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(American Journal of Botany. 2004;91:1936-1938.)
© 2004 Botanical Society of America, Inc.

Brief Communication

The mechanical role of the endodermis in Equisetum plant stems1

H.-Ch. Spatz2 and A. Emanns

Plant Biomechanics Group, University of Freiburg, Schaenzlestr.1, D-79104 Freiburg, Germany

Received for publication December 11, 2003. Accepted for publication May 28, 2004.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
The endodermis of different species of the genus Equisetum has different configurations, two or one continuous layers or a sheath only around the vascular bundles. The question whether the endodermis contributes to the mechanical stability of the aerial shoots is investigated in two ways: In a direct approach, the endodermis of segments of E. hyemale was dissected longitudinally and the mechanical stability against ovalization measured as a function of the orientation of the cuts with respect to the forces applied. A comparative approach tested the mechanical stability of eight different species of Equisetum against ovalization of the cross-section for samples, which were either fully turgescent or had reduced turgor pressure. The double-layer endodermis substantially contributed to the mechanical stability of E. affinis and E. hyemale. Equisetum arvense, E. pratense, E. sylvaticum, and E. telmateja are mechanically stabilized by their single layer of endodermis at least under conditions of low turgor pressure.

Key Words: endodermis • Equisetum • mechanical tests • turgor pressure


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Mechanical loads like wind loads can lead to local buckling of hollow plant stems (von Karman, 1911 ; Brazier, 1929 ; Spatz et al., 1997 ). One of the structural designs suitable to stabilize plant stems against ovalization and ultimately buckling is by way of a thin hypodermal sterome that encloses a hollow cylinder of turgescent parenchyma as in Equisetum giganteum (Spatz et al., 1998 ). This requires a permanently hygrophytic environment. For Equisetum hyemale this is not the case. A special adaptation, extracellular freezing attended by dehydration of tissues, enables the aerial shoots to survive subfreezing temperatures (Niklas, 1989 ) and to remain photosynthetically active over two seasons. Speck et al. (1998) have shown that in Equisetum hyemale resistance against ovalization is nearly independent of turgor pressure. This property was attributed to the presence of a continuous double layer of endodermis, not present in Equisetum giganteum (Spatz et al., 1998 ). It enables the stem to remain erect during the winter period and early spring, when temperatures have risen, but the ground is still frozen.

This hypothesis was tested in two ways. First, as direct proof, samples treated by cutting the endodermis of Equisetum hyemale longitudinally were subjected to transverse compression in different orientations. Second, we tested the dependence of transverse stability on turgor pressure by comparing eight different species of Equisetum having either two or one continuous layers of endodermis or an endodermis surrounding only the vascular bundles. Cross-sections of Equisetaceae have been described before (Bold, 1973 ; Spatz et al., 1998 ; Speck et al., 1998 ). The phylogenetic relations have been elucidated by Des Marais et al. (2003) .


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
Plants
Equisetum giganteum was obtained from the greenhouse of the Botanical Garden, University of Freiburg, and E. affinis and E. fluviatile were obtained from its outdoor plantings. Equisetum arvense, E. hyemale, E. pratense, E. sylvaticum, and E. telmateja were found in various locations around Freiburg, Germany. Unless otherwise stated the plant stems taken were younger than 1 yr old. They were kept wet and used within 2 d.

Cross-sections from various parts of the plant stems were cut with a scalpel and stained with phloroglucinol-HCl as described by Gerlach (1984) .

Dissection of the endodermis
Small cylindrical segments (ca. 6 mm long) of Equisetum hyemale were prepared, and the endodermis was cut longitudinally under a stereomicroscope with a specially prepared razor blade. Two cuts were made across from one another, such that each went from the inside of the hollow cylinder into a vallecular canal. As a control the parenchyma in segments of E. giganteum was cut in the same way.

Turgor pressure was reduced by submersing segments (15 mm long cylinders unless otherwise stated) in solutions of polyethyleneglycol (PEG 6000, Roth, Karlsruhe, Germany) of –10 bar osmolality for 15 h (Schopfer, 1986 ). Care was taken to replace air in the pith cavity with the PEG solution. A control segment from the same internode was placed in H2O for the same duration.

Transverse compression has been described in more detail (Spatz et al., 1995 , 1996 ). In short, the cylindrical segments prepared from internodes at various positions along the length of the stem were subjected to mechanical testing (Fig. 1): displacement and forces to deform the segment with a cross-section approximating a circular ring to an elliptical ring shape were measured in an Instron Universal Testing Machine, Model 4466 (Instron Wolpert, Ludwigshafen, Germany). Samples in which the endodermis was dissected longitudinally were oriented in such a way that the cuts were at 0° and 180°, that is, in the direction of the force applied, or at 90° and 270° angles to the applied force.



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  		Fig. 1. Schematic representation of a transverse compression test. F signifies the force. Double arrows indicate strains in the endodermis upon ovalization of the cross-section, tensile strains develop at 0° and 180°, compression at 90° and 270°

 
Data evaluation
Matched pairs of segments from the same internodes were always used. It was therefore sufficient to compare the slopes of the force-deflection curves in the corresponding ranges of strains, divided by the length of the cylinders, to obtain a relative measure of the mechanical stability against ovalization (see Fig. 2). The ratios are given as means ± SD; N is the number of independent determinations.



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  		Fig. 2. Test for the mechanical stability against ovalization. Matched cylindrical segments from an internode from the middle of a stem of Equisetum giganteum were exposed to either a polyethyleneglycol (PEG) solution or H2O (as control) and subsequently subjected to a transverse compression test. The segments measured 15 mm in length and 6.9 mm in diameter. The forces applied are plotted against the distance the crosshead traveled at a speed of 0.5 mm/min. Ovalization is defined as the difference in the diameter of the short axes of the elliptical cross-section from that of the originally circular cross-section

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
Anatomy
As judged by the appearance of Casparian rings in the phloroglucinol-HCl-stained cross-sections, Equisetum fluviatile and E. giganteum have an endodermis only around the vascular bundles. Equisetum telmateja has a single continuous endodermal layer, and E. affinis and E. hyemale have two continuous layers. Equisetum arvense, E. sylvaticum, and E. pratense have a single continuous layer (Table 1), but unlike the aforementioned species, their pith cavities are very small.


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  		Table 1. Ratio of mechanical stability in transverse compression tests of matched segments after prior exposure either to H2O or polyethyleneglycol (PEG) solutions. Means and standard deviations given with number, N, of independent experiments

 
Dissection of the endodermis
As shown schematically in Fig. 1, when the cylinder is deformed, strains develop at the site of the endodermis. The strains at 0° and 180° (parallel to the force applied) are tensile strains, those at 90° and 270° (at right angle to the force applied) are compressive strains. If the endodermis provides mechanical support for the structure, cuts in the endodermis at 0° and 180° should therefore have a destabilizing effect as compared to cuts at 90° and 270°. This is indeed the case. In matched segments of Equisetum hyemale the ratio of the mechanical stabilities (cut parallel/cut at right angle to the forces applied) was 0.358 ± 0.080 (N = 11). In Equisetum giganteum, the ratio (parallel/right angle) of the mechanical stabilities of matched pairs was 0.777 ± 0.154 (N = 8). The smaller value for the ratio in Equisetum hyemale as compared to E. giganteum indicates that a continuous double layer of endodermis contributes substantially to the mechanical stability of the aerial shoots.

Dependence on turgor pressure
Exposure to PEG solutions only slightly reduced the transverse compressibility of Equisetum hyemale (Table 1). This can have two reasons: water does not diffuse efficiently from the tissues into the PEG solution, or it is a manifestation of an inherent property of the structure. Two experiments were done to rule out the first possibility. (1) Differently sized (between 3 and 20 mm) matched pairs of segments were exposed to either PEG solutions or H2O. The length of the segments did not affect the ratio of the mechanical stabilities in transverse compression. (2) Prior to the 15-h exposure of segments (3 mm or 6 mm long) to the PEG solution, the vallecular canals were filled directly with a syringe with this solution. Again, no significant difference from the usual exposure to PEG solutions was found.

Comparison with other Equisetum species
The size of the stems of Equisetum affinis, a hybrid between E. hyemale and, presumably, E. giganteum, are much larger, but the cross-sections are very similar to E. hyemale. Like the latter, Equisetum affinis has two continuous layers of endodermis. Its susceptibility to reduction of turgor pressure is not significantly different from that in Equisetum hyemale (Table 1). Equisetum telmateja, with only one continuous layer of endodermis has a higher susceptibility compared to E. affinis (P < 0.01) as well as to E. hyemale (P < 0.001).

An even higher susceptibility is expected if only the vascular bundles are surrounded by endodermis. Figure 2 shows a comparison of force-deflection curves in transverse compression of matched Equisetum giganteum segments after PEG or H2O treatment. The mechanical behavior is quite different for the two samples. The slopes of the curves differ approximately by a factor of 4. The average for stems younger than 1 yr, listed in Table 1, confirms earlier data (Spatz et al., 1998 ). Older stems are less susceptible to a reduction of turgor pressure. Ratios approximating 2 are found.

Large standard deviations are found for Equisetum fluviatile both for stem parts above water level and for submersed parts of the stem. The reason for this biological variation is not yet known. Even with this caveat, it is clear that Equisetum fluviatile is quite susceptible to a reduction of turgor pressure.

The smaller species, Equisetum arvense, E. sylvaticum, and E. pratense, fall into a different class. Although only one continuous layer of endodermis is present, exposure to PEG reduces the mechanical stability only marginally. Even on a relative scale, the pith cavities are so small that a single layer of strengthening tissue will have a substantial stabilizing effect at least under conditions of low turgor pressure.


   FOOTNOTES
 
1 The authors are grateful to Profs. K. Niklas, P. Schopfer, and T. Speck for valuable suggestions and to Dr. O. Speck for the anatomical analysis of E. affinis. Back

2 E-mail: spatz{at}biologie.uni-freiburg.de Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Bold H. C. 1973 Morphology of plants, 3rd ed. Harper and Row, New York, New York, USA

Brazier L. G. 1927 On the flexure of thin cylindrical shells and other "thin" sections. Proceedings of the Royal Society of London, A 116: 104-114[CrossRef]

Des Marias D. L. A. R. Smith D. M. Britton K. M. Pryer 2003 Phylogenetic relationships and evolution of extant horsetails, Equisetum, based on chloroplast DNA sequence data (rbcL and trnL-F). International Journal of Plant Science 164: 737-751[CrossRef]

Gerlach D. 1984 Botanische Mikrotechnik, 3rd ed. Thieme Verlag, Stuttgart, Germany

Niklas K. J. 1989 Extracellular freezing in Equisetum hyemale. American Journal of Botany 76: 627-631[CrossRef][ISI]

Schopfer P. 1986 Experimentelle Pflanzenphysiologie. Bd. 1—Einführung in die Methoden. Springer Verlag, Berlin, Germany

Spatz H.-CH. H. Beismann F. Brüchert A. Emanns T. Speck 1997 Biomechanics of the giant reed Arundo donax. Philosophical Transactions of the Royal Society of London, B 352: 1-10[CrossRef]

Spatz H.-CH. H. Beismann A. Emanns T. Speck 1995 Mechanical anisotropy and inhomogeneity in the tissues comprising the hollow stem of the giant reed Arundo donax. Biomimetics 3: 141-155

Spatz H.-CH. H. Beismann A. Emanns T. Speck 1996 A new method to determine Young's modulus in tangential direction for hollow tubes, in particular hollow plant stems. Proceedings of the Third Biennial ESDA Conference, Montpellier 77: 221-224

Spatz H.-CH. L. Köhler T. Speck 1998 Biomechanics and functional anatomy of hollow stemmed sphenopsids: I. Equisetum giganteum (Equisetaceae). American Journal of Botany 85: 305-314[Abstract]

Speck T. O. Speck A. Emanns H.-CH. Spatz 1998 Biomechanics and functional anatomy of hollow stemmed sphenopsids. III. Equisetum hyemale. Botanica Acta 111: 366-376[ISI]

Von Karman T. 1911 Über die Formänderung dünnwandiger Rohre, insbesondere federnder Ausgleichsrohre. Zeitschrift der Vereinigung deutscher Ingenieure 55: 1889-1895




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This Article
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