Abstract
A heat girdle, applied to the base of flag leaves of pearl millet, caused a rapid decline in stomatal conductance (
g
s). Stomata began to close within minutes of girdling (with
g
s reaching a minimum after about 5 min) but then partly reopened before closing again 20\2-30 min from the start of treatment. The initial closing response could be deferred and initially counteracted by enclosing the leaf in a polyethylene bag. Cell death in the girdled zone appeared to be necessary for the response as mechanical pressure alone was ineffective. Only stomata «downstream» of a girdled zone (relative to the direction of xylem flow) were affected by the treatment; there was no lateral or 《backward》 transmission of a closing stimulus. No immediate differences between control and girdled leaves could be detected in bulk leaf water potential or in abscisic acid content. The rapid effects of girdling on
g
s are thus ascribed to a transient, localised disturbance in epidermal water relations probably induced by a temporary interruption in xylem flow.