O-3-INDUCED DEGRADATION OF RUBISCO PROTEIN AND LOSS OF RUBISCO MESSENGER-RNA IN RELATION TO LEAF AGE IN SOLANUM-TUBEROSUM L

The effect of ozone (O-3) on Rubisco degradation was investigated by dark incubation of potato plants (Solanum tuberosum L. cv. Norland) following exposure to charcoal-filtered air or 0.30 mu l l(-1) O-3 for 6 h. Rubisco small subunit (rbcS) mRNA levels declined dramatically in control foliage within 10-16 h of dark incubation; thus declines in Rubisco protein following a 48-h dark period were ascribed to proteolytic degradation. Foliage sampled from the 4th and 6th leaves from the apex, designated immature and mature, respectively, showed no effect of O-3 on Rubisco protein content immediately following the exposure. However, the decline in Rubisco quantity during the dark incubation was significantly enhanced by prior treatment with O-3 in the mature leaves. The immature leaves sustained a similar decline in Rubisco quantity in both O-3 and control treatments. O-3 had a significant effect on the relative quantity of rbcS mRNA in the immature leaves (sampled immediately following the O-3 exposure). Levels of rbcS mRNA in mature leaves and Rubisco large subunit (rbcL) mRNA in both leaf ages were not significantly affected by O-3. There was no effect of O-3 on Rubisco quantity in immature or mature leaves of plants maintained in the greenhouse under a 16 h photoperiod for 48 h following the exposure. Thus the effect of shortterm O-3 exposure on processes affecting Rubisco synthesis and degradation may have been transient, and a more prolonged exposure would be necessary to effect a decline in Rubisco protein quantity in plants grown under a 16 h photoperiod. We concluded that O-3 caused enhanced degradation of Rubisco in mature leaves of plants induced to senesce by dark incubation following O-3 exposures. The potential for O-3-induced reduction in synthesis of Rubisco also exists.