out in the leaf surface and it is absolutely necessary in
leaf surface microbiology in future in order to obtain a more realistic and
complete picture of the species composition in the phyllosphere. Results of
one of the first approaches, comparing the bacterial species identified using
PCR versus cultivation-based techniques (Yang et al. 2001), in fact, yielded
two quite different pictures of the species composition of the phyllosphere.
This proves that our knowledge of the species composition on leaf surfaces
obtained from cultivation-based techniques is still rather limited and needs
further research.
4 Alteration of Leaf Surface Wetting
Investigations of the seasonal development of leaf surface wetting have shown
several times that leaf surfaces become more and more wettable with increasing
leaf age (Cape 1983; Turunen and Huttunen 1989; Cape and Percy 1993;
Neinhuis and Barthlott 1998). This was normally attributed to chemical
changes in the physico-chemical properties of the waxy leaf surface at the
leaf/atmosphere interface caused by environmental pollution. In addition, it
was shown that wax erosion due to the constant exposure of the leaf surface to
wind, rain and the deposition of dust particles from the atmosphere to the leaf
surface also occurs (van Gardingen et al. 1991), and may be further contributed
to these observed increases in wetting. However, epiphyllic microorganisms
as a further parameter contributing to an increased wetting of the
leaf surface may not be neglected here.
In simple model experiments, silanised glass surfaces, which are rarely wetted
by water due to their high hydrophobicity, were colonised by bacteria and
wetting properties were quantified by measuring contact angles (Knoll and
Schreiber 1998, 2000). From these experiments, it became obvious that already
at a coverage of 10 % of the total surface, contact angles decreased by 25°