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  • 1 Climate Stress Laboratory, ARS, U. S. Department of Agriculture, Beltsville, MD 20705-2350

Roots exposed to drying soil have been shown to generate non-hydraulic signals which can be communicated to the shoot. Such `root signals' can cause an inhibitory effect on leaf growth without causing detectable water deficits in the shoots. Plants grown in restricted root zone volumes also typically show a reduction in leaf and shoot growth. Although water stress and root restriction both impair growth, their effects on photosynthesis, leaf initiation, and C, N, and P metabolism may be quite different. Abscisic acid (ABA) has been shown to be produced in the roots after only mild dehydration and to play a major role in signal transduction from the roots to the shoots. Whether root-restricted plants are capable of generating `root signals' such as ABA or other plant hormones, which can be communicated to the shoot, remains to be determined. The application of new tools, such as gas chromatography/mass spectrography for hormone analysis, nuclear magnetic resonance imaging, and photoacoustic spectroscopy, should help to identify the nature of `root signals' generated during root restriction and clarify their regulatory role in shoot behavior.

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