Effects of phosphorus nutrition on the response of photosynthesis to CO2 and O 2, activation of ribulose bisphosphate carboxylase and amounts of ribulose bisphosphate and 3-phosphoglycerate in spinach leaves.

Phosphorus-deficient spinach plants were grown by transferring them to nutrient solutions without PO4. Photosynthetic rates were measured at a range of intercellular CO2 partial pressures from 50-500 μbar and then the leaves were freeze-clamped in situ to measure ribulose bisphosphate carboxylase (Rubisco) activity and metabolite concentrations. Compared with control leaves, deficient leaves had significantly lower photosynthetic rates, percentage activation of Rubisco, and amounts of ribulose bisphosphate and 3-phosphoglycerate at all CO2 partial pressures. After feeding 10 mM PO4 to the petioles of detached deficient leaves, all these measurements increased within 2 hours. At atmospheric CO2 partial pressure the photosynthetic rate was stimulated in 19 mbar O2 compared with 200 mbar. At higher CO2 partial pressures this stimulation was less but the percentage stimulation in deficient leaves was no different from controls in either CO2 partial pressure. It was concluded that phosphorus deficiency affects both Rubisco activity and the capacity for ribulose bisphosphate regeneration, and possible causes are discussed.