| Summary: | The effects of root cooling treatments on leaf growth of seedlings of <i>Phaseolus vulgaris</i> L. were examined. Plants were germinated in vermiculite and then grown with their root systems cooled to 10<SUP>o</SUP>C or 15<SUP>o</SUP>C, or kept at the same temperature as the serial parts (23<SUP>o</SUP>C). Leaf elongation rates, recorded continuously using Rotary Variable Displacement Transducers, were shown to be very sensitive to root cooling treatments, and at 10<SUP>o</SUP>C, elongation rates fell to zero growth within 10 min after start of treatment. When root systems were returned to 23<SUP>o</SUP>C, leaf elongation rates increased almost immediately to overshoot the control rates within 5-10 min, subsequently declining to control values over the next 50-60 min. Leaf turgor pressure fell at the beginning of the root cooling treatment and was believed to cause the instantaneous growth reduction, and to trigger abscisic acid (ABA) synthesis in leaves. Recovery in bulk leaf turgor occurred within 3-4 h from the commencement of root cooling and remained virtually constant throughout the experiment. No detectable changes in leaf turgor were observed when root systems were returned to 23<SUP>o</SUP>C. A rise in abscisic acid occurred in rapidly expanding leaves of <i>Phaseolus vulgaris, Hordeum vulgare</i> L. and <i>Helianthus annuus</i> L. Root cooling treatment did not lead to a rise in leaf ABA content in the fully expanded first leaf of <i>H.vulgare</i>. In <i>H.annuus</i> as well as in <i>H.vulgare</i> the levels of abscisic acid declined as leaves aged. Analysis of abscisic acid present in xylem sap of <i>P.vulgaris</i> seedlings showed that the amount of this hormone coming from the roots was insufficient to account for the observed rise in leaf ABA. However, the responses involving high concentration of abscisic acid in the leaves, may be indirectly mediated by ABA coming from the root systems.
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