Photosynthesis and Ribulose 1,5-Bisphosphate Levels in Intact Chloroplasts

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  • RJRichard G. Jensen
    Richard G. Jensen
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Abstract
The response of ribulose 1,5-bisphosphate levels and CO2 fixation rates in isolated, intact spinach [Spinacia oleracea cv. Viroflay] chloroplasts to pyrophosphate, triose phosphates, DL-glyceraldehyde, O2, catalase and irradiance during photosynthesis was studied. Within 1 min in the light, a rapid accumulation of ribulose bisphosphate was measured in most preparations of intact chloroplasts, and this subsequently dropped as CO2 fixation increased. Pyrophosphate, triose phosphates and catalase increased CO2 fixation and also the levels of ribulose bisphosphate. CO2 fixation was inhibited by DL-glyceraldehyde and O2 with corresponding decreases in ribulose bisphosphate. When the rate of photosynthesis decreased at limiting irradiances (low light), the level of ribulose bisphosphate in the chloroplast did not always decrease, suggesting that ribulose bisphosphate was not limiting CO2 fixation under these conditions. When triose phosphates (fructose bisphosphate plus aldolase) were added to suspensions of chloroplasts at low irradiances, ribulose bisphosphate increased while CO2 fixation decreased. These observations provide considerable evidence that high ribulose bisphosphate levels clearly are not solely sufficient to permit rapid rates of CO2 fixation, but that factors other than ribulose bisphosphate concentrations are overriding the control of photosynthesis. Isolated chloroplasts are capable of using C reserves to produce considerable ribulose bisphosphate. Upon illumination in the absence of CO2 and O2, intact chloroplasts produced up to 13 mM ribulose bisphosphate.

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