The behaviour of the 32 kDa QB protein of the chloroplast thylakoid membrane was investigated in senescing leaves of Festuca pratensis. Bf 993, a non-yellowing Festuca mutant, is known to be unable to degrade at least some of its thylakoid proteins during senescence. A high rate of QB protein turnover under illumination was observed in both Bf 993 and a normally yellowing cultivar. In the dark, however, the QB protein of Bf 993 exhibited the unusual stability of other thylakoid proteins that have been investigated in this genotype, whereas the normal cultivar was able to degrade its QB protein at an appreciable rate. These observations suggest that two independent processes exist for the degradation of the QB protein, one of which is impaired in mutant genotype Bf 993.
