Leaf tensile strength was measured for the drought‐tolerant grass Eragrostis curvula and the desiccation‐tolerant grass E . nindensis when fully hydrated, partially dehydrated, naturally air‐dried, and flash‐dried. Leaf tensile strength increased in intact, air‐dried leaves of E . curvula but not for similarly treated leaves of E . nindensis . Examination of leaf cross‐sections by light microscopy and histochemical staining for lignins failed to show any significant structural differences between the two species in the hydrated state. When leaves were flash‐dried, the tensile strength of E . curvula remained unchanged from leaves dried naturally, while there was a marked increase in the tensile strength of flash‐dried leaves of E . nindensis . Proton NMR indicated that the desiccation‐tolerant E . nindensis retained mobile water when leaf relative water content was less than 20% if dried naturally but not if flash‐dried, whereas no mobile water was detected in leaves of E . curvula when dried either naturally or with flash‐drying to below 20% relative water content. This behaviour suggests a fundamental difference in strategy for surviving water loss in vegetative tissues between desiccation‐tolerant species and drought‐tolerant species.
During dehydration, numerous metabolites accumulate in vegetative desiccation‐tolerant tissues. This is thought to be important in mechanically stabilizing the cells and membranes in the desiccated state. Non‐aqueous fractionation of desiccated leaf tissues of the resurrection grass Eragrostis nindensis (Ficalho and Hiern) provided an insight into the subcellular localization of the metabolites (because of the assumptions necessary in the calculations the data must be treated with some caution). During dehydration of the desiccant‐tolerant leaves, abundant small vacuoles are formed in the bundle sheath cells, while cell wall folding occurs in the thin‐walled mesophyll and epidermal cells, leading to a considerable reduction in the cross‐sectional area of these cells. During dehydration, proline, protein, and sucrose accumulate in similar proportions in the small vacuoles in the bundle sheath cells. In the mesophyll cells high amounts of sucrose accumulate in the cytoplasm, with proline and proteins being present in both the cytoplasm and the large central vacuole. In addition to the replacement of water by compatible solutes, high permeability of membranes to water may be critical to reduce the mechanical strain associated with the influx of water on rehydration. The immunolocalization of a possible TIP 3;1 to the small vacuoles in the bundle sheath cells may be important in both increased water permeability as well as in the mobilization of solutes from the small vacuoles on rehydration. This is the first report of a possible TIP 3;1 in vegetative tissues (previously only reported in orthodox seeds).
Pressure-volume (PV) curves of the desiccation-tolerant angiosperms, Eragrostis nindensis , Craterostigma wilmsii and Xerophyta humilis , and the desiccation-sensitive species, E. curvula , were compared. The shape of curves for E. nindensis and C. wilmsii differed from the usual curvilinear form. Over the relative water content (RWC) range of approx. 70 to 25%, PV curves indicated water potentials higher than directly measured water activity on frozen-thawed tissue. Anatomical studies showed considerable cell wall folding and a consequent reduction in cell volume in these two species; this was not seen in X. humilis or E. curvula which showed normal PV curves. It is suggested that this wall folding may have prevented the development of negative turgor and physical stress in the cells, and contributed to desiccation tolerance. Copyright 2001 Annals of Botany Company
The ability of tissues to survive desiccation is common in seeds but rare in vegetative tissues. In this study the ultrastructure of hydrated and dehydrated tissues were examined at different stages of the life cycle of the resurrection grass, Eragrostis nindensis Ficalho & Hiern. Conventional fixation techniques are unsuitable for dry tissues as rehydration occurs during fixation in aqueous fixatives. Thus a cryofixation and freeze-substitution method was developed. As a result of the improved fixation methods, it was possible to identify the stage and nature of the damage in the desiccation-sensitive tissues. E. nindensis has desiccation-tolerant orthodox seeds, but the young seedlings are not tolerant to extreme water loss. However, like the seeds, most of the leaves of the adult plant are tolerant to desiccation (only the oldest outermost leaf on a tiller are not). Desiccation-induced damage in these outer leaves was observed in the later stage of dehydration, dominated by the appearance of abundant cell wall fractures (1 wall fracture per 50 μm2). Unlike the outer leaves, the leaves of seedlings appeared similar to those of the hydrated ones upon desiccation. Irreparable damage occurred on rehydration of these tissues possibly as a result of the absence of protection mechanisms observed during desiccation of the inner desiccation-tolerant leaves of the mature plants. The mesophyll tissues of these leaves become compact with extensive cell wall folding on drying. The bundle sheath cells maintained their shape with desiccation but became packed with small vacuoles.
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
ABSTRACT Under water‐limiting conditions excitation energy harnessed by chlorophyll can lead to the formation of reactive oxygen species (ROS). Resurrection plants minimize their formation by preventing the opportunity for light–chlorophyll interaction but also quench them via antioxidants. Poikilochlorohyllous species such as X erophyta humilis break down chlorophyll to avoid ROS formation. Homoiochlorophyllous types retain chlorophyll. We proposed that leaf folding during drying of Craterostigma wilmsii and Myrothamnus flabellifolius shades chlorophyll to avoid ROS (Farrant, Plant Ecology 151, 29–39, 2000). This was tested by preventing leaf folding during drying in light. As controls, plants were dried without light, and X. humilis was included. Craterostigma wilmsii did not survive drying in light if the leaves were prevented from folding, despite protection from increased anthocyanin and sucrose and elevated antioxidant enzyme activity. Membranes were damaged, electrolyte leakage was elevated and plastoglobuli (evidence of light stress) accumulated in chloroplasts. Restrained leaves of M. flabellifolius survived drying in light. Leaf folding allows less shading, but the extent of chemical protection (anthocyanin content and antioxidant activity) is considerably higher in this species compared with C. wilmsii . Chemical protection appears to be light regulated in M. flabellifolius but not in C. wilmsii . Drying in the dark resulted in loss of viability in the homoiochlorophyllous but not the poikilochlorophyllous species. It is hypothesized that some of the genes required for protection are light regulated in the former.
The sections in this article are Introduction Water Transport Measurements: Principles and Methods Aquaporins at the Level of Molecules, Cells and Tissues Mechanisms of Regulation Conclusion
• Background and Aims Previous studies on grass leaf tensile properties (behaviour during mechanical stress) have focused on agricultural applications such as resistance to trampling and palatability; no investigations have directly addressed mechanical properties during water stress, and hence these are the subject of this study.
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