Growth and phytohormonal status as affected by relative humidity

French bean (Phaseolus vulgaris L.) plants were grown for 25 days under controlled environmental conditions at low (50%) and high (95%) relative humidity (RH) at night. Growth parameters (leaf area, shoot and root length, and dry weight) affected by the low RH increased significantly, whereas, at the high RH, an opposite response was recorded. RH-treated plants also differed in their phytohormonal status. Declining activity of auxins (leaf, stem, root) and rising activities of gibberellin-like compounds (leaf) and cytokinins (leaf, stem, root) at 50% RH were accompanied by an earlier initiation of flowering by 4-5 days. The results indicate that the growth/flowering responses to RH at night can be mediated through changes in the activities of endogenous phytohormones. The hypothetical role of the free energy balance of moving water in modifications of hormonal status is discussed.