An Ultrastructural Study of the Effects of 2,4-D on Tobacco Leaves

Tobacco (Nicotiana tabacum L. ‘Samsun N. N.’) leaves were treated with (2,4-dichlorophenoxy)acetic acid (2,4-D) at a concentration of 500 mg/L by the syringe infiltration technique and then sampled at regular intervals. Electron microscopic examination of the treated tissue revealed that the response of the mesophyll cells to 2,4-D was related to the age of the leaf at the time of treatment. Expanding leaves from the upper portion of the plant exhibited a high degree of tolerance to 2,4-D. Mature, fully-expanded leaves from the lower part of the plant were highly sensitive to the herbicide, with the mesophyll cells undergoing rapid structural breakdown. This breakdown involved a rupturing and disintegration of the tonoplast, plasmalemma, and membranes of chloroplasts and mitochondria.