The physiology of poikilohydric plants
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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 and M. 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 (F V /F M ) did not recover on rehydration. Rapidly dried leaves of M. flabellifolius did not fold tightly against the stem and F V /F M did 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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To understand mechanisms of osmoprotection, the composition of sugars and related compounds were analyzed in extracts of fully hydrated and desiccated leaves of the desiccation‐tolerant resurrection plant Myrothamnus flabellifolia . During the dehydration process the concentrations of fructose and glucose decrease, whereas sucrose, arbutin and glucopyranosyl‐β‐glycerol increase. The substances were identified by GC‐MS and NMR‐analyses. This is the first report of large amounts of glucopyranosyl‐β‐glycerol in higher plants which may act as an osmoprotectant. Significant levels of the nonreducing sugar trehalose were present in all samples tested.
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Myrothamnus flabellifolius Welw. is a desiccation-tolerant ('resurrection') plant with a woody stem. Xylem vessels are narrow (14 μm mean diameter) and perforation plates are reticulate. This leads to specific and leaf specific hydraulic conductivities that are amongst the lowest recorded for angiosperms (k s 0.87 kg m −1 MPa −1 s −1 ; k l 3.28×10 −5 kg m −1 MPa −1 s −1 , stem diameter 3 mm). Hydraulic conductivities decrease with increasing pressure gradient. Transpiration rates in well watered plants were moderate to low, generating xylem water potentials of -1 to -2 MPa. Acoustic emissions indicated extensive cavitation events that were initiated at xylem water potentials of -2 to -3 MPa. The desiccation-tolerant nature of the tissue permits this species to survive this interruption of the water supply. On rewatering the roots pressures that were developed were low (2.4 kPa). However capillary forces were demonstrated to be adequate to account for the refilling of xylem vessels and re-establishment of hydraulic continuity even when water was under a tension of -8 kPa. During dehydration and rehydration cycles stems showed considerable shrinking and swelling. Unusual knob-like structures of unknown chemical composition were observed on the outer surface of xylem vessels. These may be related to the ability of the stem to withstand the mechanical stresses associated with this shrinkage and swelling. Copyright 1998 Annals of Botany Company
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The rehydration characteristics of the desiccation-tolerant plants Craterostigma wilmsii and Myrothamnus flabellifolia (homoiochlorophyllous) and Xerophyta 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 (F V /F M ), photosynthetic pigments and chloroplast ultrastructure as well as damage to the plasmamembrane were studied.
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