Application of 'traditional' breeding methods

Plant varieties have not been selected to date on the basis of nutritional qualities but there are wide natural variations that can be found in the gene pool of crop plants. Examples of where significant variations in the nutrient content of geno types have been documented include a:

• 2-fold variation in calciumconcentrationin beans [6]
• 4-fold variation in β-caroteneconcentrationsin broccoli [7]
• 4-fold variation in folatesin beetroot [8]
• 2–3 fold variation in iron andzinc levels in maize [9]

In the case of the pro-vitamin A carotenoids, plants provide highly variable amounts depending on their colour. Varieties of sweet potato may contain levels varying from 0.13mg to 11.3mg g^-1 dry weight of β-carotene [10]. Similar variations in levels can be found in carrots and cassava. In the case of the tomato, genes have been identified that are associated with high and low lycopene content. Incorporation of genes that increase lycopene content and or elimination of genes that decrease the lycopene content, can be achieved by pedigree selection and backcross programmes. Such techniques have produced hybrids with a three-or four-fold contentof lycopenein tomato fruits [11].