Effects of Plant-Growth Regulators on Shoot Development and Field Survival of Forest-Tree Seedlings

Abstract

1. A series of seven experiments, conducted between October, 1942, and October, 1943, involving about 25,000 individual trees and several hundred feet of nursery bed, was undertaken to survey several potential applications to forest-planting practice of the control of top development of seedlings by means of growth regulators. Seedlings of red, loblolly, shortleaf, pitch, and table-mountain pines, red spruce, tuliptree, and white ash were treated in the nursery bed, before storage or before planting. Napthaleneacetic acid, naphthalenemethylacetate, napthalene acetamide, and mixtures of naphthalene compounds with or without indolebutyric acid were applied in various ways. Methods included dipping, soaking, or spraying with water solutions and carrier emulsions of lanolin or commercial wax, and exposure to vapors of the compounds. 2. Nursery-bed treatments were applied for purposes of prolonging dormancy and restricting the amount of shoot growth. The normal surge of leader growth of pine seedlings was not delayed or substantially lessened by concentrations of 200 or 600 mg./l., sprayed on the expanding buds and old foliage. Needle development, once started, could be completely arrested shortly after treatment and "frozen" for the remainder of the season on red and table-mountain pines. Undesirable curving of the new leaders resulted after treatment of table-mountain pine but not of red pine. 3. At the end of the growing season, red pine seedlings which had been sprayed in May with 600 mg./l. naphthalene acetamide had somewhat lower top-root ratio and much lower average dry weight than untreated seedlings. Inhibiting treatments were close to the lethal concentration, for they occasionally killed some of the trees. More than one application during the period of shoot growth of a spray containing 600 mg./l. naphthaleneacetic acid or similar growth regulators resulted in death of most of the seedlings. 4. Red pine seedlings having only one-half to one-third the normal quantity of new foliage, as a result of inhibition by growth regulators, were no more resistant to artificial drought than untreated seedlings. Top pruning of loblolly pine during early spring resulted in significant decrease in resistance to artificial drought and to transplanting during the same summer and therefore did not appear to be a desirable alternative to growth regulators for checking top growth in nursery beds. 5. Vapors and sprays of naphthalenemethylacetate and a mixture of growth regulators were effective in preventing or restricting the development of etiolated shoots of white ash seedlings held until June in unrefrigerated storage. These treatments also stimulated initiation of new roots, but a high proportion of the new roots appeared on the stem. Inhibition of shoots and stimulation of roots were much less distinct in treated trees heeled-in outdoors. Pre-storage treatments with growth regulators did not affect the survival of white ash seedlings planted in May when controls were still largely dormant, but they resulted in higher initial survival and greater root growth of trees planted in June. 6. For 2-year-old seedlings of white ash, a successful pre-storage treatment was naphthalenemethylacetate applied as vapor (0.3 or 0.5 mg./cu. ft. of space for 18 hours at 70⚬ F.) or as a spray (100 mg./l. in dilute commercial-wax emulsion). Similar pre-storage treatments did not improve survival of loblolly or short-leaf pine seedlings. 7. Growth regulators were also applied just before planting in an attempt to improve initial survival by holding back top growth and transpiration while the roots become established. Pre-planting treatments were applied to tops of seedlings of red, shortleaf, and loblolly pines, and to roots, tops, and whole plants of red spruce and tuliptree. Various treatments with 80-1000 mg./l. naphthaleneacetic acid and other compounds failed to improve the field survival of conifers, and usually lowered it. 8. Seedlings of tuliptree dipped in emulsions containing 200 or 400 mg./l. of a mixture of growth regulators just before planting in the spring leafed out more slowly than untreated trees. However, survival was so uniformly high for all treatment groups (planted in May) that no advantage resulted from the prolonged dormancy. Tuliptree seedlings treated with growth regulators showed slight stimulation of root growth in the heeling-in bed and excelled controls in total dry weight by the end of the first season in the plantation. 9. Application of emulsions of lanolin (30-50 gm./l.) or commercial wax to foliage of conifers immediately before planting resulted in higher survival during the spring months. After a long and severe summer drought (1943), the survival of coated seedlings exceeded that of controls in some plantations but not in others.