The role of water in the regulation of plant development

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Abstract
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.