Pretreatment with antioxidants decreases the effects of salt stress on chloroplast ultrastructure in rice leaf segments (Oryza sativa L.)

We investigated the kinds of active oxygen species leading to the destruction of chloroplast ultrastructure in salt-stressed rice plants. After the seedlings were grown for 3 wks, leaf segments (5 mm square) were cut from the middle portion of the 5th leaves. Leaf segments were incubated in 200 mM NaCl under dark or light conditions for 24 hr. The chlorophyll content in the leaf segments drastically decreased in light between 12- and 24 hr in 200 mM NaCl, but, no reduction was observed in the dark. In electron microscopic studies, 200 mM NaCl caused swelling of thylakoids and destruction of thylakoid membranes in light. On the other hand, no ultrastructural changes were observed under dark condition. In one experiment, leaf segments were incubated in 200 mM NaCl for 24 hr in light after preincubation with antioxidants for 12 hr in light. Pretreatment with ascorbate and benzoate, which scavenge H2O2 and (.) OH, respectively, effectively suppressed the reduction of chlorophyll content and the destruction of chloroplasts by NaCl in light. However, Tiron and DABCO, which scavenge O-2(-) and O-1(2), respectively, could not suppress the effects of salt stress in light. Fe-SOD activity was increased about eight time by salt stress (200 mM NaCl), but, catalase activity was reduced to 69% of the control and ascorbate peroxidase activity was not affected by NaCl. These results suggested that salt-induced injury in chloroplasts is dependent on light, and that H2O2 and (.) OH are responsible for the deleterious effects of salt stress on chlorophyll content and chloroplast ultrastructure.