8.2.5 Nastic movements
Nastic responses differ from tropisms because the direction of movement is not related to the stimulus direction but is instead dictated by the plant. Many legumes with divided leaves such as Leucaena (Figure 8.15), Phaseolus beans and the pasture species Siratro (Macroptilium atropurpureum), widely grown for forage in Queensland, are good examples. Early in the morning on hot days, leaflets are oriented horizontally, but as temperature and solar radiation levels rise the leaflets move to a vertical position perpendicular to the sunlight. This is helionasty, which cuts down radiation absorption and consequently reduces water use and overheating. When solar radiation declines towards dusk, leaflets return to their former horizontal position. In legumes, movement is controlled by reversible turgor changes in a small fleshy elbow, the pulvinus, located at each leaflet or pinnule base, which can flex back and forth as water flows in or out of the pulvinus cell vacuoles.
Seismonastic or thigmonastic movements are rapid responses to vibration, touch or flexure. Examples are the high-speed bending of leaf pulvini in the sensitive plant Mimosa sensitiva (Figure 8.16), and the curvature of hairs of insectivorous plants. In the case of the Venus fly trap, sensory hairs coupled to an electrical signalling system require stimulation at least twice within a 30 s period (Simons 1992). This appears to allow the plant to discriminate single pieces of debris from an insect crawling within the trap. Most seismonastic movements result from the explosive loss of water from turgid ‘motor’ cells, causing the cells temporarily to collapse and inducing very quick curvature in the organ where they are located.
(b) Nocturnal ‘sleep’ movements
Leaves and leaflets that become vertical at night are called nyctinastic. This is commonly termed a ‘sleep’ movement, although these plants do not actually slow down their metabolism at night. The ‘Prayer Plant’ (Maranta) is a good example (Figure 8.17). Sleep movements are either growth based, and therefore cease at leaf maturity, or are caused by reversible turgor changes in the pulvinus.
Turgor-based pulvinus flexure
Turgor-based sleep movements are exhibited by many legumes. Examples are clover (Trifolium), bean (Phaseolus), Bauhinia, coral tree (Erythrina) and many tropical legume trees, such as Pithecelobium saman and Leucaena. Turgor-based sleep movements occur mainly in compound leaves with a mechanism similar to helionasty. The daily rhythm of water movement results from a flux of potassium ions from one side of the pulvinus to the other, either increasing or decreasing the water potential of cell vacuoles in each half.
Growth-based petiole epinasty
Other species follow a daily rhythm of leaf movement due to differential growth of upper and lower halves of the petiole. The day–night rhythmic curvature of the petiole is not related to a directional stimulus and is termed ‘epinastic’. Like turgor-based sleep movements, magnitude varies with the amount of solar radiation intercepted. Epinastic growth movements may be caused by diurnal changes in production of the plant hormone ethylene, which promotes growth of cells on the upper side of the petiole, inducing downward curvature (Figure 8.18). Leaves constantly produce small amounts of ethylene and, according to one hypothesis, production in-creases towards the end of the day, moving the lamina from horizontal to vertical. The opposite would occur towards the end of the night, allowing the lamina to return to the horizontal daytime position. Supporting evidence comes from petiole cells where ability to respond to ethylene is blocked by silver thiosulphate, and the epinastic leaf movement subsequently disappears.
Nocturnal leaf folding may help plants to conserve water by promoting dew formation, since the air and soil beneath the canopy cool more rapidly after the canopy has folded up or become vertical. The lower temperature then promotes dew development, which falls to the ground around the base of the plant, supplementing rainfall.
Growth-based epinasty is also seen in many dicotyledonous seedlings during germination, when the end of the shoot is bent over in a plumular hook. The hook is a temporary structure which protects the apical bud as the shoot pushes through the soil. It is created by cells on one side of the plumule expanding more rapidly than cells on the opposite side, possibly in response to ethylene, which is produced by the plumule in darkness. On reaching the soil surface, the plumule is exposed to daylight which appears initially to reverse and then to cancel the differential response to ethylene, and consequently the stem straightens.