Phosphorus recycling in photorespiration maintains high photosynthetic capacity in woody species

Leaf photosynthetic CO2 responses can provide insight into how major nutrients, such as phosphorus (P), constrain leaf CO2 assimilation rates (A(net)). However, triose-phosphate limitations are rarely employed in the classic photosynthesis model and it is uncertain as to what extent these limitations occur in field situations. In contrast to predictions from biochemical theory of photosynthesis, we found consistent evidence in the field of lower A(net) in high [CO2] and low [O-2] than at ambient [O-2]. For 10 species of trees and shrubs across a range of soil P availability in Australia, none of them showed a positive response of A(net) at saturating [CO2] (i.e. A(max)) to 2kPa O-2. Three species showed >20% reductions in A(max) in low [O-2], a phenomenon potentially explained by orthophosphate (P-i) savings during photorespiration. These species, with largest photosynthetic capacity and P-i>2mmol Pm-2, rely the most on additional P-i made available from photorespiration rather than species growing in P-impoverished soils. The results suggest that rarely used adjustments to a biochemical photosynthesis model are useful for predicting A(max) and give insight into the biochemical limitations of photosynthesis rates at a range of leaf P concentrations. Phosphate limitations to photosynthetic capacity are likely more common in the field than previously considered.