During night hours, leaves of some kinds of plants/trees are seen closed. Why?

P. HABEEB RAHMAN

Madurai, Tamil Nadu

Leaves of some plants, trees and petals of flowers close at night with the onset of darkness. This is called Nyctinasty. Nyctinasty is a non-directional response of plants, animals due to changes in temperature or light in the surrounding environment.

It is a circadian (diurnal) rhythmic movement of certain plants, mainly members of Fabaceae (bean family) and certain Cucurbits (gourd family). Also, leaves of plants belonging to the bean family (leguminosae ) exhibit this nyctinasty. The word nyctinasty has been derived from two Greek words, nux/nukt means night+ nasti meaning pressed.

These movements are associated with the diurnal variation of light and temperature and are controlled by the circadian clock and light receptor chemicals such as cytochromes present in the plants. Several leaf opening and closing factors are in turn controlled by the plant biochemical pathways. In plants belonging to leguminosae, the leaf is not a simple leaf but a compound leaf wherein small leaflets are arranged on the branched or non-branched rachis (leaf stalk). The leaf base is called a pulvinus leaf base as it is swollen.

The compound leaf is attached to the stem by means of this swollen leaf base. Anatomically this pulvinate leaf base is responsible for controlling the opening and closure of the entire leaf depending on the temperature and light conditions of the surrounding environment.

As darkness sets in the turgor pressure of the leaf base changes which will be transmitted till the tip of the leaf along the entire rachis. Hence the entire leaf folds up resulting in the closure of the leaf. Here changes of potassium flux along the dorsal and ventral cells of the pulvinus due to the far red aerobic phytochromal responses bring about the changes of turgor pressure in the cells of the pulvinus. Hence the entire leaf along with the leaf base contracts during nights or under high temperature conditions. In the daylight the reverse phenomenon occurs and the leaf opens up.

Dr. T. BHAVANI

Bangalore

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