Leaf inorganic phosphorus as a potential indicator of phosphorus status, photosynthesis and growth of Eucalyptus grandis seedlings

In eucalypts the reduction in CO2 assimilation and total leaf area at low phosphorus (P) supply is not associated with lower leaf total P concentrations. We tested the hypothesis that the leaf concentration of inorganic phosphorus ([Pi]) may be a better indicator of P nutrition status in Eucalyptus grandis W. Hill ex Maiden by growing seedlings in P deficient soil supplemented with P supplies ranging from 3 to1000 mg kg?1. Height, biomass accumulation, gas exchange, chlorophyll fluorescence and concentrations of total P ([Pt]), organic P ([Po]) and [Pi] of the last fully expanded leaves were measured when harvested at 19 weeks. All parameters of growth increased with larger applications of soil P with most becoming saturated at additions of 500 mg P kg?1 soil. Soil P supply had larger effects on biomass and canopy leaf area, by both increased in leaf initiation and expansion, than on CO2 assimilation (A). Leaf [Pt] and [Po] concentrations were largely invariant to soil P supply and were not correlated with any of the measured growth and photosynthetic parameters. By contrast, leaf [Pi] increased from 171 to 398 mg kg?1 with increasing soil P supply. Furthermore, number of leaves, total leaf area and seedling biomass increased exponentially with leaf [Pi], while individual leaf area and A increased linearly with leaf [Pi], and quantum yield of photosystem II similarly increased, and non-photochemical quenching decreased, with increasing leaf [Pi]. The response of A to internal CO2 concentration (Ci) indicated that at lower P supplies A became increasingly restricted by limitations associated with Rubisco and RuBP regeneration. Stomatal limitation may in part be masking the full effect of P supply on A as Ci declined with either increasing soil P or leaf [Pi] supply. We conclude that leaf [Pi] was a potentially better indicator than [Pt] or [Po] for correlating the effects of soil P supply on growth and photosynthesis of E. grandis. Furthermore, as A achieved at saturating Ca increased with increasing P supply leaf [Pi], these findings suggest that a greatly increased rate of canopy assimilation could be achieved at higher P supply in response to the expected increase in global CO2 levels.