SEVERITY OF MUTANT PHENOTYPE IN A SERIES OF CHLOROPHYLL-DEFICIENT WHEAT MUTANTS DEPENDS ON LIGHT-INTENSITY AND THE SEVERITY OF THE BLOCK INCHLOROPHYLL SYNTHESIS

Analyses of a series of allelic chlorina mutants of wheat (Triticum ae stivum L.), which have partial blocks in chlorophyll (Chl) synthesis a nd, therefore, a limited Chl supply, reinforce the principle that Chl is required for the stable accumulation of Chl-binding proteins and th at only reaction centers accumulate when the supply of Chl is severely limited. Depending on the rate of Chl accumulation (determined by the severity of the mutation) and on the rate of turnover of Chl and its precursors (determined by the environment in which the plant is grown) , the mutants each reach an equilibrium of Chl synthesis and degradati on. Together these mutants generate a spectrum of phenotypes. Under th e harshest conditions (high illumination), plants with moderate blocks in Chl synthesis have membranes with very little Chl and Chl-proteins and membrane stacks resembling the thylakoids of the lethal xantha mu tants of barley grown at low to medium light intensities (which have m ore severe blocks). In contrast, when grown under low-light conditions the same plants with moderate blocks have thylakoids resembling those of the wild type. The wide range of phenotypes of Chl b-deficient mut ants has historically produced more confusion than enlightenment, but incomparable growth conditions can now explain the discrepancies repor ted in the literature.