The effects of limiting phosphate on photosynthesis and growth of Lotus japonicus

• P deficiency did not affect the root:shoot ratio in Lotus japonicus.
• Quantum yield, respiration rates and stomatal conductance were not affected by low P.
• P deprivation decreased triose-P transport and the photosynthesis:leaf respiration.
• This caused lower C availability for growth.

Photosynthesis is an integral part of plant metabolism for the production of carbohydrates, which are in turn used for plant biomass production. Phosphate (P) plays an important role in photosynthesis and the production of carbohydrates; however, it is also one of the most difficult macronutrients for plants to obtain. The effects of P deprivation on photosynthesis and respiration in the model legume Lotus japonicus were studied using gas exchange measurements and the A:Ci model of photosynthesis. Phosphate deprivation decreased plant biomass, overall photosynthetic reactions, photosynthesis:respiration ratio and triose-P utilization. The root:shoot ratio, the hallmark of P deprivation, did not show any significant increase under P deprivation, which is in contrast to the bulk of previous findings. Consistent allocation to both roots and shoots could explain this phenomenon. Direct photosynthetic and respiration measurements did not show any significant decrease under P deprivation, although the effect of P on photosynthesis has been shown to be largely species-specific. The ratio of photosynthesis to respiration, however, showed a marked decrease and therefore was affected by P deprivation. Changes in photosynthesis, respiration and triose-P utilization explain the decrease in plant biomass.