An object of the present invention is to determine novel means of activating plants more particularly, means of controlling plant growth, such as means of promoting growth, means of controlling dormancy, means of imparting tolerance against stress for plants (dryness, high or low temperatures, osmotic pressure, etc.), and means of preventing aging. The present inventors have found that the above object can be achieved by providing plant activators containing, as an active ingredient, C4-C24 ketol fatty acids, in particular, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid.
