Mechanisms of desiccation tolerance in cryptogams.
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Date
2002
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Abstract
In this study adaptations of bryophytes and lichens to desiccation stress were
examined. The aim was to test whether desiccation tolerance in the selected species is
constitutive or if desiccation tolerance could be induced by various hardening
treatments. In addition, some putative tolerance mechanisms were investigated,
including the accumulation of sugars, increase in ROS scavenging systems and other
mechanisms e.g. energy dissipating processes. To determine if hardening treatments
prior to desiccation stress increased desiccation tolerance, mosses and lichens were
partially desiccated or treated with ABA. The effect of hardening treatments on the
physiology of the moss Atrichum androgynum and lichens Peltigera polydactyla, Ramalina celastri and Telochistes capensis during a desiccation-rehydration cycle was investigated. Photosynthesis, respiration and chlorophyll fluorescence
measurements were used as rapid tools to determine the metabolic activities in these
lichens and moss species. In A. androgynum partial desiccation following slow drying
at 52% RH increased the rate of recovery of net photosynthesis. Net photosynthesis
recovered almost completely following slow drying in the material that was partially
dehydrated and/or treated with ABA. This suggests that partial dehydration hardens the
moss, and that ABA can fully substitute for partial dehydration. In R celastri and P.
polydactyla both partial dehydration and ABA treatments displayed some
improvement in desiccation tolerance depending on the duration and severity of
stress. The reduction in the re-saturation respiration burst in P. polydactyla, although
not quite significant, strongly suggests that hardening increases mycobiont tolerance.
However, it is more difficult to establish whether the hardening treatments improve
photobiont performance.
In the moss A. androgynum ABA treatment increased the rate of recovery of
photosynthesis and PSII activity, and also doubled non-photochemical quenching
(NPQ). Increased NPQ activity will reduce ROS formation, and may explain in part
how ABA hardens the moss to desiccation. In ABA treated, but not untreated mosses,
desiccation significantly increased the concentration of soluble sugars in A.
androgynum. Sugar accumulation may promote vitrification of the cytoplasm and
protect membranes during desiccation. Starch concentrations in freshly collected
A. androgynum and R. celastri were only c. 40 and 80 mg g ¯¹ dry mass respectively, and slightly rose during desiccation, but were only slightly affected by ABA
pretreatment. ABA did not reduce chlorophyll breakdown during desiccation. In P.
polydactyla ABA pretreatment had little effect on any of these parameters.
Changes in the activities of the free radical scavenging enzymes ascorbate
peroxidase, catalase and superoxide dismutase were measured during wetting and
drying cycles in the moss A. androgynum and in the lichens P. polydactyla, R. celastri
and T capensis. These species normally grow in the understorey of the Afromontane
forest, moist, xeric, and extremely xeric miicrohabitats respectively. In A.
androgyum, enzyme activity was measured shortly after collection, after 3 d storage
following hardening by partial dehydration and/or 1 h treatment with ABA or distilled
water and during desiccation and rehydration. In A. androgynum enzyme activities of
CAT and SOD in untreated material were always higher than in the hardened
treatments, while both partial dehydration and ABA treatments tended to reduce both
CAT and the induction of SOD activity, although these effects were not significant
between the treatments. This suggests that ABA may not be involved in the induction
of free radical scavenging enzymes and probably these enzymes are not important in
desiccation tolerance of A. androgynum. In lichens, the enzyme activity was measured
shortly after collection, after hydration for 48 hat 100% RH, after desiccation for 14 d and 28 d, and during the first 30 min of hydration with liquid water. Enzyme activities
tended to rise or stay the same following rehydration in all the species tested. After
desiccation for 14 d, enzyme activities decreased, and then decreased further to very
low values after 4 weeks desiccation. In all species, including T capensis from an
extremely xeric habitat, the activities of all enzymes remained at very low values
during the 30 min following rehydration, and were therefore unavailable to remove
any reactive oxygen species accumulating in lichen tissues as a result of desiccation.
Results suggests that the enzymic antioxidants are more likely to be involved in
removing reactive oxygen species produced during the normal metabolic processes of
lichens than having a role in desiccation tolerance.
The Afromontane understorey moss Atrichum androgynum displayed an
oxidative burst of H₂O₂ during rehydration following desiccation. Maximum rates of
H₂O₂ production occur during the first 15 min of rehydration. While the production of
H₂O₂ increases with increasing desiccation times, the moss produced significant amounts of H₂O₂ during rehydration after desiccation for times that did not inhibit
photosynthesis or cause K⁺ leakage. A. androgynum may produce more H₂O₂ during
desiccation than rehydration, because desiccation artificially induced using
polyethylene glycol strongly stimulates production. Experiments involving inhibitors
and exogenously supplied reductants indicate that peroxidases are responsible for the
synthesis of H₂O₂. Factors that influence the rate of H₂O₂ production during
rehydration include light and the hormone ABA. Patterns of H₂O₂ production are discussed in terms of their possible role as a defence against pathogenic fungi and bacteria.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2002.
Keywords
Cryptogams., Bryophytes., Plants, Effect of environment on., Plants, Effect of stress on., Theses--Botany.