RELATIONSHIPS BETWEEN GROWTH RATES AND NUCLEIC ACID CONTENTS IN THE ROOTS OF INBRED LINES OF CORN

WOODSTOCK, LOWELL W., and FOLKE SKOOG. (U. Wisconsin, Madison.) Relationships between growth rates and nucleic acid contents in the roots of inbred lines of corn. Amer. Jour. Bot. 47(9): 713-716. Illus. 1960.-A study has been made of the rates of elongation and nucleic acid contents of the roots of inbred lines of Zea mays, var. W-158, A-374B, W-58, W-28, 8A, and W-148. A direct proportionality was found between the ribosenucleic acid contents (4g./root) of the apical 3 or 6 mm. of the roots (roughly equivalent to their growing tips) and their rates of elongation. This strict, quantitative relationship to growth rate appears to be specific for RNA; it was not found for DNA, and it was independent of the size of the root. A detailed study of one line (W-58) has shown, furthermore, that the ribosenucleic acid content in the root tip decreases with time in proportion to the decrease in the growth rate of the root. THERE are many indications, summarized by Brachet (1957), that ribosenucleic acid (RNA)2 is necessary for protein synthesis and that this may account for correlations between growth and RNA contents observed in various tissues of plants and animals. Potapow and Maroti (1956), in a comparative study of the meristems of the root tips and shoot apices of bean seedlings, found that the more rapid rates of cell division, growth, and differentiation in the roots were associated with higher rates of nucleic acid-phosphorus metabolism. Conversely, Clowes (1956) has found a metabolically quiescent center in the apical meristem of the Zea mays root, the cells of which are characterized by relatively small nucleoli and low cytoplasmic RNA contents. Tamiya et al. (1955) found that in Chlorella, the increase in RNA contents paralleled the increase in dry weight. Earlier work in this laboratory, summarized by Skoog, (1954), has also indicated that the nucleic acids may play a decisive role in the processes of growth and differentiation in higher plants. In particular, Porter (1953), studying possible interrelationships between RNA and auxin effects on growth of corn roots, obtained data which suggested that RNA might be the limiting, factor in the cessation of cell elongation and that the inhibition of root growth by indoleacetic acid (IAA) might be due to an effect on ribosenucleic acid metabolism. The present investigations are a continuation of these studies of relationships between RNA and the 1 Received for publication August 24, 1959 This work was supported in part by the University Research Committee with funds from the Wisconsin Alumni Research Foundation and in part by grants-in-aid from the American Cancer Society and the Nat'l Science Foundation. The authors also wish to express their gratitude to Professor Norman P. Neal, Agronomy Dept., University of Wisconsin, for providing the seeds used in these experiments. 2 The term ribosenucleic acid is used throughout, although some of the tests are only specific for pentoses. growth of the corn root. Of particular concern is the question of whether or not a quantitative relationship exists between RNA contents in the growing parts of the root and the rates of root elongation. Comparisons have been made of the growth rates and nucleic acid contents of the roots of 6 inbred lines of corn, and the changes in the rate of growth and in the contents of nucleic acids with time have been studied in a single line. More detailed analyses of the distribution of growth and nucleic acids in the roots, and studies of the effects of chemical substances such as IAA on the growth and nucleic acid contents will be presented in later papers. MATERIALS AND METHODS.-Seeds of inbred lines of Zea mays, var. W-158, A-374B, W-58, W-28, 8A, and W-148 were soaked in distilled water at room temperature for 2 hr., then placed, embryo down, on plexiglass strips wrapped in filter paper and allowed to germinate in Pyrex baking trays. The germination trays were filled with distilled water to a level that kept the filter paper moist without flooding the seeds, covered with glass plates to reduce evaporation, and kept in a dark room at 2627?C. and 85% relative humidity. The period of germination varied from 45 to 60 hr. depending on the line of corn used, because the time was selected which would furnish the largest number of plants with primary root lengths in the range from 12 to 18 mm. Roots were selected in such a way that the average root length at the start was very close to 15 mm. for each treatment. For the determinations of the rates of root growth, lots of 10 corn seedlings were placed on a cardboard cover over a 150-ml. beaker in such a way that the roots extended vertically through small holes into the water. The root lengths were determined to the nearest 0.5 mm. by placing the seedlings on a plastic ruler with the roots extended over the calibrated scale. The lengths of the 10 roots in each beaker were determined before and after a 3-hr. Deriod of [The JOURNAL for October (47: 609-711) was issued October 6, 1960] AMERICAN JOURNAI. OF BOTANY, Vol. 47, No. 9, November, 1960