Ozone-responsive proteins in a tolerant and sensitive clone of white clover (Trifolium repens)

Two clones of white clover (Trifolium repens L. cv. Regal), NC-R (O3-resistant) and NC-S (O3-sensitive), were used to investigate biochemical responses associated with O3 tolerance. Plants were exposed in CSTR chambers to a step-function O3 profile (minimum concentration of 50 nl liter−1, maximum concentration of 150 nl liter−1 ) for 3 days, 8 h day−1. No visible foliar symptoms were apparent on the sensitive genotype after fumigation. Total soluble leaf protein and the activity of superoxide dismutase isoenzymes either declined or was not affected by O3 in both NC-S and NC-R. Catalase activity increased by 15–20% in the clones. Two-dimensional gel electrophoresis demonstrated the differential, exposure-dependent accumulation of a small group (two to four) of O3-responsive proteins (ORPs). These polypeptides were not present at detectable levels in control plants. During the 3-day fumigation with O3 , the ORPs progressively accumulated in both clones; however, the increase in NC-S was delayed and protein content never attained comparable levels to those observed in NC-R. The proteins have apparent molecular masses of 21.5–23 kD and pIs in the range of 4.2–4.5. The identity of these polypeptides is not known.