Effects of Phosphorus Nutrition on Ribulose-1,5-Bisphosphate Carboxylase Activation, Photosynthetic Quantum Yield and Amounts of Some Calvin-Cycle Metabolites in Spinach Leaves

Abstract

When spinach plants were transferred to nutrient solutions without phosphorus, the photosynthetic rate per unit leaf area gradually declined. Stomatal conductance also decreased but was not the sole cause of the decreased photosynthetic rate because the partial pressure of CO₂ in the intercellular spaces (C_I) was unaltered. Measurements of the photosynthetic rate as a function of C_I indicated reductions in both ribulosebisphosphate (RuP₂) carboxylase activity and RuP₂ regeneration capacity. From assays of RuP₂ carboxylase activity in vitro and 'percentage activation', it was concluded that low-P leaves had less enzyme per unit area than controls and that the enzyme was also less activated. The photosynthetic quantum yield was reduced by phosphorus deficiency with no effect on leaf absorptance or chlorophyll content. The reduced quantum yield was accompanied by changes in chlorophyll fluorescence of photosystems I and II measured at 77K. However, since phosphorus deficiency did not affect the uncoupled rate of whole-chain electron transport in vitro, some factor(s) other than photoinhibition probably contributed to the reduced quantum yield. The lack of effect on this electron-transport rate also indicates that the maximal RuP₂ regeneration rate in low-P leaves was not limited by the amount of electon-transport components. At ambient [CO₂], low-P leaves had significantly less RuP₂ and 3-phosphoglycerate (PGA) than controls and the response of photosynthesis to low [O₂] was similar to control leaves. Therefore photosynthesis did not appear to be limited by triose-phosphate utilization. The low concentrations of RuP₂ and PGA (and presumably other Calvin-cycle intermediates) might have reduced the rate of the Calvin cycle. After returning low-P plants to nutrient solutions with PO₄, the percentage activation of RuP₂ carboxylase, amounts of RuP₂ and PGA, quantum yield and maximal RuP₂ regeneration rate increased within 24 h. The quantum yield and photosynthetic rate at higher irradiance also increased when leaf discs from low-P plants were floated on 10 mM PO₄ solutions for 2 h.