Comparison of spectrophotometric and radioisotopic methods for the assay of Rubisco in ozone‐treated plants

Radioisotopic and spectrophotometric assays for ribulose‐1,5‐bisphosphate carboxy‐lase/oxygenase (Rubisco) initial and final activities and Rubisco content were compared in plants chronically exposed to ozone (O 3 ) in a greenhouse and the field. In a greenhouse experiment, Glycine max was treated in exposure chambers with either charcoal‐filtered air (CF air) or 100 nl O 3 1 −1 for 6 h daily during vegetative growth. Samples were collected after 7 days of exposure. In a field experiment, G. max was treated in open‐top chambers with either CF air or nonfiltered air with O 3 added at 1.5 times ambient O 3 for 12 h daily. Average daily O 3 concentrations were 21 and 92 nl T1 in the CF and O 3 treatments, respectively. Samples were collected during vegetative and reproductive growth. Both assays generally yielded comparable Rubisco initial and final activities for greenhouse‐grown plants regardless of the O 3 treatment. However for field‐grown plants, Rubisco initial and final activities averaged 15 and 23% lower when assayed by the spectrophotometric rather than the radioisotopic method. For Rubisco content estimated by the spectrophotometric method, lower r 2 values for the regression of Rubisco activity vs concentratio of carboxyarabinitol‐1,5‐bisphos‐phate were observed in O 3 than in CF‐treated plants. Both assays yielded comparable Rubisco contents in the greenhouse and in the field although the variation was larger with the spectrophotometric method in field‐grown plants. Growth conditions, field vs greenhouse, were more critical to the spectrophotometric assay performance than the O 3 treatments for measurement of Rubisco activity and content.