Diaheliotropic Responses of Leaves of Macroptilium atropurpureum cv. Siratro

Abstract

Diaheliotropic movements in Macroptilium atropurpureum cv. Siratro caused the adaxial surfaces of leaflets to orient towards a light by the flexing of secondary pulvini at the base of the leaflets, and to a lesser extent by petiole bending and flexing of the primary pulvinus at the base of the petiole.

Secondary pulvini flexed more in response to light than did primary pulvini. This flexing was in response to blue light such that the illuminated side of the pulvinus contracted, and the leaflet reoriented with its adaxial surface facing the light. This response was enhanced by the following: a light gradient along the length of the illuminated surface of the pulvinus, the presence of additional low-intensity diffuse light, a photon irradiance greater than 0.5 mmol m^-2 s^-1, and a change of photon irradiance. The abaxial surface of a pulvinus was more sensitive to light than was its adaxial surface.

The stimulus for movement was transmitted from an illuminated primary pulvinus to the shaded secondary pulvini of the same leaf, and this caused re-orientation of both individual leaflets and the whole leaf. Similarly, a stimulus was transmitted to the primary pulvinus from secondary pulvini (or from the adjacent petiole) when illuminated, resulting in a deflection of the petiole by the primary pulvinus and re-orientation of the adaxial surface of the whole leaf towards the light.

Secondary pulvini have the ability to move individual leaflets in three dimensions because the plane of the leaflet is at right angles, in two dimensions, to the axis of the distal end of the pulvinus. Terminal leaflets showed a greater three-dimensional movement than did lateral leaflets.

Movement of a light through large angles resulted in leaflet re-orientation proportional to, but less than, the change in angle of the light. On the other hand, secondary pulvini caused leaflets to track, though imperfectly, a light source that simulated the movement of the sun. A circumnutational circadian rhythm is partly responsible for the diaheliotropic movements of Siratro leaves outdoors, and the ability of an individual leaflet to track the sun depends upon its orientation with respect to the sun.