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

Introduction From genes to stomata, (+)-ABA plays an important Abscisic acid (ABA) is involved in many of the responses of plants to environmental stress. This role in stress physiology (Davies and Jones, 1991; Munns and Cramer, 1996). (+)-ABA concentrations usually study focuses on the inhibitory effect of ABA on leaf expansion. In addition, the effects of (+)-ABA, the increase when the plant is exposed to a stress, particularly stresses which disturb water relations (Munns and Sharp, natural form of ABA, were compared to the effects of (−)-ABA. Leaf elongation rates (LER) were measured 1993). Some positive effects that (+)-ABA provides to the plant are reduced stomatal conductance and maintenfor the 3rd leaf of maize plants. ABA concentrations were measured by RIA for total ABA and an ELISA ance of root growth under stress (see reviews by Munns and Cramer, 1996; Munns and Sharp, 1993). In addition, specific for (+)-ABA. ABA was added to the hydroponic solution and changes in LER were measured over time. (+)-ABA-induced gene products may play a role in stress tolerance, but to date there is no direct evidence to ABA could inhibit LER within 30 min and reached steady-state LER within 4 h. Internal ABA concentrasupport this (Chandler and Robertson, 1994). Under many circumstances, (+)-ABA appears to tions in the growing zone of the leaf also reached steady-state concentrations after 4 h. This effect of inhibit plant growth; in particular, ABA often inhibits leaf expansion (Dale, 1988; Dodd and Davies, 1996; ABA was reversible, because LER was fully restored upon removal of externally applied ABA, and internal Munns and Cramer, 1996; Munns and Sharp, 1993). This concentrations of ABA in the growing zone returned is an appropriate strategy for a plant which is exposed to to normal levels, whereas ABA concentrations drought, because a reduced leaf surface area will reduce remained elevated in mature tissue. Thus, steady-state water loss and improve the plant’s chances for survival. LER was highly correlated with the steady-state On the other hand, the reduction in leaf expansion may internal ABA concentration of the growing zone. ABA not benefit salt-stressed plants (there is usually no shortinhibited leaf expansion by increasing the apparent age of water, at least under irrigated conditions). In fact, cell wall yield threshold; no other growth parameters the major limitation of growth to moderately salt-stressed were affected. The (−)-enantiomer of ABA had much maize plants is restriction of leaf expansion (Cramer et less effect on LER than (+)-ABA when compared upon al., 1994). The increase of ABA concentration in plants an external concentration basis. Internal ABA concenby moderate salt-stress (He and Cramer, 1996) and water trations rationalized the response, showing that stress (Dodd and Davies, 1996; Munns and Sharp, 1993) (−)-ABA accumulation was very low, most likely due is correlated with reduced leaf expansion. to low uptake rates. From this analysis, it was deterIn a review of how drought, salinity and temperature mined that LER was equally sensitive to internal conlimit cell expansion (Cramer and Bowman, 1993), it was centrations of (+)or (−)-ABA. found that there were no universal mechanisms of control. The mechanisms of control vary with genotype and the stress imposed. For example, the response of the cell wall