I postulate that certain of the effects of nitrate on plant development are mediated by the combination of an osmotic effect on water uptake and a nutritional effect on protein synthesis. This hypothesis is discussed with reference to effects on seed germination, apical dominance, lateral root initiation, flowering and leaf senescence. The postulated osmotic effect of nitrate is consistent with the well-established role of both nitrate and reduced forms of N as major osmotica in plant cells, and also with the similarity and interaction between developmental effects of nitrate and water. Evidence of the nutritional component of developmental effects of nitrate is provided by a comparison of responses induced by nitrate and by other osmotica of less nutritional significance. Carbohydrate has also been reported to influence development by a combination of osmotic and nutritional effects. The proposed hypothesis is a unifying concept which provides a similar physiological explanation for a wide range of diverse developmental responses that are usually attributed to the effect or interaction of different hormonal factors.