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Control and Adjustment of the Rate of Photosynthesis Above Present CO(2) Levels.

DE2002762794

Publication Date	2002
Page Count	34
Abstract	The adjustment of photosynthesis to different environmental conditions and especially to elevated CO(sub 2) is often characterized in terms of changes in the processes that establish (limit) the net CO(sub 2) assimilation rate. At slightly above present ambient pCO(sub 2) light-saturated photosynthetic responses to CO(sub 2) depart limitation by the catalytic capacity of tissue rubisco content. An hypothesis attributing this departure to limited thylakoid reaction/electron transport capacity is widely accepted, although we find no experimental evidence in the literature supporting this proposition.. The results of several tests point to the conclusion that the capacity of the thyiakoid reactions cannot be generally responsible for the deviation from rubisco limitation. This conclusion leaves a significant gap in the interpretation of gas exchange responses to CO(sub 2). Since the inputs to the photosynthetic carbon reduction cycle (CO(sub 2) and photon-capture/electron-transport products) do not limit photosynthesis on the shoulder of the A=f(c(sub i)) curve, the control of photosynthesis can be characterized as: due to feedback. Several characteristics of gas exchange and fluorescence that occur when steady-states in this region are perturbed by changes in CO(sub 2) or O(sub 2) suggest significant regulation by conditions other than directly by substrate RuBP levels. A strong candidate to explain these responses is the triose-phosphate flux/ inorganic phosphate regulatory sequence, although not all of the gas exchange characteristics expected with 'TPU-limitation' are present (e.g. oxygen-insensitive photosynthesis). Interest in nitrogen allocation between rubisco and light capture/electron transport as the basis for photosynthetic adjustment to elevated CO(sub 2) may need to be reconsidered as a result of these findings. Contributors to the feedback regulation of photosynthesis (which may include sucrose phosphate synthase and fructose bisphosphatase activities, phloem loading, and 'sink-strength') may play a large role in the adjustment of photosynthesis to elevated CO(sub 2). The continuing rise in atmospheric CO(sub 2) elevates the need to understand the regulation of photosynthesis that is not related to rubhsco capacity.
Keywords	Carbon dioxide Photosynthesis Plants Ecological concentration Fructose Phrosphates
Source Agency	Technical Information Center Oak Ridge Tennessee
Corporate Authors	Desert Research Inst., Reno, NV.; Department of Energy, Washington, DC.
Supplemental Notes	Sponsored by Department of Energy, Washington, DC.
Document Type	Technical Report
NTIS Issue Number	200220
Contract Number	DE-FG03-95NV62026

Control and Adjustment of the Rate of Photosynthesis Above Present CO(2) Levels.

Control and Adjustment of the Rate of Photosynthesis Above Present CO(2) Levels.
DE2002762794

Publication Date	2002
Page Count	34
Abstract	The adjustment of photosynthesis to different environmental conditions and especially to elevated CO(sub 2) is often characterized in terms of changes in the processes that establish (limit) the net CO(sub 2) assimilation rate. At slightly above present ambient pCO(sub 2) light-saturated photosynthetic responses to CO(sub 2) depart limitation by the catalytic capacity of tissue rubisco content. An hypothesis attributing this departure to limited thylakoid reaction/electron transport capacity is widely accepted, although we find no experimental evidence in the literature supporting this proposition.. The results of several tests point to the conclusion that the capacity of the thyiakoid reactions cannot be generally responsible for the deviation from rubisco limitation. This conclusion leaves a significant gap in the interpretation of gas exchange responses to CO(sub 2). Since the inputs to the photosynthetic carbon reduction cycle (CO(sub 2) and photon-capture/electron-transport products) do not limit photosynthesis on the shoulder of the A=f(c(sub i)) curve, the control of photosynthesis can be characterized as: due to feedback. Several characteristics of gas exchange and fluorescence that occur when steady-states in this region are perturbed by changes in CO(sub 2) or O(sub 2) suggest significant regulation by conditions other than directly by substrate RuBP levels. A strong candidate to explain these responses is the triose-phosphate flux/ inorganic phosphate regulatory sequence, although not all of the gas exchange characteristics expected with 'TPU-limitation' are present (e.g. oxygen-insensitive photosynthesis). Interest in nitrogen allocation between rubisco and light capture/electron transport as the basis for photosynthetic adjustment to elevated CO(sub 2) may need to be reconsidered as a result of these findings. Contributors to the feedback regulation of photosynthesis (which may include sucrose phosphate synthase and fructose bisphosphatase activities, phloem loading, and 'sink-strength') may play a large role in the adjustment of photosynthesis to elevated CO(sub 2). The continuing rise in atmospheric CO(sub 2) elevates the need to understand the regulation of photosynthesis that is not related to rubhsco capacity.
Keywords	Carbon dioxide Photosynthesis Plants Ecological concentration Fructose Phrosphates
Source Agency	Technical Information Center Oak Ridge Tennessee
Corporate Authors	Desert Research Inst., Reno, NV.; Department of Energy, Washington, DC.
Supplemental Notes	Sponsored by Department of Energy, Washington, DC.
Document Type	Technical Report
NTIS Issue Number	200220
Contract Number	DE-FG03-95NV62026