Macromolecular physiology of plastids XIV.Viridis mutants in barley: Genetic, fluoroscopic and ultrastructural characterisation

Abstract

A sample of 42viridis mutants of barley has been localised by diallelic crosses to 32 nuclear genes. They have been grouped into five different categories on the basis of their fluorescence induction kinetics; this was used as a rapid method for the determination of their photosynthetic capacities. The mutants within each category were further characterised by low temperature fluorescence emission spectroscopy, chlorophyll content, visible fluorescence under UV light, viability, and chloroplast ultrastructure as seen by thin-section electron microscopy. The photosystem I-type mutants have a high initial fluorescence, variable fluorescence, but no fluorescence decline. They fluoresce under UV light and are seedling lethals, but in general have well-developed thylakoid systems. Photosystem II-type mutants, which have a high initial fluorescence, no variable fluorescence, and no fluorescence decline, fluoresce brightly under UV light and are all seedling lethals. Their chloroplast ultrastructure is characterised by giant grana, but as a group, their chlorophyll content is low. The nature of the photosynthetic defects of mutants in the third category is not known, but they are unusual in having a steady-state fluorescence lower than the initial fluorescence. Many of these mutants have a near-normal chloroplast ultrastructure, high chlorophyll content, and are viable under favourable conditions. The fourth group of mutants have fluorescence induction kinetics similar to those of the wild-type, and they are not considered to have major photosynthetic defects, as might be expected from the fact that many will survive under suitable growth conditions. The fifth category contained mutants with extremely low levels of chlorophyll under the growth conditions used.The low temperature fluorescence emission peak at long wavelengths is in part due to antennae chlorophyll of photosystem I. In the mutants deficient in photosystem I, this peak was present, but its wavelength was shifted from 739 nm (wild-type) to 729–731 nm. However, photosynthetically competent mutants with reduced amounts of chlorophyll also showed similar or greater wavelength shifts, and it is concluded that at present, fluorescence emission spectra can not be used to predict the nature of the photosynthetic defects. Similarly, it was not possible to discern a usable relationship between chloroplast ultrastructure and specific photosynthetic deficiencies.