Abstract. Deuterium‐labelled ABA‐aldehyde was fed to various tomato genotypes. Normal and notabilis mutant plants incorporated substantial amounts of the label into ABA. In contrast, two ABA‐deficient mutants, flacca and sitiens, reduced ABA‐aldehyde to a mixture of cis‐ and trans‐ABA alcohol rather than oxidizing it to ABA. It was concluded that ABA‐aldehyde is the immediate precursor of ABA in higher plants. It appears that the flacca and sitiens lesions both act to block the last step of the ABA biosynthetic pathway. The mutant gene loci are likely to be involved in coding for different sub‐units of the same dehydrogenase enzyme.

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SUMMARYThe metabolism of deuterium-labelled analogues of ABA by normal and flacca mutant tomato plants was investigated. Comparison of the biological activity of ABA, ABA alcohol, ABA aldehyde and their 1-trans isomers was made in both mutant and non-mutant genotypes. While in normal plants ABA alcohol and ABA aldehyde were as effective as ABA in inducing stomatal closure, in the^acca mutants only ABA itself was biologically active. Both ABA alcohol and ABA aldehyde were converted to the inactive compound trans-ABA alcohol instead of ABA when fed to fiacca plants. As trans-ABA aldehyde was also readily converted to trans-ABA alcohol hy flacca plants, it was not possible to establish whether isomerization precedes reduction or vice versa in the synthesis of trans-ABA alcohol from ABA aldehyde.