Superoxide dismutase activity in C3 and C3/CAM intermediate species of Clusia

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The C3-CAM intermediate Clusia minor L. and the C3 obligate Clusia multiflora H.B.K. plants were exposed for 7 d to a combination of drought stress and high irradiance of about 1200 μmol m−2 s−1 for 12 h per day. In both species under these conditions a strong decrease in stomatal conductance was observed at dawn and dusk. Changes in stomatal behaviour of C. minor were accompanied by only a low nocturnal accumulation of malate and citrate. Thus, in C. minor drought stress applied in combination with high irradiance limited CAM expression, and possibly this is the main reason why C. minor prefers semi-shaded sites in the field. The mitochondrial MnSOD, in both well watered and stressed plants of two species showed strong diurnal oscillations with maximum activity at dusk. These oscillations can be explained by the engagement of mitochondria in dissipation of an excess of reducing equivalents. In plants which are able to carry out CAM metabolism tricarboxylic acid cycle is expected to be down regulated in the dark period to prevent breakdown of the entire malate and citrate.

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