CHLOROPHYLL BREAKDOWN IN SENESCENT LEAVES - IDENTIFICATION OF THE BIOCHEMICAL LESION IN A STAY-GREEN GENOTYPE OF FESTUCA-PRATENSIS HUDS

Chlorophyll breakdown in senescent leaves proceeds in essentially thre e steps: dephytylation by the action of chlorophyllase; conversion of chlorophyllide to phaeophorbide by Mg-dechelatase; and oxygenolytic cl eavage of the chlorin-macrocycle by a newly discovered dioxygenase. Th e metabolic lesion responsible for high retention of chlorophyll durin g foliar senescence in a mutant genotype of meadow fescue (Festuca pra tensis Huds.) was located in the third step of the breakdown pathway. Senescent leaves of both the normally yellowing reference genotype, c. v Rossa, and the non-yellowing mutant Bf993 were shown to be competent with regard to chlorophyllase and Mg-dechelatase. On the other hand, thylakoids isolated from senescent leaves of cv. Rossa were able to ca rry out oxygenolysis of phaeophorbide into a colourless fluorescent ca tabolite in vitro, whereas Bf993 thylakoids were deficient in this act ivity. It is concluded that the Sid locus, a mutant allele of which is responsible for the stay-green character, encodes or regulates the ge ne for, phaeophorbide a dioxygenase.