Studies on Ultrastructure and Function of Photosynthetic Apparatus in Rice Cells : IV. Effect of low dose and intermittent fumigation of ozone on the ultrastructure of chloroplasts in rice leaf cells

Young rice plants (Oryza sativa L. cv. Koshihikari), which are 4 week old plants grown in a potting soil under natural light at 28/27°C in day/night, were intermittently fumigated with 0.1 ppm ozone for 4 h (AM 10-14)/day for 4 days, and the ultrastructural changes of chloroplasts in leaf cells were examined by electron microscopy. A visual symptom of small reddish brown flecks appeared on an abaxial leaf surface fumigated for 2 days. After fumigation for 3 days or more, the flecks increased in both size and number and also brightness of green leaves slightly diminished. On observation by electron microscopy, the first indication of ozone injury was swelling of the thylakoid membranes in chloroplasts of abaxial leaf cells fumigated for 2 days. The swelling began first at the outermost thylakoid in grana stacks and was followed by a distortion in the arrangement of internal thylakoids. After fumigation for 3 days or more, the changes of thylakoid membranes became serious and then vacuolization occurred in chloroplast stroma. In leaves which were fumigated for 4 days and afterward kept in a fumigation chamber for a day, destruction of chloroplast envelope was observed. The ultrastructural changes occurred first on the leaf tip area and then extended to the middle region. Ozone injury was more serious in abaxial leaf cells than in adaxial ones. A characteristic was no increase of osmiophilic globules in chloroplasts throughout this ozone fumigation.