Composition of the Photosynthetic Apparatus of Normal Barley Leaves and a Mutant Lacking Chlorophyll b

Abstract

Comparison of the composition of the energy‐trapping apparatus of normal barley photo‐synthetic membranes and those lacking chlorophyll b shows that not only is chlorophyll b absent from the mutant, but all constituents (chlorophyll a, chlorophyll b, carotenoids and the protein moiety) of the major chlorophyll‐protein complex of normal higher plant chloroplast membranes are also missing; other chlorophyll‐containing components, as far as can be ascertained, are essentially unaffected by the mutation. The nuclear gene which codes for the protein moiety of this complex is suggested as a possible site of the mutation in this barley mutant. This chlorophyll α/β‐protein complex which accounts for such a large proportion of the chlorophyll and protein of the photosynthetic apparatus of higher plants is thus not essential for a plant to grow photosynthetically, and therefore a new name, light‐harvesting chlorophyll‐protein, is proposed for the complex, which was formerly termed the photosystem II chlorophyll‐protein. The biosynthetic machinery in the mutant is unable to compensate for the loss of the light‐harvesting chlorophyll‐protein by adding extra chlorophylls into alternative sites in the membrane, thus the chlorophyll/P700 ratio in the mutant is two‐thirds that of the normal plant. The light‐harvesting chlorophyll‐protein is the major location for chlorophyll b in higher plant membranes. The function of the complex is thought to be analogous to that of the algal biliproteins, i.e. light energy absorbed by the complex is fed preferentially, but not exclusively, to the photosystem II reaction‐center or trap. It is reasoned that the complex exerts a strong influence on the organization of photosynthetic lamellae in higher plant chloroplasts by maintaining lamellae in contact with each other.