RELATION BETWEEN THE DEVELOPMENT OF ROOT SYSTEM AND SHOOT UNDER LONG- AND SHORT-DAY ILLUMINATION

Abstract

The relative development of roots and tops of 8 spp. grown under 7-hr. and 15-hr. daily illumination, respectively, was determined. The plants were grown for about 7 weeks in soil in 30 X 30 X 80 cm. containers which permitted of rather normal root development. Red clover, radish, iris, and oats, all long-day plants as regards flowering, developed large tops and proportionately extensive root systems under the 15-hr. day; under short-day illumination the growth of both tops and roots was greatly retarded and approximately to the same degree. Their development was similar to that of the long-day plants when the latter were only 3.5 weeks old. Sunflower, whose time of flow ering is less modified by the length of day, developed in a manner similarly. Dahlia, the great ragweed, and cosmos, all short-day plants, attained their greatest size and greatest root development under the 15-hr. day; under short-day illumination the dwarfed tops were furnished with a correspondingly meager absorbing sys tem, although more food was accumulated in the short- day dahlia. Thus in all cases development of the root systems was in direct correlation with the development of tops. The effect of length of daily illumination on photosynthetic activity showed that red clover, radish, and sunflower are either more active in photosynthesis or less active in removing or using the product under long-day illumination than they are under a 7-hour day. Great ragweed and dahlia were about equally active photosynthetically under the 2 light periods, or less active in removal or use of the products under short-day illumination.