The use of abscisic acid (ABA)-deficient mutants has contributed to elucidating the connection between the capacity to synthesize ABA and the drought response of plants. This review summarizes the present state of knowledge of two ABA-deficient mutants, recently isolated in sunflower (Helianthus annuus L.): nondormant-1 (nd-1) and wilty (w-1). The former is an albino mutant defective in carotenoid biosynthesis, inducào uyin vino tissue cullure. This mutation causes photobleaching or colorless pig."nt., absence of seed dormancy, and inability to accumulate ABA in cotyledons and leaves. Among albino mutants of dicotyledon species, nd-1 is the first mutânt characterized by ABA deficiency. The latter (w-1) is a genotype in which a spontaneous mutation confers a wilty phenotype. The wilting conditiôn of the nr-1 mutant is due to abnormal stomatal behaviour, associâted with low levels of endogenous ABA. Exogenou. agÀ rreatments can induce a phenotypic reversion of the mutant. Partial phenotypic reversion was also observed in mutânt scions grafted onto wild-type rootstocks. Detached leaves of w-1 were strongly dehydrated with a slight ABA increase. When water stress was imposed to potted mutant plants, significant changes in ABA content in stomal conductance were found only at very low water potentials. M: Moreover,w-1 started to accumulate ABA in the xylem sap and to close stomata when soil water content and leaf water potential were dramatically reduced. The results suggest thât the low endogenous AIIA levels and the inability to synthesize the hormone rapidly either in the leaves or in the roots are responsible for the high susceptibility of w-1 to water stress