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