Cooling of the roots was shown here to cause marked and rapid inhibition of leaf growth in wheat seedlings, as in some other species. The root-shoot signal involved in this response was investigated. Displacement transducers were used for the simultaneous monitoring of leaf growth rate and leaf thickness in individual seedlings. Leaf water status was inferred from leaf thickness. Kinetics of changing water status were compared with those of changing leaf growth rate in the same seedling. Leaf water status was found to decline markedly beginning immediately after root cooling. This occurs because cooling reduces the hydraulic conductivity of roots almost to zero. Apparent leaf growth rate was extremely sensitive to small changes in leaf water status, and the initial decrease in leaf water status after root cooling was sufficient to cause cessation of leaf growth. Complications arising from leaf elasticity and other sources were considered. Prolonged root cooling caused a sustained depression in both leaf water status and leaf growth rate. The effects of root cooling on leaf thickness and growth rate were completely prevented if water was made freely available to the shoot. It is therefore concluded that hydraulic factors (signals) can explain the short-and (at least in part) the medium-term effects of root cooling on leaf growth rate in wheat.