Comparative Plant Growth-Retarding Activity of Amo-1618, Phosfon, and CCC

Growth responses of fifty-five species of agronomic or horticultural interest were studied following soil and foliar application of Amo-1618, phosfon, or CCC. 2. Most plants responded in several ways to the growth-retarding substances, the most obvious response being lowered stem elongation. There were marked differences in responsiveness of different genera. 3. Treatment with Amo-1618 resulted in retarded growth of only six species; with phosfon, of all plants responsive to Amo-1618 plus twelve additional species; and with CCC, of most species (the exceptions included foliage plants, petunia, and cleome). At present, no explanation is possible for these different responses. 4. The side effects of the three chemicals were different even though the primary action (retarding internode elongation) was similar. Amo-1618 induced few side effects. Overtreatment with phosfon resulted in veins and stems being permanently cleared of chlorophyll. In contrast, treatment with CCC induced a temporary paling of the color between the veins; at maturity, this side effect was not obvious. 5. Amo-1618 and phosfon persisted in soil, affecting more than one crop of plants. CCC did not persist in the soil, affecting only one crop or less. A seasonal variation in response to the chemicals was observed: growth retardation following application of Amo-1618 was slightly greater in summer, of phosfon much greater in summer, and of CCC much less in summer than in winter. 6. The following responses by different parts of the plants resulted from application of the growth-retarding substances: germination of seeds was delayed, the initiating process unaffected. Root development was initially inhibited, but effects were seldom apparent at flowering time. Internode length was less on most plants; the main stem of certain treated plants grew in essentially a rosette fashion. Overtreatment with phosfon resulted in reduced wall thickening of sclerenchymatous cells in stems of Lilium longiflorum, and when the flowers developed, the stems became top-heavy and easily bent. Leaves on most treated plants were much darker green than those on untreated plants; varying degrees of chlorosis followed excessive treatments. Blade expansion was often slow, but the leaves eventually were similar in size to those of untreated plants. Control of flower initiation by photoperiod and temperature was unaffected; although flowering time of most plants was not noticeably altered, that of chrysanthemum and cleome was delayed. Flower-bud initiation of azaleas was promoted. The reduction in fresh and dry weights observed for treated plants resulted primarily from the reduced stem length.