ADDING the genes of broccoli, cabbage or cauliflower to those of rapeseed could yield an oil that can be used to make superstrong plastics. Presently, rape-seed oil yields only 66 per cent erucic acid and is expensive to purify. Researchers at the National Research Council of Canada's Plant Biotechnology Institute say adding broccoli or cauliflower genes can result in oils with more than 90 per cent erucic acid. The researchers say future products might include a high-temperature lubricant, a substance to prevent plastic film from sticking to itself and a tough, nylon-based plastic that might be used for car parts.