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 without significant changes in leaf conductance, whereas W-1 responded rapidly to water loss by reducing stomatal aperture. After 2h stress ABA increased slightly in w-1, while W-1 leaves showed a 20-fold increase. When water stress was imposed to potted plants by water withholding, w-1 quickly dehydrated, and lost turgor, while W-1 maintained positive turgor values for a longer period. Wild-type plants responded to small 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 closed 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 concentration. By contrast the mutant started to accumulate ABA in the xylem sap and to close stomata when soil water content and leaf water potential were dramatically reduced. The low endogenous ABA levels and the 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 to water stress.