Aging progression involving dwarfism and its acceleration by red light in bean hypocotyls

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 maximum 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 tall and dwarf hypocotyl units were reversed by subsequent exposure to far red light regardless of the duration and intensity of red light, thus indicating that the hastened aging progression of the hypocotyl by red light is mediated 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.