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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.

It is postulated that the changes in growth and metabolic activity that characterize many features of plant development are produced by the interaction between the hydraulic conductivity of the tissues and the influence of environmental factors, transpiration and osmotic gradients on the uptake, distribution, or loss of water. Mechanisms based on this concept are proposed for six aspects of development. These are (i) seed germination and stem elongation (water uptake), (ii) apical dominance and tropisms (water distribution), and (iii) leaf senescence and abscission (water loss). The postulated mechanisms are based on the hypothesis that water is the factor that normally limits the rate of growth and metabolic activity in the intact plant. The validity of the concept is supported by the unique capacity of water to perform three basic functions. These functions are (i) to mediate environmental effects on growth and metabolism, (ii) to correlate the growth of different parts of the plant, and (iii) to integrate growth and metabolic activity at the cellular level. It is also suggested that the responses induced by the application to plants of certain growth-regulating substances can be mainly attributed to the capacity of these substances to promote the uptake of water.

The effects of various environmental conditions on bud development in seedlings of Agropyron repens L. Beauv. were investigated. The bud in the axil of leaf 1 was the most responsive. At a high nitrogen level (315 p.p.m.) most of the buds at this position developed as tillers, and at a low level (2.1 p.p.m.) as rhizomes. A transition from rhizome to shoot development was induced by increasing the nitrogen supply after the rhizomes were initiated. Temperature had a similar effect, tillers being produced at a high (day) temperature (27 °C) and rhizomes at a low temperature (10 °C). Reducing the daylength from 18 to 9 h strongly promoted tillering and almost completely prevented rhizome development. A reduction of light intensity from 4000 to 2000 ft-c did not affect bud development but appeared to increase the tendency for rhizomes to form a terminal shoot. Similar effects were shown by the buds at the coleoptile and leaf 2 positions but the inherent tendency for the former to develop as rhizomes and the latter as tillers limited their response.There was some evidence from these results that bud and rhizome development is controlled by a similar mechanism and that the carbohydrate level may be an important factor in determining the inherent pattern of bud development.