Photosynthetic induction phenomena in spinach chloroplasts in relation to the nature of the isolating medium

1. Measurements were made of photosynthetic CO₂ fixation and O₂ evolution by spinach chloroplasts isolated in sorbitol media containing 2-(N-morpholino)ethanesulphonate (MES).

2. The chloroplasts isolated in MES-sorbitol media exhibited induction phenomena which were similar to those shown by chloroplasts isolated in orthophosphate-sugar mixtures. Added ribose 5-phosphate shortened the lags which preceded the attainment of maximal rates of CO₂ fixation and O₂ evolution. O₂ evolution reached its maximum rate almost immediately in the presence of 3-phosphoglycerate. Induction periods were shortened by pre-illumination of the parent tissue prior to separation of the chloroplasts.

3. In the absence of added substrate (other than CO₂) lags exhibited by chloroplasts isolated in MES-sorbitol were shorter than those observed with chloroplasts prepared in orthophosphate-sorbitol. These shorter lags could be extended by briefly exposing the chloroplasts to sugar media containing orthophosphate, malate or acetate or to Tris-NaCl.

4. The results are discussed in relation to photosynthetic induction phenomena and current methods of chloroplast isolation.