Geoscience Reference
In-Depth Information
thought to provide a long-term protection against stress dam-
age. According to Bohnert and Shen (1999), a nearly universal
reaction under stress conditions, including WD, is the accumu-
lation of 'compatible solutes', many of which are osmolytes (i.e.
metabolites whose high cellular concentration increases the
osmotic potential significantly) considered to lead to osmotic
adjustment. These observations indicate that 'compatible sol-
utes' may have other functions as well, namely in the protection
of enzyme and membrane structure and in scavenging of radi-
cal oxygen species.
Desiccation induces a zeaxanthin + anteraxanthin-mediated
photo-protective mechanism in desiccation-intolerant
Frullania
dilatata
(Deltoro et al. 1998). They propose that when CO
2
fixa-
tion and, therefore, ATP consumption are decreased at low rela-
tive water content (RWC), the functioning electron flow gives
rise to an acidification of the thylakoid lumen that induces
zeaxanthin and anteratxanthin synthesis. It has been proposed
that the photo-protective process results in the diversion of
energy away from the reaction centre (Ruban and Horton 1995;
Medrano et al. 2002). There are, however, experimental data
which do not support the statement that the xanthophyll cycle
plays a major or specific role in the direct energy dissipation
of absorbed light energy (Schindler and Lichtenthaler 1994).
According to Tambussi et al. (2002), the non-photochemical
fluorescence quenching (qN), as well as the content of zeaxan-
thin and anteraxanthin after moderate water stress, increased
significantly. However, at severe water stress a further rise in
these xanthophylls was not associated with any increase in
qN. In addition, the β-carotene content rose significantly dur-
ing severe WD, suggesting an increase in antioxidant defence.
One tentative scheme of photosynthetic control under drought
is proposed by Medrano et al. (2002, Figure 14.5).
Acclimatisation
of plants
exposed to cold
temperatures
The ability to tolerate freezing temperatures under natural
conditions varies greatly among tissues. Seeds and other par-
tially dehydrated tissues, as well as fungal spores, can be kept
indefinitely at temperatures near absolute zero (0 K or −273°C),
indicating that these very low temperatures are not intrinsically
harmful. Hydrated, vegetative cells can also retain viability at
freezing temperatures, provided that ice crystal formation can
be restricted to the intercellular spaces and cellular dehydration
is not too extreme.
Temperate plants have the capacity for
cold acclimation
—a
process whereby exposure to low but nonlethal temperatures (typ-
ically above freezing) increases the capacity for low-temperature
