untitled

and more variable pattern was found in Metarhizium,
but mannitol was again one of the main compatible
solutes in the spores, supplemented by erythritol and
arabitol as the solute concentration of the medium
increased.
A different type of adaptation to water stress is
found among the fungi that commonly grow as sapro-
trophs on the surfaces of living or senescing plant leaves

• the environment termed the phyllosphere(Fig. 8.13).
  These fungi (Cladosporium, Alternaria, Sydowia, etc.) have
  darkly pigmented (melanized) hyphae and spores.
  They do not grow at low (negative) water potentials but
  they have a remarkable ability to withstand periodic
  wetting and drying, which few other fungi can tolerate.
  Park (1982) investigated this by growing these fungi
  on sheets of transparent cellulose film (Cellophane)
  placed on top of malt extract agar plates. Then he
  removed the pieces of film bearing the fungal colonies
  and suspended them over saturated solutions of
  NaNO 2 or KNO 3 in closed containers. These solutions
  generate equilibrium relative humidities of 66% and
  45%, respectively, equivalent to about −70 MPa and
  −95 MPa. Even after 2 or 3 weeks in these severe
  drought conditions, the fungi started to regrow within
  an hour when the Cellophane films were returned to
  the agar plates, and they did so from the original
  hyphal tips. By contrast, a range of common soil fungi
  (e.g. Fusarium, Trichoderma, Gliocladium) or typical

food-spoilage fungi (Penicillium spp.) never regrew

from their original hyphal tips, although many of

them could regrow after 24 hours, from spores or sur-

viving hyphal compartments behind the tips. Clearly,

the phyllosphere fungi are naturally and specially

adapted to the fluctuating moisture conditions in

their normal habitat. They are, of course, the same fungi

that grow as sooty moulds on kitchen and bathroom

walls, where they experience the same wide fluctuations

in moisture levels.

Light

Light in the near-ultraviolet (NUV) and visible parts of

the spectrum (from about 380 to 720 nm) has relatively

little effect on vegetative growth of fungi, although

it can stimulate pigmentation. In particular, blue

light induces the production of carotenoid pigments

in hyphae and spores of several fungi, including

Neurospora crassa. These carotenoids, which also occur

in algae and bacteria, are known to quench reactive

oxygen species, discussed earlier. The pigments serve

to minimize photo-induced damage. Melanins similarly

protect cells against reactive oxygen species and ultra-

violet radiation.

Light has a much more profound effect on fungal

differentiation, acting as a trigger for the production

ENVIRONMENTAL CONDITIONS 155

Fig. 8.12Changes in the compatible solutes
of the conidia of two insect-pathogenic
fungi (Beauveria bassianaand Metarhizium
anisopliae). Conidia were harvested from
agar plates that contained increasing levels
of glucose or trehalose. The compatible
solutes found in the conidia were: mannitol
(white squares), erythritol or arabitol (black
circles), and trehalose (black squares).
(Adapted from Hallsworth & Magan 1994.)