Leaves of maize (Zea mays L.) seedlings were supplied with different concentrations of abscisic acid (ABA). Its effects on the levels of superoxide radical (O₂^–), hydrogen peroxide (H₂O₂) and the content of catalytic Fe, the activities of several antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), the contents of several non-enzymatic antioxidants such as ascorbate (ASC), reduced glutathione (GSH), α-tocopherol (α-TOC) and carotenoid (CAR), and the degrees of the oxidative damage to the membrane lipids and proteins were examined. Treatment with 10 and 100 µM ABA significantly increased the levels of O₂^– and H₂O₂, followed by an increase in activities of SOD, CAT, APX and GR, and the contents of ASC, GSH, α-TOC and CAR in a dose- and time-dependent pattern in leaves of maize seedlings. An oxidative damage expressed as lipid peroxidation, protein oxidation, and plasma membrane leakage did not occur except for a slight increase with 100 µM ABA treatment for 24 h. Treatment with 1,000 µM ABA led to a more abundant generation of O₂^– and H₂O₂ and a significant increase in the content of catalytic Fe, which is critical for H₂O₂-dependent hydroxyl radical production. The activities of these antioxidative enzymes and the contents of α-TOC and CAR were still maintained at a higher level, but no longer further enhanced when compared with the treatment of 100 µM ABA. The contents of ASC and GSH had no changes in leaves treated with 1,000 µM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 µM) induced an antioxidative defence response against oxidative damage, but a high concentration of ABA (1,000 µM) induced an excessive generation of AOS and led to an oxidative damage in plant cells.

The interrelationship among water‐stress‐induced abscisic acid (ABA) accumulation, the generation of reactive oxygen species (ROS), and the activities of several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) was investigated in leaves of detached maize (Zea mays L.) plants exposed to –0.7 MPa water stress induced by polyethylene glycol (PEG 6000). Time‐course analyses of ABA content, the production of ROS, and the activities of antioxidant enzymes in water‐stressed leaves showed that a significant increase in the content of ABA preceded that of ROS, which was followed by a marked increase in the activities of these antioxidant enzymes. Pretreatment with an ABA biosynthesis inhibitor, tungstate, significantly suppressed the accumulation of ABA, and also reduced the increased generation of ROS and the up‐regulation of these antioxidant enzymes in water‐stressed leaves. A mild oxidative stress induced by paraquat, which generates O₂^– and then H₂O₂, resulted in a significant enhancement in the activities of antioxidant enzymes in non‐water‐stressed leaves. Pretreatment with some ROS scavengers, such as Tiron and dimethylthiourea (DMTU), and an inhibitor of NAD(P)H oxidase, diphenyleneiodonium (DPI), almost completely arrested the increase in ROS and the activities of these antioxidant enzymes induced by water stress or ABA treatment. These data suggest that water stress‐induced ABA accumulation triggers the increased generation of ROS, which, in turn, leads to the up‐regulation of the antioxidant defence system.

Lodging-resistant rice (Oryza sativa) cultivars usually show slow grain filling when nitrogen is applied in large amounts. This study investigated the possibility that a hormonal change may mediate the effect of water deficit that enhances whole plant senescence and speeds up grain filling. Two rice cultivars showing high lodging resistance and slow grain filling were field grown and applied with either normal or high amount nitrogen (HN) at heading. Well-watered and water-stressed (WS) treatments were imposed 9 days post anthesis to maturity. Results showed that WS increased partitioning of fixed ¹⁴CO₂ into grains, accelerated the grain filling rate but shortened the grain filling period, whereas the HN did the opposite way. Cytokinin (zeatin + zeatin riboside) and indole-3-acetic acid contents in the grains transiently increased at early filling stage and WS treatments hastened their declines at the late grain filling stage. Gibberellins (GAs; GA₁ + GA₄) in the grains were also high at early grain filling but HN enhanced, whereas WS substantially reduced, its accumulation. Opposite to GAs, abscisic acid (ABA) in the grains was low at early grain filling but WS remarkably enhanced its accumulation. The peak values of ABA were significantly correlated with the maximum grain filling rates (r = 0.92**,P < 0.01) and the partitioning of fixed¹⁴C into grains (r = 0.95**,P < 0.01). Exogenously applied ABA on pot-grown HN rice showed similar results as those by WS. Results suggest that an altered hormonal balance in rice grains by water stress during grain filling, especially a decrease in GAs and an increase in ABA, enhances the remobilization of prestored carbon to the grains and accelerates the grain filling rate.