ATP SYNTHESIS DURING PHOTOSYNTHETIC INDUCTION STUDIED WITH A VERY HIGH TIME RESOLUTION

Subcellular ATP/ADP ratios as well as 3-phosphoglycerate (PGA) and tri osephosphate (TP) levels were determined in barley protoplasts using r apid fractionation by membrane filtration. When the protoplasts were i lluminated, ATP production started virtually momentarely. The ATP/ADP ratio in the chloroplasts increased from 1 to 4 within 2 s. Also, the export of ATP to the cytosol was very rapid giving a dramatic increase in the cytosolic ATP/ADP ratio within a few seconds, the maximum was reached after about 15 s. In contrast to ATP production, about one min ute elapsed until oxygen evolution started and the maximum rate was re ached after four to five minutes. The ATP/ADP ratios in the chloroplas ts and the cytosol stayed very high until photosynthetic oxygen evolut ion started and then decreased gradually when the photosynthetic rate reached its maximum. In photorespiratory and non-photorespiratory cond itions the subcellular ATP/ADP ratios were very similar during photosy nthetic induction, also in both cases PGA decreased and triosephosphat es increased in the chloroplasts as well as in the cytosol. In steady- state the subcellular ATP/ADP ratios were higher in photorespiratory c onditions as compared to non-photorespiratory conditions.