An analysis of seed development in Pisum sativum .19. Effect of mutant alleles at the r and rb loci on starch grain size and on the content and composition of starch in developing pea seeds

The effects of mutant alleles at the r and rb loci on starch grain size and the levels of starch and amylose in developing pea (Pisum sativum L,) seeds have been examined, Four lines, near-isogenic except for genes at these loci, have been used to show that both mutations reduce levels of starch throughout embryo development and reduce levels still further when combined in the 'double mutant', The reduction in starch content was due, at least in part, to a reduction in starch granule size, Although the proportion of starch in mature embryos was similar in the rrRbRb and RRrbrb lines, the starch content differed between these two lines during development, as a percentage of embryo dry weight, This difference was due to a reduction in the absolute growth of the embryo caused by the rb mutation, Lines homozygous for the mutant r allele with either wild-type (RbRb) or mutant (rbrb) alleles at the rb locus contained increased proportions of amylose in their starch throughout development, due to a reduced production of amylopectin. The presence of the rb mutation, however, also reduced the amount of amylose in relation to the reduction in total starch levels. Mutant alleles at both loci also reduced starch levels in the testa during development, the reduction due to rb being more extreme, Reciprocal crosses showed a maternal effect of the rb mutation on final seed size and on the absolute amount of starch in the embryo.