PHYTOMECHANICS // Mechanism of defense of the Mimosa pudica

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Giving continuity to the order of publication by topic, this time we will address PHYTOMECHANICAL aspects, specifically on the defense mechanisms that use certain plant groups, in the post, the phytomomechanical model by TIGMONASTY of the Mimosa pudica will be detailed, and each biohydraulic component will be outlined involved in the rapid folding of its leaves.

Introduction

The movements, that execute the vegetal species, obey to the capacity that have the plants to perceive environmental stimuli, this motor action is regulated by physiological reactions, that induce these phyto-mechanical answers that guarantee the survival of the vegetables [3].

However, these external environmental stimuli that induce mechanical responses in plants, has long captured the interest of biologists, physicists and mechanical engineers to explain these reactions phyto-mechanical based on the fact that the plants do not have muscles, nerves or hydraulic structures that help to emit movements.

Consequently, in order to expand the knowledge about the motor capacity available to plant species, in the present post we will describe the phytomomechanical model by TIGMONASTY that presents the Mimosa pudica.

Phytomomechanical ability

Tigmonasty, is interpreted as the rapid movements exerted by the organographic structures of some plant groups, as a mechanical response sensitive to external stimuli such as touch [1].

Fig. 2 Biohydraulic events and phyto-mechanical responses of Mimosa pudica. Author: @lupafilotaxia.

These rapid movements by tigmonasty, are perceptible to the human eye and are based on mechanical and biohydraulic precepts that deploy the plants as a defense mechanism against predatory elements, basically the touch is the action of mechanical stimulation as a threat signal that the plant species receive, triggering a differential water transport at the cellular level, specifically in the xylematic tissue.

In this sense, it is important to note that the water transport initiated in the xylem, generates changes in pressure and state of turgor in the cells, with the consequent increase, extension and contraction (loss of water) in the vacuoles, which subsequently deploy hydraulically movements in plant organs.

Biomechanical species

To explain the movement by tigmonasty, the Mimosa pudica was taken, as an indicator species attending to its motor and defensive ability to touch, this species is known in Venezuela as a dormant or opium poppy belongs to the division Magnoliophyta, class Magnoliopsida, order Fabales and family Fabaceae, morphologically characterized by having herbaceous biotype stems, compound leaves of bipinnate order and alternate phyllotaxy, tiny flowers arranged in inflorescences like heads and dehiscent fruits with seeds located in legumes.

Fig. 3 Pinnules and leaflets of Bipinnado-composed sheet of Mimosa pudica. Author: @lupafilotaxia.

Phytomechanical organ morphology

Structure fitomechanics

The phytomechanical element is not a proper organ, it is a structure of thickened morphology called PULVINUS, which is not more than the basal segment of the leaves, with the capacity to expand or contract to generate movements.

Fig. 4 Pulvinus as a phytomechanical structure of Mimosa pudica. Author: @lupafilotaxia.

Phytomechanical DNA

The pulvinus, is a connection structure between the petiole and the stem, whose cells store genetic information that induce the loss of cell turgor, triggering variable changes in volume and shape, biohydraulic aspect that results in the movement or closure of the leaf blades, product of the loss of water and solutes in the cells at the level of pinnules and leaflets.

Defense mechanism

The plants have adopted multiple mechanisms of defenses, as a result of evolutionary processes, among them the rapid folding of the leaves as defensive tactics, natural ability used by certain plant groups in order to scare off herbivorous organisms, mainly phytophagous insects, in general terms this strategy consists of making violent movements of foliar closure exposing in most cases thorns or sharp stingers.

Mechanical defense strategy

The tigmonasty, is the movement of defense that holds Mimosa pudica (Dormidera), as a sensitive motor strategy, this biological capacity is implemented by rapid movements that exert the leaves by bending sharply their pinnules and leaflets when they come into contact with external elements.

Fig. 5 Organographic structures of bipinnate-composed leaf of Mimosa pudica. Author: @lupafilotaxia.

Strategy I: Primary pulvinus formation

The meristematic cells, located in the axillary buds of the stalk of Mimosa pudica when divided, generate an enlarged organographic structure of circular and elastic morphology called pulvinus, which allows to connect the petiole of the leaf with the stem knots and whose phyomechanical function consists in generating movements of total bending of the bipinnado-composite sheet.

Strategy II: Formation of secondary pulvinus

The bipinnate-compound leaf of Mimosa pudica, generates secondary pulvinus located at the apex of the petiole, this with the purpose of folding by means of contraction movements the leaflets exposed to contact with external elements.

Strategy III: Formation of tertiary pulvinus

Mimosa pudica, has tertiary pulvinus along the leaflet rachis, specifically at the base of each leaflet, this with the aim of making movements by contraction in the leaflet when receiving tactile stimuli.

Indicators of phytomechanical nastic

Fig. 6 Biohydraulic events and indicators of phytomechanical nastic in Mimosa pudica. Author: @lupafilotaxia.

To explain the biohydraulic phenomena, which are generated as a defense response by tigmonasty, in this segment the biological events at the cellular level will be illustrated, concretely what is related to water transport, whose flows are implied in the phytomechanical movements executed by the; pulvinus, pinnule and leaflet of the leaves of Mimosa pudica.

Biohydraulic event 1

The stem, petioles, pulvinus, pinnule and leaflet of Mimosa pudica, comprise a biohydraulic circuit formed by an interconnected set of tubular cells of xylem morphology (vegetal tissue that transports water and solutes), under natural conditions and in the absence of tactile stimuli, these structures maintain an osmotic equilibrium at cellular level, which indicates the differential nonexistence in fluid pressure, finding a uniform turgor at a cellular level.

Fig. 7 Diagram of biohydraulic event 1, absence of tactile stimuli in Mimosa pudica. Author: @lupafilotaxia.

Biohydraulic event 2

The tactile stimuli of external elements, induce in the TERTIARY PULVINUS of Mimosa pudica, an osmotic imbalance that triggers a loss of turgor in the cells, this by the rapid exit of water product of a high concentration of solutes in the cells of the petiole spine, generating in this way a movement in the form of rotation in the leaflets.

Fig. 8 Diagram of biohydraulic event 2, movements in the form of rotation in the leaflets of Mimosa pudica. Author: @lupafilotaxia.

Biohydraulic event 3

The prolonged tactile stimuli of external elements, cause an osmotic imbalance in the cells of the PRIMARY and SECONDARY PULVINUS, generating a water deficit, mediated by high concentrations of solutes in the cells of the stem, facilitating movements in the form of flexion, in each bipinnate-compounds leaf of Mimosa pudica.

Fig. 9 Diagram of biohydraulic event 3, movements in the form of flexion in the bipinnate-compound leaves of .Mimosa pudica. Author: @lupafilotaxia.

SCIENTIFIC CONTRIBUTIONS OF THIS PUBLICATION

The post outlines the strategies of the phytomechanical model by TIGMONASTY, observed in Mimosa pudica, as a defense mechanism, synthesis that also allows to provide a didactic vision of the biohydraulic reactions that occur in the pulvinus, pinnules and leaflets, when they are exposed to tactile stimuli, On the other hand, the article outlines in a descriptive way the structures adopted organographically by M. pudica, to adjust its defense mechanism.

BIBLIOGRAPHICAL REFERENCES CONSULTED AND CITED:

[1] Biro R., and Jaffe M. Thigmomorphogenesis: ethylene evolution and its role in the changes observed in mechanically perturbed bean plants. Physiologia Plantarum. 1984; 62: 289-296.

[2] Darwin C. The Power of Movement in Plants. London: William Clowes. 1980.

[3] Gentry A. The distribution and evolution of climbing plants. En: The biology of vines, Putz FE & Mooney HA (eds.), Cambridge University Press, Cambridge, RU. 1991; 3-52.

[4] Hafsa A., Sehgal A., Mishra A., Mishra R., and Rajiv G. Mimosa pudica L. (Laajvanti): An overview. Pharmacognosy Reviews. 2012;6:12:115-24.

[5] Hill B., and Findlay G. The power of movement in plants: the role of osmotic machines. Q Rev Biophys. 1981; 14:173-222.

[6] Taiz L., and Zeiger E. Plant Physiology. 4nd ed. Sinauer Associates Inc., Sunderland, MA, U.S.A. 2006.

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