KINETICS OF MAIZE LEAF ELONGATION IV - EFFECTS OF (-ABSCISIC-ACID 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 A BA 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 conce ntrations 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 reach ed steady-state LER within 4 h, Internal ABA concentrations in the gro wing 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 concen trations remained elevated in mature tissue, Thus, steady-state LER wa s highly correlated with the steady-state internal ABA concentration o f the growing zone. ABA inhibited leaf expansion by increasing the app arent cell wall yield threshold; no other growth parameters were affec ted, The (-)-enantiomer of ABA had much less effect on LER than (+)-AB A when compared upon an external concentration basis, Internal ABA con centrations rationalized the response, showing that (-)-ABA accumulati on was very low, most likely due to low uptake rates. From this analys is, it was determined that LER was equally sensitive to internal conce ntrations of (+)- or (-)-ABA.