Field water relations of a compass plant, Lactuca serriola L.

Abstract Field water relations of Lactuca serriola serriola and L. serriola integrifolia were examined. Leaf conductance to water vapour was high early in the morning and declined rapidly during the midday hours. Leaf water potentials decreased to their minima early in the morning and remained low all day. Afternoon recovery of leaf conductance occurred occasionally. Leaf conductance was shown to have a linear response to vapour concentration difference. No differences were seen between L. serriola serriola and L. serriola integrifolia. The pattern of diurnal gas-exchange activity appeared to be complemented by the pattern of intercepted solar irradiance which results from the compass plant leaf orientation observed in L. serriola.