Kinetics of maize leaf elongation IV. Effects of (+)- and (−)-abscisic acid

Abscisic acid (ABA) is involved in many of the responses of plants to environmental stress. This study focuses on the inhibitory effect of ABA on leaf expansion. In addition, the effects of (+)-ABA, the natural form of ABA, were compared to the effects of (−)-ABA. Leaf elongation rates (LER) were measured for the 3rd leaf of maize plants. ABA concentrations were measured by RIA for total ABA and an ELISA specific for (+)-ABA. ABA was added to the hydroponic solution and changes in LER were measured over time. ABA could inhibit LER within 30 min and reached steady-state LER within 4 h. Internal ABA concentrations in the growing zone of the leaf also reached steady-state concentrations after 4 h. This effect of ABA was reversible, because LER was fully restored upon removal of externally applied ABA, and internal concentrations of ABA in the growing zone returned to normal levels, whereas ABA concentrations remained elevated in mature tissue. Thus, steady-state LER was highly correlated with the steady-state internal ABA concentration of the growing zone. ABA inhibited leaf expansion by increasing the apparent cell wall yield threshold; no other growth parameters were affected. The (−)-enantiomer of ABA had much less effect on LER than (+)-ABA when compared upon an external concentration basis. Internal ABA concentrations rationalized the response, showing that (−)-ABA accumulation was very low, most likely due to low uptake rates. From this analysis, it was determined that LER was equally sensitive to internal concentrations of (+)- or (−)-ABA.