GENETIC, ANATOMICAL AND BREEDING STUDIES OF SHATTER-RESISTANCE IN RAPESEED (BRASSICA NAPUS L. AND B. CAMPESTRIS L.) (SILIQUA STRENGTH, INTERSPECIFIC HYBRIDIZATION, MUTATION BREEDING)

The thesis study involved the development of a laboratory test of shatter-resistance of rapeseed for use in basic studies of the mechanism of shattering, of its inheritance and of breeding for shatter-resistance. The test developed involves loading the siliqua in a testing machine as a cantilever with its replum in the horizontal plane. The strength of the siliqua was measured as the maximum bending moment at, and the energy absorbed by the siliqua up to, dehiscence. Field experiments have shown a reasonably close positive correlation between shatter-resistance in the field and siliqua strength. Anatomical studies suggested a passive dehiscence mechanism and that shatter-resistance of the B. campestris ssp. oleifera vars. Yellow Sarson and Brown Sarson is due to the absence of abscission layers in the sutures of the siliqua. However, there did not appear to be discrete anatomical differences in the zone of dehiscence of the siliqua between the intermediate-shattering and shatter-susceptible accessions of Brassica. Genetical studies in B. campestris using the Sarsons as shatter-resistant parents, in crosses with cv. Torch, indicated dominance of shatter-susceptibility, measured as siliqua strength, and the possible presence of 2-3 major genes controlling shattering. Selection studies indicated little genetic variation for shatter-resistance within cv. Midas (B. napus). Recovery of high siliqua strength plants in the backcross F(,2) populations of crosses between cv. Torch (shatter-susceptible) and the Sarsons (shatter-resistant) revealed the possibility of transferring shatter-resistance to it using simple backcrossing. Both hybridization of B. napus with the Sarsons and mutation breeding using cv. Midas produced variation for siliqua strength embracing intermediate levels of shatter-resistance. Further breeding and selection using these sources of variation may provide a good potential for developing shatter-resistant B. napus cultivars.