Effects of low concentrations of ozone on the fine structure of radish leaves

Radish (Raphanus sativus L.) plants were fumigated with 0.1 or 0.05 μl l^-l O₃ for 8 or 24 h a day for 6 to 18 d and the leaf tissues examined by light and electron microscopy. Ultrastructural damage was apparent in the leaves fumigated with as low as 0.05 μl l^-1 for 8 h a day for 6 d. Ozone induced an increase in both the number and size of the plastoglobules but a decrease in chloroplast dimensions. These changes in the chloroplasts developed further even after O₃ fumigation had been discontinued. The plastoglobules were electron dense in the early stages of exposure to O₃ but subsequently became electron translucent. Finally large plastoglobules were extruded into the vacuole, a phenomenon which may partly account for the reduction in chloroplast size. Ozone also caused disruption of the tonoplast and this was followed by collapse of the cells. Low concentrations of O₃ appear to accelerate senescence of the chloroplasts.