Interactions between the effects of atmospheric CO 2 content and P nutrition on photosynthesis in white lupin ( Lupinus albus L.)

Phosphorus (P) is a major factor limiting the response of carbon acquisition of plants and ecosystems to increasing atmospheric CO 2 content. An important consideration, however, is the effect of P deficiency at the low atmospheric CO 2 content common in recent geological history, because plants adapted to these conditions may also be limited in their ability to respond to further increases in CO 2 content. To ascertain the effects of low P on various components of photosynthesis, white lupin ( Lupinus albus L.) was grown hydroponically at 200, 400 and 750  µ mol mol −1 CO 2 , under sufficient and deficient P supply (250 and 0.69  µ m P, respectively). Increasing growth CO 2 content increased photosynthesis only under sufficient growth P. Ribulose 1,5‐biphosphate carboxylase/oxygenase (Rubisco) content and activation state were not reduced to the same degree as the net CO 2 assimilation rate ( A ), and the in vivo rate of electron transport was sufficient to support photosynthesis in all cases. The rate of triose phosphate use did not appear limiting either, because all the treatments continued to respond positively to a drop in oxygen levels. We conclude that, at ambient and elevated CO 2 content, photosynthesis in low‐P plants appears limited by the rate of ribulose biphosphate (RuBP) regeneration, probably through inhibition of the Calvin cycle. This failure of P‐deficient plants to respond to rising CO 2 content above 200  µ mol mol −1 indicates that P status already imposes a widespread restriction in plant responses to increases in CO 2 content from the pre‐industrial level to current values.