Seed storage proteins provide a source of amino acids and reduced N necessary for germination and early growth of the seedling. Because the long term aim of much of the current research in this area is to modify the composition of the storage protein fraction, it is of interest to ask what kinds of changes might be tolerated by the developing seed without affecting this physiological role. For example, glycosylation and many of the post-translational modifications seen in some legume storage proteins may not be essential and major alterations in the relative amounts of the component proteins in the storage protein fraction are also tolerated. Some nutrient deficiencies result in very extensive changes in this latter category and nutrient deficient plants provide a useful tool for the study of some of the cellular mechanisms that regulate the composition of the storage protein fraction. Sulphur deficiency and potassium deficiency have contrasting effects on the relative proportions of legumin and vicilin in pea seeds. These changes are mainly the result of altered levels of their respective mRNAs together with a change in the pattern of synthesis and accumulation of these two proteins during seed development.