The following points highlight the two main types of nastic movements in plants.The types are: 1. Oscillatory Movements 2. Single-Event Movements.
Nastic Movement: Type # 1.
Oscillatory Movements:
Oscillatory nastic movements may be based on either diurnal periods or on shorter periods. Nyctinastic movements are the diurnal movements. The leaflets of Trifolium repens show nyctinastic movement. The terminal leaflet closes at night and opens during the day through an angle of 1800 rotation.
The oscillatory leaflet movement of Trifolium is caused by the pulvinus. It consists of two types of cells—extensor motor cells and flexor motor cells. The movement of the pulvinus is brought about by the periodic swelling and shrinking of the extensor and flexor motor cells.
The bending motion of the pulvinus is obviously an osmotic mechanism, K+ together with other unidentified anions being the major components of the osmoticum.
The potassium content of the extensor cells is higher at the time of leaflet opening than that in the flexor cells and vice versa. So, there is a net translocation of K+ ions back and forth between the flexor and extensor tissues during opening and closing of the leaflet.
Concomitant with the rhythmic changes in K+ concentration there are rhythmic change or oscillations in the membrane potential difference of both extensor and flexor cells. During their swelling the membrane potential difference becomes largely negative with the K+ build-up in both extensor and flexor cells, and with the release of K+ ions it gets less negative.
The situation is still obscure regarding the transport of anions. At the time of swelling there must be some active transport of solutes into the motor cells at the expense of metabolic energy. The mechanism of transport is still unknown but probably proton extrusion may be involved.
In the species of Albizzia, Mimosa and Samanea, the leaves are doubly compound with primary pulvini attaching the rachilla to the stem and secondary pulvini connecting pinnae to rachilla, and tertiary pulvini connecting pinnules to pinna. Their leaves show photonasty.
They open in light and close in the dark. The opening is brought about by bending motion in secondary and tertiary pulvini showing circadian rhythms. Each pulvinus is cylindrical in shape with a central vascular supply, surrounded by a few cell-thick parenchymatous cortex.
The motor cells are present in the outer region of cortex. The mechanism of pulvinus movement is osmotic, consisting of alternating swelling and shrinking of extensor and flexor motor cells.
Here, also the major osmoticum is K+ and anions including Cl–. Movement of both K+ and CI– between the extensor and the flexor regions in the pulvinus of Samanea has been reported by Campbell et al.
Nastic Movement: Type # 2.
Single-Event Movements:
The insectivorous plants like Drosera, Dionaea, Aldrovanda, etc., show this type of movement. In Drosera, there are multicellular glandular hairs on the inner surfaces of leaves, the tips of which secrete drops of a sticky liquid.
When an insect gets adhered to the sticky hairs, they immediately start bending. The touch of an insect stimulates the outer hairs to bend rapidly inwards to place the insect to the center of the leaf, followed by slow bending movement of the outer tentacles. They straighten out again after several hours.
The hairs are excitable and can develop a series of action potentials (i.e., electrical fluctuations) after being stimulated by various types of stimuli which may be chemical, electrical or mechanical. The action potentials at the hair tips are propagated to the base causing bending. The duration of the action potential is 10-30 seconds. The ionic basis of the action potential is still not known.
In Dionaea, the leaf is a two-lobed structure joined at the midrib with spines at the periphery. An appropriate mechanical stimulus quickly brings about the closure of the lobes within 300 milliseconds. Obviously, the insects make the stimulation and are trapped. The stimulation is received by the sensitive hairs. The action potentials produced in the hairs are propagated to the cells of the trap lobes.
The aquatic Aldrovanda trap is a lobed structure inside which there are 20 sensitive hairs. The cells of the lobe are excitable either electrically or by movement of sensitive hairs. Nastic movements found in Mimosa pudica result from touch and are called thigmonasty.
The bending movement is very fast in the primary pulvini taking about 1-2 sec causing fold up of the leaflets, followed by slow recovery to the original position in TO-15 min. The excitable cells in the extensor region are responsible for the actual movement.
The movement is caused by water transport out of the motor cells of the pulvinus, suggesting the osmotic model. The action potentials are developed in the motor cells owing to a net efflux of K+ and CI– ions, which is similar to that occurring in animal nerve cells but much slower. The recovery is light-dependent, and thus dependent on photosynthesis.