Abstract
Six nuclear gene mutants of barley, heat-sensitive for chloroplast development, are described. These conditional lethal mutants in six different genes can be grown as homozygous viable plants in the field, their mutant phenotype only being expressed with development at high temperature. Chlorophyll accumulation in the mutants but not in the wild type is inhibited by a growth temperature of 31°C. The degree of temperature sensitivity varies amongst the mutants. For example, partial inhibition of chlorophyll production is evident in the mutantvir-zf ts4 above 20° while inhibition is complete at a growth temperature of 29°. The mutantvir-zi ts49 remains green at 29° but is bleached at 31°. The reduction in chlorophyll content of the leaves at high growth temperatures is accompanied by the appearances of structural abnormalities in the chloroplasts and reduced photochemical activity per mg chlorophyll. The inhibitory effect of elevated temperatures is confined to some early stage of plastid development as the light-dependent conversion of etioplasts into chloroplasts in the mutants is not inhibited at 32°. Leaf elongation in the mutants compared with the wild type is not affected by high growth temperatures. When the mutants are grown at temperatures permitting normal chlorophyll accumulation, the chloroplast thylakoid membranes appear to be no less heat stable than in the wild type.
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