Effects of phosphorus nutrition on the response of photosynthesis to CO2 and O2, 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 bar 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.Abbreviations A CO₂ assimilation rate - C_i intercellular CO₂ partial pressure - PGA 3-phosphoglycerate - RuP₂ ribulose 1,5-bisphosphate - Rubisco RuP₂ carboxylase/oxygenase