a compound of formula : wherein : r 1 is — sr 10 , — o — c — r 11 , — nr 12 r 13 , where r 10 is a c 1 - 8 - alkyl group or a phenyl group unsubstituted or substituted by a c 1 - 4 - alkyl group , r 11 is a thiophenenyl , furanyl or pyrrolyl group , r 12 is h or a c 1 - 4 - alkyl group and r 13 is a c 1 - 8 - alkyl group or a phenyl group unsubstituted or substituted by a c 1 - 4 - alkyl group ; r 2 is h or a c 1 - 4 - alkyl group ; r 3 and r 4 are independently h or c 1 - 4 - alkyl groups ; r 5 and r 6 are independently h , oh or or 14 , or taken together are ═ o , where r 14 is a protecting group ; r 7 is h or a c 1 - 4 - alkyl group ; and , r 8 is h , r 9 is oh and r 15 is h , or r 15 is h and r 8 and r 9 taken together are — o —, or r 9 is oh and r 8 and r 15 taken together form a bond ; and , r 18 and r 19 are both h , or r 18 and r 19 taken together form a bond , or a plant physiologically acceptable salt thereof is useful for inhibiting 9 - cis - epoxycarotenoid dioxygenase in a plant or seed and is therefore useful for regulating aba biosynthesis in the plant or seed .