CHLOROPLAST ULTRASTRUCTURE AND FLUORESCENCE RESPONSE OF OILSEED RAPE CONTAINING MALE-STERILE RADISH CYTOPLASM

The production of hybrid seed is facilitated if one parent possesses a male-sterile cytoplasm. Introduction of the cytoplasm of male-sterile radish (Raphanus sativus L.) into rapeseed (Brassica napus L.) result s not only in transfer of the desirable male-sterile trait but induces a chlorophyll defect in the backcrossed male-sterile plants. In this study we show that the defect manifests itself in two different ways i n the alloplasmic plants: a) smaller and fewer chloroplasts with an im paired ultrastructure and b) an increase in chlorophyll fluorescence. Defective chloroplasts were characterized by a reduction in both the n umber and size of grana, the latter due to poor stacking of thylakoids and with frequent discontinuity in the intergranal thylakoid systems. The changed chloroplast morphology and the increase in chlorophyll fl uorescence are probably the cause of the lowered photosynthetic effici ency associated with the alloplasmic plants. We propose that the defic iency is the result of incompatibility between the genomes of the radi sh chloroplast and the rapeseed nucleus. Supporting this hypothesis ar e studies of male-sterile rapeseed plants in which, by protoplast fusi on, the radish chloroplasts were substituted by those of normal male-f ertile rapeseed. Such plants showed complete restoration of their phot osynthetic potential and displayed both normal chloroplast ultrastruct ure and normal levels of chlorophyll fluorescence.