GROWTH RETARDANT ACTIVITY OF PACLOBUTRAZOL ENANTIOMERS IN WHEAT SEEDLINGS

The resolved enantiomers of paclobutrazol appeared to have different p rimary modes of action as plant growth retardants in rht3 (tall) wheat seedlings. 2S,3S-Paclobutrazol reduced shoot growth more effectively than root growth, whereas the opposite was the case with the 2R,3R-ena ntiomer. Low concentrations (0.03-1.0 mu M) of 2S,3S-paclobutrazol spe cifically inhibited gibberellin A(1) (GA(1)) production in Rht3 (dwarf ) seedlings without affecting shoot growth, confirming that inhibition of GA biosynthesis is the primary mode of action of this enantiomer. Reductions in shoot growth of rht3 (tall) wheat treated with 2S,SS-pac lobutrazol were associated with reductions in GA(1) content, an effect that could be reversed by gibberellic acid (GA(3)) application, showi ng that GAs are important regulators of light-grown shoot growth in wh eat. The inhibition of root growth of wheat seedlings following treatm ent with 2R,3R-paclobutrazol was associated with a decline in de novo synthesis of major sterols, a decrease in stigmasterol: sitosterol rat io and an accumulation of the 14 alpha-methyl sterol, obtusifoliol. Co ncentrations >3 mu M 2S,3S-paclobutrazol also affected de novo sterol production in wheat roots, suggesting that root growth is more respons ive to interference with sterol than GA biosynthesis. There was a decl ine in abscisic acid (ABA) content in Rht3 (dwarf) shoots treated with relatively high concentrations of 2S,3S-paclobutrazol but no effect w ith its optical isomer.