Browsing by Author "Sherwin, H"

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    Differences in rehydration of three desiccation-tolerant angiosperm species.
    (Oxford University Press, 1996) Sherwin, H; Farrant, J
    The rehydration characteristics of the desiccation-tolerant plantsCraterostigma wilmsii andMyrothamnus flabellifolia (homoiochlorophyllous) andXerophyta viscosa (poikilochlorophyllous) were studied to determine differences among them. A desiccation-sensitive plant (Pisum sativum) was used as a control. Recovery of water content, quantum efficiency (FV/FM), photosynthetic pigments and chloroplast ultrastructure as well as damage to the plasmamembrane were studied. P. sativum did not recover after desiccation and considerable damage occurred during rehydration. The desiccation-tolerant plants appeared to differ in their responses to dehydration and rehydration. The small herbaceousC. wilmsii generally showed little damage in the dry state and recovered faster than the other tolerant species.M. flabellifolia took longer to recover thanC. wilmsii probably due to the presence of a woody stem in which dehydration-induced xylem embolisms slowed the rate of recovery. The poikilochlorophyllous speciesX. viscosa took the longest to recover because it took longer to reconstitute the chloroplasts and the photosynthetic pigments. Quantum efficiency recovered in all species before water content and chlorophyll content recovered to control levels. The significance of these different responses to desiccation and recovery from desiccation is discussed.
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    The effect of drying rate on the survival of three desiccation-tolerant angiosperm species.
    (Oxford University Press, 1999) Farrant, J; Cooper, K; Kruger, L; Sherwin, H
    The effect of drying rate on the survival of three angiosperm resurrection plants, Craterostigma wilmsii (homoiochlorophyllous), Xerophyta humilis (poikilochlorophyllous) and Myrothamnus flabellifolius (homoiochlorophyllous) was examined. All species survived slow drying, but only C. wilmsii was able to survive rapid drying. C. wilmsii was rapidly able to induce protection mechanisms such as folding of cell walls to prevent mechanical stress and curling of leaves to minimize light stress, and thus survived fast drying. Rapid drying of X. humilis andM. flabellifolius appeared to allow insufficient time for complete induction of protection mechanisms. In X. humilis, there was incomplete replacement of water in vacuoles, the photosynthetic apparatus was not dismantled, plasma membrane disruption occurred and quantum efficiency of photosystem II (FV/FM) did not recover on rehydration. Rapidly dried leaves of M. flabellifolius did not fold tightly against the stem and FV/FMdid not recover. Ultrastructural studies showed that subcellular damage incurred during drying was exacerbated on rehydration. The three species co-occur in environments in which they experience high desiccation pressures. C. wilmsii has few features to retard water loss and thus the ability for rapid induction of subcellular protection is vital to survival. X. humilis and M. flabellifolius are able to retard water loss and protection is acquired relatively slowly. Copyright 1999 Annals of Botany Company.
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    Protection mechanisms against excess light in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa.
    (Springer, 1998) Sherwin, H; Farrant, J
    Mechanisms of avoidance and protection against light damage were studied in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa. In C. wilmsii, a combination of both physical and chemical changes appeared to afford protection against free radical damage. During dehydration leaves curled inwards, and the abaxial surface became exposed to light. The tissue became purple/brown in colour, this coinciding with a three-fold increase in anthocyanin content and a 30% decline in chlorophyll content. Thus light-chlorophyll interactions are progressively reduced as chlorophyll became masked by anthocyanins in abaxial layers and shaded in the adaxial layers. Ascorbate peroxidase (AP) activity increased during this process but declined when the leaf was desiccated (5% RWC). During rehydration leaves uncurled and the potential for normal light-chlorophyll interaction was possible before full hydration had occurred. Superoxide dismutase (SOD) and glutathione reductase (GR) activities increased markedly during this stage, possibly affording free radical protection until full hydration and metabolic recovery had occurred. In contrast, the leaves of X. viscosa did not curl, but light-chlorophyll interactions were minimised by the loss of chlorophyll and dismantling of thylakoid membranes. During dehydration, free radical protection was afforded by a four-fold increase in anthocyanin content and increased activities of AP, GR and SOD. These declined during rehydration. It is suggested that potential free radical damage may be avoided by the persistence of anthocyanins during the period of thylakoid membrane re-assembly and full chlorophyll restitution which only occurred once the leaves were fully rehydrated.
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    Use of metabolic inhibitors to elucidate mechanisms of recovery from desiccation stress in the resurrection plant Xerophyta humilis.
    (Springer, 1998) Dace, H; Sherwin, H; Illing, N; Farrant, J
    Xerophyta humilis (Bak.) Dur. and Schinz is a poikilochlorophyllous resurrection plant in that it is tolerant of considerable water loss (< 5% relative water content [RWC]) and thylakoid membranes are dismantled and chlorophyll is lost during dehydration. In this paper we examined the processes associated with recovery from desiccation upon rehydration. Dried leaf explants were rehydrated in water (control) or in solutions of actinomycin-D or cyclohexamide in order to determine to what extent initial recovery was dependant on de novo transcription and translation respectively. Our results suggest that considerable protection of subcellular organisation and components of metabolism occurs during drying such that the initial recovery of metabolism on rehydration is virtually independent of de novo transcription of nuclear genes. However recovery does require the synthesis of new proteins. The plasmalemma remains intact and macromolecular synthesis is not required for maintenance of its integrity. Messenger RNA's for chlorophyll biosynthesis appear to be stored in a stable form in the dried leaves and are translated on rehydration. Similarly most of the mRNA's necessary for recovery of electron transport in the chloroplast (as determined by measuring the quantum efficiency of photosystem II [FV/FM] using chlorophyll fluorescence) appear to be stabily present in the dried leaves. However, for total recovery of FV/FM new genomic transcription is necessary.