DEVELOPMENT OF THERMOLUMINESCENCE BANDS DURING GREENING OF WHEAT LEAVES UNDER CONTINUOUS AND INTERMITTENT ILLUMINATION

Abstract— Mature wheat leaves excited by 1-min illumination at a low temperature of -60° C showed five thermoluminescence bands at -45, -10, +25, +40 and +55° C (denoted as Z_u, A, B₁ B₂ and C bands, respectively). The development of these bands during greening of etiolated wheat leaves under continuous and intermittent illumination was investigated, and the following results were obtained. (1) Etiolated leaves showed only the C band. When these leaves were greened under continuous light, the B₁ and B₂ bands appeared at 3 h and the Z_u band appeared at 10 h. The B₁ and B₂ bands were intensified during prolonged greening under continuous illumination to be the strong bands observed for mature leaves. The A band and the group of B₁ and B₂ bands were alternative: Similar experiments by excitation of thermoluminescence at -20° C showed the development of the A band instead of these B₁ and B₁ bands. (2) When the etiolated leaves were greened under intermittent illumination (1-ms light + 5-min dark), the Z_u band first appeared after 5 h of illumination (60 flashes) and was gradually intensified during further illumination with 340 flashes but, interestingly, neither the B₁ nor the B₂ band appeared even after 24–28 h of illumination with 280–340 flashes. (3) On exposure of such leaves greened under intermittent illumination to continuous light, the B₁ and B₂ bands were rapidly developed. The appearance of these bands was accompanied by the generation of the Hill activity (DCIP photoreduction with water as electron donor). (4) These results were discussed in relation to the previous study on photoactivation of the latent water-splitting system accumulated in the leaves greened under intermittent illumination. It was deduced that the structure responsible for the A band or the group of B₁ and B₂ bands is involved in the evolution of oxygen in chloroplasts, and probably involves cations of the Mn²⁺ catalyst generated by the action of light.