Effect of Auxin on Chlorella vulgaris and Studies on the Movement of Leaves

Abstract

[Part 1] Chlorella vulgaris of a re-isolated clone was cultured in Knop's solution to which different amounts of pure heteroauxin were added. The results show that auxin promotes the enlargement of the individual cells. Particularly in the young cultures the cell volume is proportional to the logarithm of the auxin concentration. In older cultures the matter is complicated by the shortage of another factor, presumably food. At high concentrations auxin retards cell division. This probably due to a decreased amount of chlorophyll and a decreased photosynthetic activity. In agar medium heteroauxin in very low concentrations stimulates growth, but at high concentrations inhibits grown of the alga. Auxin does not affect respiration but it markedly affects the rate of photosynthesis. The effect is indirect through the changes in the chlorophyll content and the cell size. In the young cultures the rate of photosynthesis is proportional to the chlorophyll concentration, while in the old cultures it is proportional to both the chlorophyll concentration and the extent of the cell surface. [Part 2] Malva leaves are diaphototropically sensitive, i.e., the leaves orient themselves transversely to the sun's rays. They follow the sun's course during the day and return to their original position at night. No such movement can be observed in diffuse light or in darkness. Light is perceived by the lamina. The movement consists of a curvature of the laminar joint which is capable of reversible contraction and extension. A difference in osmotic pressures on the two sides of the petiole is found to be responsible for the curvature. The leaves of Carica papaya assume a horizontal position in the day and droop abaxially at night. The movement has a normal periodicity of twenty four hours due to some internal cuase (autonomic). The mechanism of the movement has been shown to be of the following sequence: (1) There is a differential distribution (or production) of auxin on the apical and basal lobes of the leaf blade. (2) Because of its special vasicular structure the auxin from the apical lobe goes to the lower (abaxial) side of the petiole, while that from the basal lobe goes to the upper (adaxial) side of the petiole. (3) Because of the differential supply of auxin, the two sides of the petiole grow unequally and give rise to nyctinastic curvature.