A Comparison of the Effectiveness of 2,4-Dichlorophenoxyacetic Acid and Four of Its Salts in Inducing Histological Responses in Bean Plants

Abstract

1. Young kidney-bean plants were decapitated in the second internode. 2,4-Dichlorophenoxyacetic acid or one of four salts-ammonium, copper, calcium, or magnesium 2,4-dichlorophenoxyacetate-was applied to the cut surface in 0.5% concentration by weight in lanolin paste. Observations were made of material grown for a period of 30 days after treatment. 2. Gross response showed characteristic differences among the tumors. Treatments with 2,4-D and with the ammonium salt resulted in relatively deep, club-shaped tumors. Proliferation was entirely below the cut surface. The tumor induced by the copper salt was likewise formed below the cut surface but Was somewhat shallower. Those induced by calcium and magnesium salts were broadly flared at the cut surface and gradually tapered below. Growth above the cut surface occurred with both of the latter substances but to a more marked degree with the magnesium salt. 3. Histological responses in the early stages showed considerable similarity. In parenchymatous tissues of the inner cortex including the endodermis, of the pericycle, phloem, cambial zone, xylem, and rays, proliferation was initiated soon after treatment. Response induced by the magnesium salt was earlier and more marked than in other treatments, less so with the calcium salt, and least with the ammonium salt and with 2,4-D. 4. Study of the tumor as a whole disclosed a pattern of zonation for all treatments. At upper levels a zone of limited proliferation was found in the responses to 2,4-D and to the ammonium and copper salts. This zone was lacking in the other responses. A zone of major proliferation occupied the intermediate levels below the zone of limited proliferation in treatments with 2,4-D and with the ammonium and copper salts but was found at upper levels in the tumors induced by the calcium and magnesium salts. In response to the magnesium salt this zone Was only poorly represented. With all substances a root zone was formed at next lower levels, and below the roots a zone of minor proliferation terminated the tumor. Comparison on the basis of zonation disclosed a consistent sequence among the substances according to the characteristics of the responses induced: 2,4-D and the ammonium, copper, calcium, and magnesium salts. This, same series was duplicated in gross responses and in details of histological responses. 5. Maturation was earliest and most marked in the treatment with the magnesium salt and least with the ammonium salt and with 2,4-D. In the zone of limited proliferation maturation occurred early, was mainly as parenchyma, and was soon followed by collapse. Maturation in the zone of major proliferation was longest delayed. Many of the derivatives differentiated as parenchyma except at levels of transition to lower zones. In the root zone the rays organized as root primordia remained active until the death of the tumor 3 weeks or more after treatment. Proliferated tissues of vascular bundles and associated endodermis and cortical parenchyma both in the root zone and in the zone of minor proliferation exhibited complicated patterns of differentiation into tracheids, parenchyma, vascular bundles, and meristematic areas. 6. Characteristic differences in histological responses to the different substances are discussed. The most pronounced differences were found in decapitated stems treated with the magnesium salt. However, no response induced by one substance was entirely absent in the reaction to another substance. All responses appeared to fall within the range of effects distinctive for 2,4-D. Independent effects of the different cations on stem responses were also considered. 7. Zonation is interpreted as an expression of a concentration gradient. On the basis of this interpretation the effectiveness of the growth-regulating substance was greatly modified by the form of the salt which was applied.