Effects of CO2 and O2 on Photosynthesis and Growth of Autotrophic Tobacco Callus

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Mean inhibition of net photosynthesis in autotrophic tobacco callus by 21 and 40% O2 was 30 and 47%, respectively, similar to intact leaves. Increasing CO2 concentrations (500-2000 microliters per liter) produced a steady decline in percent inhibition at both O2 concentrations, indicating that O2 inhibition resulted primarily from photorespiration. Net photosynthetic rate was plotted as a function of CO2 concentration at 1, 21, and 40% O2 for calculation of kinetic constants. Values for Vmax were similar at all O2 concentrations (x̄ = 5.27 μmol CO2 per gram fresh weight per hour), indicating that O2 inhibition of net photosynthesis was fully reversible by CO2. To determine whether CO2 and O2 produced similar effects on growth, autotrophic callus was incubated for three weeks in atmospheres of normal air, high CO2, high O2 and high CO2/high O2. Growth in high CO2 was nearly double that in normal air. High O2 decreased growth significantly relative to air, but growth in high CO2/high O2 was similar to that in air. Lack of CO2 reversal of growth inhibition by O2 indicates that prolonged exposure to high O2 results in toxicity arising from a nonphotorespiratory source. Growth of heterotrophic callus (2% sucrose), however, was not inhibited by 40 or 60% O2, suggesting that O2 toxicity in autotrophic callus results primarily from disruption of photosynthetic functions.

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