MECHANICAL DAMAGE RESISTANCE OF SNAP BEAN (PHASEOLUS-VULGARIS L) SEEDS - PROTECTION OF THE EMBRYONIC AXIS BY THE COTYLEDONS

Mechanical damage resistance was studied in three genotypes of snap be an (Phaseolus vulgaris L.) seeds, a white-seeded commercial variety an d two near isogenic lines (white and dark-seeded) selected for mechani cal damage resistance. The effect of impact at a velocity of 30 mis, w as investigated on eleven specific locations on the seed surface. Aver aged over genotype, impacts at the top, front or side of the embryonic axis lobe reduced germination move than corresponding impacts at the chalazal lobe, while impacts on the back of either lobe had little det rimental effect. Impacts on the front or top of the axis had a greater deterimental effect on the commercial variety than the dark-seeded br eeding line. Differential susceptibility of the genotypes to impact wa s studied at the morphological level. Coverage/protection of the embry onic axis by the cotyledons were allocated to categories by visual exa mination after removal of the seed coat, and the embryonic axis expose d area anti length was measured by an image analysis system. The comme rcial variety had a larger exposed axis area than the breeding lines. One mechanism responsible for the greater resistance to mechanical dam age in the breeding lines compared to the commercial variety was bette r embryonic axis protection by the cotyledons.