The Influence of Light Quality on Levels of Ahscisic Acid in Tomato Plants, and Evidence for a Novel Abscisic Acid Metabolite

Tomato plants were grown in growth cabinets under two different light sources. One source consisted of white and red fluorescent tubes (Red cabinet), while the second consisted of both tungsten filament and fluorescent lamps (Far red cabinet). Energy fluence rates were adjusted to give equal photosynthetic rates in the two cabinets. Extension growth was approximately three times greater in the Far-red cabinet. No difference could be detected in abscisic acid (ABA) levels in leaves or petioles of plants grown under the two light regimes, but levels were 40–90% greater in stems of the slower growing plants illuminated with fluorescent light only. Exogenously applied ABA effectively reduced the growth rate of the plants in the Far-red cabinet to that of plants in the Red cabinet. However, it was shown that light-induced changes in growth rates occurred before any change in endogenous ABA could be demonstrated, thus precluding a role for ABA in the initial response to a change in light quality. The changed ABA levels appeared to be the result of a modified pathway of ABA degradation. Evidence is presented for a novel metabolite of ABA which yields free ABA on basic hydrolysis.