CHARACTERIZATION OF A WILTY SUNFLOWER (HELIANTHUS-ANNUUS L) MUTANT .2. WATER RELATIONS, STOMATAL CONDUCTANCE, ABSCISIC-ACID CONTENT IN LEAVES AND XYLEM SAP OF PLANTS SUBJECTED TO WATER DEFICIENCY

The response of w-1, a wilty sunflower (Helianthus annuus L.) mutant, to water stress is described in comparisonwith the control line (W-1). Detached leaves of w-1 strongly dehydrated during the first 30 min wi thout significant changes in leaf conductance, whereas W-1 responded r apidly to water loss by reducing stomatal aperture. After 2h stress AB A increased slightly in w-1, while W-1 leaves showed a 20-fold increas e. When water stress was imposed to potted plants by water withholding , w-1 quickly dehydrated, and lost turgor, while W-1 maintained positi ve turgor values for a longer period. Wild-type plants responded to sm all changes in leaf water potential by accumulating ABA and by closing stomata, whereas in the mutant significant changes in ABA content and in stomatal conductance were found only at very low water potentials. in another experiment in which water was withheld under high relative humidity, when soil water content started to decrease W-l rapidly clo sed stomata in the absence of any change in leaf water status and the reduction in conductance was paralleled by a rise in xylem sap ABA con centration. By contrast the mutant started to accumulate ABA in the xy lem sap and to close stomata when soil water content and leaf water po tential were dramatically reduced. The low endogenous ABA levels and t he inability to synthesize the hormone rapidly either in the leaves or in the roots seem to be responsible for the high sensitivity of w-1 t o water stress.