Reversibility of Photosynthetic Inhibition in Cotton after Long-Term Exposure to Elevated CO2 Concentrations

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Abstract
Cotton (Gossypium hirsutum L. cv. Stoneville 213) was grown at 350 and 1000 .mu.l per liter CO2. The plants grown at elevated CO2 concentrations contained large starch pools and showed initial symptoms of visible physical damage. Photosynthetic rates were lower than expected based on instantaneous exposure to high CO2. A group of plants grown at 1000 .mu.l per liter CO2 was switched to 350 .mu.l per liter CO2. Starch pools and photosynthetic rates were monitored in the switched plants and in the 2 unswitched control groups. Photosynthetic rates per unit leaf area recovered to the level of the 350 .mu.l per liter CO2 grown control group within 4-5 days. To assess only nonstomatal limitations to photosynthesis, a measure of photosynthetic efficiencies was calculated (moles CO2 fixed m-2 s-1 per mol-1 intercellular CO2). Photosynthetic efficiency also recovered to the levels of the 350 .mu.l l-1 CO2 grown controls within 3-4 days. Recovery was correlated to a rapid depletion of the starch pool, indicating that the inhibition of photosynthetic is primarily a result of feedback inhibition. Complete recovery may involve the repair of damage to the chloroplasts caused by excessive starch accumulation. The rapid and complete reversal of photosynthetic inhibition suggests that the appearance of large, strong sinks at certain developmental stages could result in reduction of the large starch accumulation and that photosynthetic rates could recover to near the theoretical capacity during periods of high photosynthetic demand.