Some Histological Effects of 2,4,5-Trichlorophenoxyacetic Acid Applied to Mature Apricot Leaves

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Apricot trees were sprayed with 2,4,5-trichlorophenoxyacetic acid at concentrations of 50, 100, and 200 ppm on May 22, 1965, when spur leaves were fully expanded. Some effects of 2, 4,5-T were studied in leaves collected up to October 2. Increase in petiole diameter resulted from enlargement of cells of the ground tissue and from greatly stimulated development of secondary xylem and phloem. Increase in leaf blade thickness was caused primarily by increased depth of palisade tissue and bundle sheaths and their extensions, but evidence of limited induced cell division in both mesophyll tissues contributed in some leaves; in a few leaves from the 50-ppm treatment, much cell division without cell enlargement in mesophyll tissues accounted for all the increase in leaf thickness between vascular bundles. In the mesophyll of leaves treated with the higher concentrations, many cells and nuclei were larger than in controls. Distributions of frequencies of cell widths and of nuclear diameters in palisade cell populations indicated that the general degree of endopolyploidy increased following 2,4,5-T treatment and that some of the comparable cells in controls were also polyploid. The number of stomata per unit length of the lower epidermis was almost one-half less in leaves given the highest 2,4,5-T concentration than in the controls. The immediate causes were (a) a great increase in treated plants in the cell size in bundle sheath extensions and correspondingly of some epidermal cells, and (b) stretching, or (c) a few cell divisions of other epidermal cells. Tyloses in control petioles and midveins were not numerous until October, but in petioles of treated leaves many developed early. It is suggested that the actions of 2,4,5-T in stretching cell walls and in stimulating endopolyploidy combined to cause tylosis formation, the stretched pit membranes of the vessels not having been able to withstand the pressure of enlarging polyploid xylem parenchyma. Aside from exceptional instances of limited cell proliferation in phloem ray cells, no abnormalities were induced. The general effects of 2,4,5-T as applied here were concluded to have been acceleration or stimulation of processes occurring normally in untreated leaves.