Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts

Chloroplasts were isolated from 10 d old wheat seedlings and illuminated at 5 °C or 25 °C in various concentrations of $PO43-$ . Photosynthetic oxygen evolution, ATP content, and export of triose phosphates and 3-phosphoglycerate were measured. Incorporation of ¹⁴C from NaH¹⁴CO₃ into pentose monophosphates, fructose monophosphate, and glucose monophosphates was determined. The ATP content in illuminated chloroplasts decreased when the $PO43-$ concentration in the medium was low. The ATP content increased when the $PO43-$ concentration was increased. A higher $PO43-$ . concentration in the medium was needed to increase the ATP at 5 °C than at 25 °C. This would suggest that $PO43-$ deficiency occurs more readily at low than at high temperatures. More ¹⁴C was incorporated into photosynthetic metabolites within the chloroplasts at 5 °C than at 25 °C, indicating decreased assimilate export when the temperature was low. Dihydroxyacetone phosphate was preferentially exported when the $PO43-$ concentration enabled a high rate of photosynthesis at 25 °C. However, under conditions of $PO43-$ deficiency, either due to low $PO43-$ concentration in the medium or due to low temperature, 3-phosphoglycerate was preferred for export. The results suggest that the relatively high photosynthetic rates at low temperature are due to increased concentrations of photosynthetic metabolites. The assimilate export at low temperature seems to be decreased due to decreased concentrations of dihydroxyacetone phosphate in the stroma. Preferential export of 3-phosphoglycerate at low temperature or at low $PO43-$ concentration in the medium may be a consequence of high stromal concentrations of this metabolite. On the other hand, it could also be due to decreased stromal pH.