Submitted to: Photosynthetica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2003
Publication Date: 1/1/2004
Citation: Bunce, J.A., Sicher Jr, R.C. 2004. Daily irradiance and feedback inhibition of photosynthesis at elevated carbon dioxide concentration in Brassica oleracea. Photosynthetica. 41:481-488. Interpretive Summary: The concentration of carbon dioxide in the atmosphere is increasing exponentially. A higher concentration of carbon dioxide often increases the rates of photosynthesis and growth of plants only temporarily. Reasons why the increase in photosynthesis is only temporary are being sought so that new crop varieties can be developed which will yield better as atmospheric carbon dioxide continues to rise. In this study, it was found in collards that it was the accumulation of too much sugar in leaves which slowed photosynthesis at elevated carbon dioxide. Too much leaf sugar only occurred after sunny days, and was caused by the plants not being able to use all of the extra sugar resulting from stimulated photosynthesis at elevated carbon dioxide. A very close relative, kohlrabi, which has an enlarged stem in which to store the extra sugars produced, grows better at higher carbon dioxide than collards. This information will be of interest to scientists developing varieties of crop plants for expected future environmental conditions.
Technical Abstract: The fundamental cause of down-regulation of photosynthesis at elevated carbon dioxide concentration is thought to be a slower rate of utilization of carbohydrates than their stimulated rate of production, but there are few studies directly supporting this idea. We hypothesized that down-regulation would not occur in kohlrabi because it has a large sink for carbohydrates in an enlarged stem, but would occur in collards, which lack this sink. Field tests were consistent with this hypothesis. In collards, the degree of down-regulation of photosynthesis in plants grown at elevated carbon dioxide varied depending on the solar radiation of the day prior to the measurement of photosynthetic capacity, as did leaf carbohydrate content. Elevated carbon dioxide did not result in lower leaf contents of chlorophyll or soluble protein, nor was there any evidence of a triose phosphate utilization rate limitation of photosynthesis. Experiments in controlled environment chambers confirmed that down-regulation of photosynthesis only occurred above a minimum level of radiation. The degree of down-regulation of photosynthesis was highly correlated with leaf glucose and fructose contents, and less well correlated with starch or sucrose contents. The results suggest that down-regulation of photosynthesis at elevated carbon dioxide in collards in the field represented feedback inhibition from the accumulation of soluble carbohydrates and that day-to-day variation in its occurrence was predictable from the weather