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.