Photosynthetic characteristics of light-sensitive, chlorophyll-deficient leaves from sectorially chimeric stinging-nettle

Mutated leaves of sectorially chimeric stinging nettle (Urtica dioica L.) showed decreased pigment content and reduced photosynthetic activity. Relative to their wild type siblings, the mutated leaves were not only largely depleted in total chi and car, suggestive of a significant loss of photosynthetic units, they also exhibited much higher chi a/b ratios, indicative of a major reduction in the antenna size of the remaining photosynthetic units. Light and electron microscopy confirmed a notable decrease in chloroplast number in mutated leaves, and analysis of polypeptide composition revealed a large depletion of the apoprotein of PSII antenna in these chloroplasts. The photosynthetic units present in mutated leaves showed an intrinsic photochemical efficiency, measured as the variable to maximum fluorescence ratio of dark-adapted leaves, only slightly lower than controls, and their concentration correlated strongly with the leaves's net CO2 uptake capacity. It is concluded that mutated leaves of stinging nettle underwent a gradual chloroplast loss during development, but the remaining organelles preserved much of their photosynthetic competence. This chloroplast loss most likely arises from an accumulation of chi precursors that repress the synthesis of photosynthetic essential proteins and act as photosensitizers for chloroplast degradation.