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- Hermann Heilmeier, Rainer Wolf, Robin Wacker, Markus Woitke and
- T. Bornefeld and O.H. Volk:
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Observations on the anatomy of hydrated and dehydrated roots of Chamaegigas intrepidus Dinter
By Hermann Heilmeier*, Rainer Wolf**, Robin Wacker, Markus Woitke and Wolfram Hartung***
* Interdisziplinares Okologisches Zentmm, TU Bergakademie Freiberg, Leipziger Str. 29, D-09599 Freiberg, Germany
During dehydration, the length of roots of the aquatic resurrection plant Chamaegigas intrepidus is reduced by only 15%, whereas the diameter of fully differentiated roots is reduced by 30 to 35%. Lateral shrinking of roots is exclusively caused by collapsing cells in the rhizodermis and exodermis, which cover the cortex like a velamen radicum. Apart from the endodermis the root cortex is composed of a single layer of extremely large and thin-walled cells which show a high stability even when desiccated. Within the central stele there is a simple diarchic xylem. The rhizodermis is absolutely free of root hairs. The short cells within the dimorphic exodermis have exterior walls with characteristic thickenings, which act as valves closing the short cells and thus slowing down water loss during dehydration.
Während des Eintrocknens schrumpfen die haarlosen Wurzein der aquatischen Auferstehungspflanze Chamaegigas intrepidus um nur 15%, während der Durchmesser der voll ausdifferenzierten Wurzein um 30 bis 35% abnimmt. Letzteres rührt ausschließlich vom Kollabieren der Rhizodermis und Exodermis her, welche sich dann wie ein Velamen radicum der Rinde aniegen. Die Rinde besteht neben der Endodermis meistens aus einer einzigen extrem großlumigen und zartwandigen Zellschicht, welche auch im desikkierten Zustand eine erstaunliche Stabilitat aufweist. Der Zentralzylinder besitzt ein einfaches diarches Xylem. Bei der Hypodermis handelt es sich um eine Kurzzellenexodermis mit charakteristischen äußeren Zellwandverdickungen, welche die Durchlass (= Kurz-) zeilen wie ein Ventil verschließen und somit den Wasserverlust wahrend des Eintrocknens verlangsamen können.
aquatic resurrection plant, poikilohydric cormophyte, dimorphic exodermis, hypodermis, rhizodermis, root cortex, velamen radicum Dedicated to two pioneers of Namibian Botany, Willy Giess (1910-2000) and Prof. O.H. Volk (1903-2000)
Poikilohydric angiosperms or resurrection plants are characterised by most remarkable adaptations to their arid habitats. They can survive tissue water potentials of -200 MPa, even equilibration with 0% relative air humidity without damage over extended periods (Gaff & Giess 1986). Apart from a number of biochemical and physiological adaptations (Heilmeier & Hartung 2001), a unique trait of resurrection plants is their capability of an extreme shrinkage of their leaves during dehydration. The most extreme example is Chamaegigas intrepidus Dinter (formerly Lindernia intrepidus Dinter Oberm.) which only grows endemically in temporarily water-filled rock pools on granite outcrops in Central Namibia.
The distribution of this species was described by Giess (1969), the morphology and resumption of growth after rewatering ("resurrection") by Smook (1969). When the shallow pools dry out, the size of submerged leaves is reduced by 90%. Thus the plants withdraw into the sediment at the bottom of the pools. Through this behaviour leaves are protected from manifold environmental stress, among others the high ultraviolet radiation. This remarkable trait was already mentioned by Heil (1924), whilst Hickel (1967) and Schiller et al. (1999) provide more details. Figure 1 shows a hand-made drawing by Prof. O.H. Volk, which he prepared during one of his many expeditions in the former Southwest Africa to the natural sites of C. intrepidus using a plain pocket microscope. The drawing emphasises the unique anatomical traits which enable the drastic shrinkage, contractive tracheides and their endplates with very narrow holes. They can contract and expand like an accordion, making the extreme contraction of the desiccated leaf possible. Until now the roots of Chamaegigas intrepidus have only been rarely studied.
Heil (1924) and Hickel (1967) describe the dimorphic exodermis with short cells of Chamaegigas roots. However, there are no further investigations on tissues interior to the hypodermis, either in the hydrated or in the desiccated state. Heil (1924) assumes that the root cortex of Chamaegigas represents an aerenchyma with small, radially orientated cells; however, this hypothesis is not supported by any figures or detailed information on root anatomy. Therefore we will present results from studies, using scanning electron (SEM) and light microscopy, showing most remarkable features of the roots of Chamaegigas intrepidus. [...]
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Oecologia 88:457-462. STOCKING, M.A. 1994. Assessing vegetative cover and management effects. In: Lal, R. (ed.), >S'o;7 Erosion Research Methods, 2nd edition, pp. 211-232. Soil and Water Conservation Society and St. Lucie Press, Akeny / Delray Beach.