Chloroplast Ultrastructure During Senescence and Regreening of Flax Cotyledons

SUMMARY The ultrastructural changes in chloroplasts of flax seedling cotyledons were followed during normal senescence. Removal of the growing shoot apex from seedlings with yellowing cotyledons induced a regreening of the cotyledons. The ultrastructure of the plastids was studied to determine whether the regreening was the result of formation of new chloroplasts or a restructuring of the senescing plastids. No evidence of chloroplast de novo synthesis or of chloroplast division was found whereas changes in thylakoid distribution and granal stacking were associated with regreening as was a reduction in osmiophilic plastoglobuli. An increase in plastid peripheral reticulum was found in both normally senescing tissue and in cotyledons of detopped seedlings.