To understand the plant response to oxidative stresses, we studied the influence of magnesium (Mg++) deficiency on the formation of hydrogen peroxide (H2O2), malondialdehyde (MDA), and protease activity in kidney bean plants. The expression pattern of proteins under Mg++ deficiency also was examined via two-dimensional electrophoresis. The formation of H2O2 and MDA increased in the primary leaves of plants grown in a nutrient solution deficient in Mg++. Protease activity in Mg++-deficient plants was also higher than in those grown with sufficient Mg++. The expression pattern of the proteins showed that 25 new proteins were generated and 64 proteins disappeared under Mg++-deficient conditions. Therefore, a deficiency in Mg++ may cause oxidative stress and a change in protein expression. Some of these proteins may be related to the oxidative stress induced by Mg++ deficiency.

The response to oxidative stress induced by magnesium deficiency in kidney bean plants

The influence of a deficient (7μM) or sufficient (1000μM) supply of magnesium (Mg2+) on the content of Mg2+, chlorophyll and protein, and the activity of superoxide dismutase (SOD), ascorbate peroxidase (AP) and glutathione reductase (GR) were studied in cucumber (Cucumis sativus L.) and kidney bean (Phaseolus vulgaris L.) plants over an 11-day period. Tolerance of kidney bean to paraquat and oxyfluorfen dependent injury were also determined with intact primary leaves. Mg2+ content in the primary leaves of cucumber and kidney bean plants grown in nutrient solution with sufficient Mg2+ supply was increased, while that in the leaves of plants grown with insufficient Mg2+ was very low. Mg2+ deficiency caused more severe chlorosis and protein reduction in cucumber than in kidney bean. The activities of AP, GR and SOD of Mg2+-deficient cucumber leaves were greatly enhanced compared to those of Mg2+-sufficient leaves. In Mg2+-deficient kidney bean leaves, AP and SOD activities were enhanced, but GR activity was decreased. The results suggested that Mg2+ deficiency enhances the antioxidative enzymes in plants. Foliar application of paraquat and oxyfluorfen, active oxygen-generating herbicides, to kidney bean plants caused herbicidal damage. However, the chlorosis and water loss of kidney bean by the herbicides were much lower in Mg2+-deficient leaves than in Mg2+-sufficient ones. The data demonstrated that Mg2+-deficient kidney bean plants became highly tolerant to paraquat and oxyfluorfen injury.

Influence of Magnesium Deficiency on Induction of Activity of Antioxidative Enzymes and Increased Tolerance to Active Oxygen-Generating Herbicides in Cucumber and Kidney Bean Plants.

The relationship between protoporphyrin IX (Proto IX) levels and herbicidal activity of diphenyl ether herbicides in intact higher plant Lemna paucicostata Hegelm. strain 441 was determined. Rapid accumulation of Proto IX in the plant was observed after simultaneous exposure to light and oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene]. At 4hr, the accumulation reached the maximum and this was followed by a rapid decrease. The rate of decrease was slowed when the plants were transferred to darkness after 4hr light irradiation, although some Proto IX degradation continued in the dark. This suggests that not only photo-decomposition but degradation and/or metabolism which occurred in darkness are involved in the decrease. Proto IX levels accumulated in the plants and electrolyte leakage from the plants were increased with increasing concentration of oxyfluorfen. A positive correlation was found between Proto IX content and electrolyte leakage. Diphenyl ether herbicides used in the study had differential activity in the accumulation of Proto IX. A positive correlation was also found between Proto IX levels and their herbicidal activity to the plant.These data suggest that Proto IX is the principal compound responsible for the rapid electrolyte leakage from plants, and that variation in the herbicidal activity of the diphenyl ethers observed in the study is caused by differential activity in accumulation of Proto IX.

Shuuichi Kojima

Papers

Protoporphyrin IX Accumulation in Lemna paucicostata Hegelm. Caused by Diphenyl Ether Herbicides and Their Herbicidal Activity.