The effect of red light on the aging progression of the bean (Phaseolus vulgaris L.) hypocotyl segment unit was examined in relation to dwarfism using Kentucky Wonder (tall) and Masterpiece (dwarf) varieties. In both plants, red light promoted the elongation of younger zones and inhibited that of mature zones. The zone exhibiting maxmum elongation was shifted to the younger zones by red light irradiation regardless of the plant type, but its extent was greater in the dwarf than in the tall. Thus, red light hastens both the beginning of elongation in the younger portion and its termination in the mature portion of the hypocotyl, particularly of the dwarf plant. These red light responses in each zone of both the tail and dwarf hypocotyl units were reversed by subsequent exposure to far red light regardless of the duration and intensity of red Ught, thus indicating that the hastened aging progression of the hypocotyl by red light is medinted by phytochrome. However, there is no difference in the rate of decay of Pfr between the tall and dwarf hypocotyls.The increased expression of bean dwarfism seems to result from a concerted action of red light upon the maturation of younger portions of the hypocotyls, which is more rapid in the dwarf than in the tall, and upon the elongation of the relatively matured portion, which is more severely inhibited in the dwarf.We have reported that dwarf expression in the early growth of bean seedlings may be a reflection of a difference in gibberellin content between the tall and dwarf varieties (6,8 (1,3,5,11,14,15). These findings suggest a relationship between the dwarf expression and physiological aging of a shoot system. The present study demonstrates the involvement of a light-mediated tissue aging in the dwarfism of beans.MATERIALS AND METHODS Two bean cultivars, a tall (Phaseolus vulgaris L. cv. Kentucky Wonder) and a dwarf (P. vulgaris cv. Masterpiece), were used for comparison in all experiments. The general procedures followed that described previously (7): the seed coat was cracked carefully near its hilum with a razor blade in order to ensure an even water absorption rate between the two plant types, and the seeds were germinated in vermiculite moistened by Knop's solution at 25 C in the dark for 5 days. Hypocotyl units were prepared from the etiolated seedlings selected for 6 to 7 cm in height (Fig.