Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO2-free air for 3 hours with either 1% 02 in N2 or N2-only and then returned to normal air of 340 microliters per liter C02, 21% 02 in N2. Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the C02-free treatments as was photosynthetic CO2 fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N2 only treatment. During the C02-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO2-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO2-free treatments. cient 02 is present to support photorespiration (15). We have examined effects on the activity of Rubisco and on leaf RuBP levels under conditions reported to cause photoinhibition. Wheat and maize seedings, 7 d old, were illuminated in CO2-free atmospheres for 3 h. To our surprise, photoinhibition was induced only in wheat when the seedlings were exposed to high light with N2 only, free of CO2 and 02. Although Rubisco is partially deactivated in C02-free air, the major cause of photoinhibition of CO2 fixation was, most likely, the impaired ability of the photosystems to support RuBP regeneration rather than deactivation of Rubisco.Our earlier measurements of Rubisco activation were reported as percent activation, which was the ratio of the initial activity to the total activity times 100 (1 1, 12). Initial activity refers to the CO2 fixation activity of Rubisco in an extract, measured directly following grinding the leaf at 0C with rate-saturating levels of RuBP, C02, and Mg2' at 25°C. The Rubisco total activity is the CO2 fixation activity following full activation of Rubisco in CO2 and Mg2e for 10 min. The initial activity of Rubisco from wheat seedlings exposed to N2-only (no 02) was found to be near zero while the RuBP concentrations were at the apparent binding site levels of the enzyme (11). In those experiments we did not observe much change in the total activity. Recent experiments, especially under low CO2 and 02 levels, indicates that the total activity can change substantially.
MATERIALS AND METHODSLittle is known of the response of Rubisco2 activity in leaves exposed to C02-free atmospheres. The ability of 02 to support activation of Rubisco in the absence of CO2 was reported by Perchorowicz and Jensen (11). They suggested that 02 was involved in activation by turning over RuBP during photorespiration to form PGA and P-glycolate. This would cycle the NADPH and ATP and thereby maintain electron flow through the photosystems. Photorespiration could also supply cellular levels of CO2 needed to support activation of Rubisco (5).Illumination Gas excha...
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