The effect of humidity, root excision, and potassium supply on hypocotyl elongation in dark-grown seedlings of Helianthus annuus

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Abstract
When seedlings of H. annuus L. were grown in the dark with their roots in vermiculite saturated with distilled water the rate of elongation of the hypocotyl was significantly increased by increasing the relative humidity around the shoot from .apprx. 25-100%. This response was correlated with a reduction in transpiration rate of .apprx. 95% and with increases in the water potential and cell turgor in the growing region. Measurements with a transducer revealed very rapid growth responses to changes in humidity, usually preceded by a variable period of growth oscillations. Excision of the roots, either in water or in air, induced an immediate increase in rate of elongation at low humidity, but at high humidity this response was delayed and markedly reduced. The growth rate was significantly increased by supplying 10 mM KCl to the roots at both high and low humidity. The response to K was slower than the response to humidity and was correlated with a significant reduction in the osmotic potential of the growing region. A growth response was first detected .apprx. 45 min after the application of K to the roots and 10 min after application to the shoot. These results are consistent with the hypothesis that, in the intact plant, stem elongation is largely controlled by the interacting effects on cell turgor of transpiration-induced negative pressure potential in the apoplast and the osmotic potential of the growing cells.
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