Influence of ageing on photo- and thermo-control of seed germination in grand rapids lettuce (Lactuca sativa L.).

Abstract

The reversible photoreaction of R (Red light) and FR (Far-red light) controlling the seed germination in Grand Rapids lettuce (Lactuca sativa L.) varies markedly not only with germination temperature, but also with the physiological state of seed caused by ageing. The photoreversibility in Grand Rapids lettuce seeds is more evident in the dormant (fresh) and deteriorating (60 and 66 months after harvest) seeds. The reversibility in the non-dormant seeds, however, can be induced again by a prolonged expooure to FR light or high (40°C) temperature pretreatment after the onset of imbibition. The velocity and the viability in germination of the seeds exposed to the prolonged FR light or heat-sensitized at 40°C for 48 hr maintained for a longer time than in the seeds having no exposure to FR light or heat-sensitization. The photoreversible induction by the prolonged exposure to FR light is due to the secondary photo-dormant stage of the seeds. The photo-reversible induction by a heat-sensitization after the onset of imbibition, on the other hand, is due to the two different physiological states of the seed; a, the secondary thermo-dormant stage caused at 40°C for 48 hr, and b, the deteriorating stage caused by the thermal injury at 40°C for 144 hr. Germination induction effects in the heat-sensitized seeds with high temperature pretreatment after the onset of imbibition are greater in GA3 than in R light.