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

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