Photosynthetic responses to phosphorus nutrition in two-year-old maritime pine seedlings

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Abstract
We analyzed processes limiting photosynthesis in two-year-old, container-grown Pinus pinaster Ait. seedlings subjected to phosphorus (P) deficiency. After withholding P for 3 months, seedlings were supplied P at four relative addition rates (0, 0.005, 0.01 and 0.02 day−1) in a nutrient recycling system. At Weeks 12 and 22, responses of photosynthesis to CO2 and irradiance were measured and the following parameters derived: maximal velocity of carboxylation by Rubisco, Vm; apparent quantum efficiency of electron transport, α maximal electron transport rate, Jm; stomatal conductance and relative stomatal limitation of photosynthesis. At Week 22, these measurements were combined with concurrent measurements of chlorophyll fluorescence to determine the quantum yield of PSII, and a theoretical partitioning of total light-driven linear electron flow between fractions used to regenerate carboxylated and oxygenated RuBP. After 12 weeks of treatment, needle P concentrations ranged from 0.04 to 0.15 × 10−2 g gDW−1, and then remained constant until Week 22. Values of Jm, α and Vm increased with increasing needle P concentration (from 30 to 133 μmol m−2 s−1, 0.02 to 0.25 mol mol−1 and 13 to 78 μmol CO2 m−2 s−1 at the lowest and highest needle P concentrations, respectively). Under ambient conditions, net assimilation rates in P-deficient seedlings were limited by Vm under saturating irradiance, and by Jm under limiting irradiance, but not by triose-P regeneration. There was no detectable change in the partitioning of total light-driven linear electron flow between the fractions used for carboxylation and oxygenation. Predawn photochemical efficiency of PSII was significantly reduced in seedlings with low P concentrations. Although stomatal conductance tended to decrease with decreasing needle P concentration, relative stomatal limitation was not significantly affected. At Week 22, there was an attenuation of the effects of P nutrition on Vm and an increase in α and Jm that was probably related to cessation of growth and the seasonal decline in natural irradiance.

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