CO(2) Photoassimilation by the Spinach Chloroplast at Low Temperature.

Spinach chloroplasts were used to study the relationship between photosynthetic CO(2) fixation and temperature from 30 to -15 degrees C. In saturating light and high concentrations of CO(2), the temperature coefficients (Q(10)) above 20 degrees C were less than 2 in the intact chloroplast. Below 15 degrees C, the Q(10) values were greater than 2 and gradually increased with decreasing (down to 0 degrees C) temperature to approximately 4.4. Photosynthesis responded similarly to temperature in a reconstituted chloroplast preparation fortified with ribose 5-phosphate. In the intact chloroplast, temperature did not alter the Q(10) value in low light and high CO(2). Elevating the temperature to 25 degrees C after photosynthesizing at -15 degrees C (46 minutes) or 0 degrees C (17 minutes) restored the temperature-depressed photosynthetic rate without a lag in the intact chloroplast to the rate of a chloroplast continually at 25 degrees C. At 0 degrees C, the intact chloroplast photosynthetic rate responded slightly to the inorganic phosphate concentration (0.1-1.0 millimolar) and to pH (7.0-8.6). Relative to 25 degrees C, the levels of ribulose 1,5-bisphosphate and glycerate 3-phosphate were increased 1300 and 200%, respectively, whereas glycolate decreased 57% during intact chloroplast photosynthesis at 0 degrees C. Chilling temperature impeded the transport of photosynthetic intermediates from the stromal compartment to the external medium. Ethylene glycol was shown to be an appropriate additive to prevent freezing of the reaction mixture down to -15 degrees C for photosynthetic CO(2) assimilation.