The effect of 4-thiouridine (4SU) on plastids development of radish cotyledons was investigated in the early stage of greening up to 24hr-illumination.
1) Within 24hr of illumination, the formation of chlorophylls was reduced to 1/2 in the cotyledons treated with 0.25mM 4SU, to 1/5 with 0.5mM 4SU, as compared with the control.
2) Activity of photosystem I was first detectable after 30 min of illumination and the activity per plastids protein on a mg basis rapidly increased with the time of illumination. The activity of photosystem II could be detected after 2hr illumination, thereafter exhibiting a sigmoidal increase.
3) Activities of photosystem I and II, expressed on a chlorophyll basis, were that in both plastids isolated from 4SU treated cotyledons and untreated ones initially increased to a maximal activity upon illumination, and decreased slowly with the develoment of plastids. The maximum activity of photosystem I appeared in the earlier stage than that of photosystem II. The 4SU treatment caused a retardation of the appearance of maximal activity and a significant decrease in the photochemical activity.
4) Elongation of the dark incubation time of the seedlings induced the delay of the development of plastids in the untreated cotyledons, but the contrary promoted the development in the treated ones.