Reversibility of Photosynthetic Inhibition in Cotton after Long-Term Exposure to Elevated CO(2) Concentrations.

Cotton (Gossypium hirsutum L. cv Stoneville 213) was grown at 350 and 1000 microliters per liter CO(2). The plants grown at elevated CO(2) 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 CO(2).A group of plants grown at 1000 microliters per liter CO(2) was switched to 350 microliters per liter CO(2). Starch pools and photosynthetic rates were monitored in the switched plants and in the two unswitched control groups. Photosynthetic rates per unit leaf area recovered to the level of the 350 microliters per liter CO(2) grown control group within four to five days. To assess only nonstomatal limitations to photosynthesis, a measure of photosynthetic efficiencies was calculated (moles CO(2) fixed per square meter per second per mole intercellular CO(2)). Photosynthetic efficiency also recovered to the levels of the 350 microliters per liter CO(2) grown controls within three to four days.Recovery was correlated to a rapid depletion of the starch pool, indicating that the inhibition of photosynthesis is primarily a result of feedback inhibition. However, 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 accumulations and that photosynthetic rates could recover to near the theoretical capacity during periods of high photosynthate demand.

Photosynthetic inhibition after long-term exposure to elevated levels of atmospheric carbon dioxide.

The effect of long-term exposure to elevated levels of CO2 on biomass partitioning, net photosynthesis and starch metabolism was examined in cotton. Plants were grown under controlled conditions at 350, 675 and 1000 µl l(-1) CO2. Plants grown at 675 and 1000 µl l(-1) had 72% and 115% more dry weight respectively than plants grown at 350 µl l(-1). Increases in weight were partially due to corresponding increases in leaf starch. CO2 enrichment also caused a decrease in chlorophyll concentration and a change in the chlorophyll a/b ratio. High CO2 grown plants had lower photosynthetic capacity than 350 µl l(-1) grown plants when measured at each CO2 concentration. Reduced photosynthetic rates were correlated with high internal (non-stomatal) resistances and higher starch levels. It is suggested that carbohydrate accumulation causes a decline in photosynthesis by feedback inhibition and/or physical damage at the chloroplast level.