Genetic variation for fruit development of Pisum sativum L., with special reference to the effects of the rugosus locus
This study has compared various aspects of pea fruit development, using two green-podded lines near-isogenic except for the r locus and a yellow-podded mutant having the genotype gpgp. The gp gene is associated with reduced chlorophyll in the pod wall which then appears yellow. The r gene is associated with a wrinkled appearance of mature dried seed and cotyledonary starch grains which are small and fissured. An ultrastructural survey of tissues from pods, testas and cotyledons showed there to be no effect of the r locus on chloroplast structure. The structure of amyloplasts however, appeared to be affected by the r locus; starch grains in the cotyledons having a rugged outline in the wrinkled type. Chloroplasts from the inner tissues of the yellow pod were similar to those in leaves, green pods, green-podded testas and cotyledons. Chloroplasts in the yellow pod mesocarp and in the testa of yellow pods had dilated thylakoids, less starch and more lipid than the green-podded types. Growth analysis of the near-isogenic lines showed there to be no difference between the round (RR) and wrinkled (rr) lines in pod and testa growth, but embryo growth differed. The wrinkled embryo contained more water during development but had a lower final dry weight than the round embryos. Water potential (an indication of the osmotic regulation) was lower in the wrinkled embryos than in the round embryos, except very early in development when it was higher in the wrinkled embryos. In vitro culture of embryos showed that optimum growth was obtained in liquid media containing 10% sucrose as a carbon source. Replacing sucrose with mannitol determined that it was the sucrose which was important, not its resulting water potential. These gp and r gene effects and their relevance to future breeding programmes are discussed with other biochemical studies on similar genotypes.