Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : III. Responses of Protoplasts and Intact...

Protoplasts and intact chloroplasts isolated from Agropyron smithii Rybd. were utilized in an effort to determine the limiting factor(s) for photosynthesis at supraoptimal temperatures. Saturated CO(2)-dependent O(2) evolution had a temperature optimum of 35 degrees C for both protoplasts and intact chloroplasts. A sharp decline in activity was observed as assay temperature was increased above 35 degrees C, and at 45 degrees C only 20% of the maximal rate remained. The temperature optimum for 3-phosphoglycerate reduction by intact chloroplasts was 35 degrees C. Above this temperature, 3-phosphoglycerate reduction was more stable than CO(2)-dependent O(2) evolution. Reduction of nitrite in coupled intact chloroplasts had a temperature optimum of 40 degrees C with only slight variation in activity between 35 degrees C and 45 degrees C. Reduction of nitrite in uncoupled chloroplasts had a temperature optimum of 40 degrees C, but increasing the assay temperature to 45 degrees C resulted in a complete loss of activity. Reduction of p-benzoquinone by protoplasts and intact chloroplasts had a temperature optimum of 32 degrees C when measured in the presence of dibromothymoquinone. This photosystem II activity exhibited a strong inhibition of O(2) evolution as assay temperature increased above the optimum. It is concluded that, below the temperature optimum, ATP and reductant were not limiting photosynthesis in these systems or intact leaves. Above the temperature optimum, photosynthesis in these systems is limited in part by the phosphorylation potential of the stromal compartment and not by the available reductant.