The effects of nitrogen stress on the stable carbon isotope composition, productivity and water use efficiency of white spruce (Picea glauca (Moench) Voss) seedlings

Abstract

A, assimilation rate a, fractionation against 13C for CO2 diffusion through air b, net fractionation against 13C during CO2 fixation Ca, ambient CO2 concentration Cc, CO2 concentration at the chloroplast Ci, intercellular CO2 concentration D, vapour pressure deficit En, needle transpiration rate Ep, whole plant water use gw, leaf internal transfer conductance to CO2 gs, stomatal conductance to water vapour L, projected leaf area NUE, nitrogen use efficiency PEP, phosphoenolpyruvate Rubisco, ribulose‐1,5‐biphosphate carboxylase TDR, time domain reflectometry WUE, water use efficiency Δ, carbon isotope discrimination δ13C, carbon isotope abundance parameter δ13Ca, carbon isotopic composition of atmospheric CO2 θ, volumetric soil water content The effect of nitrogen stress on needle δ13C, water‐use efficiency (WUE) and biomass production in irrigated and dry land white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings, representing 10 controlled crosses, were planted in the field in individual buried sand‐filled cylinders. Two nitrogen treatments were imposed, nitrogen stressed and fertilized. The ranking of δ13C of the crosses was maintained across all combinations of water and nitrogen treatments and there was not a significant genetic versus environmental interaction. The positive relationships between needle δ13C, WUE and dry matter production demonstrate that it should be possible to use δ13C as a surrogate for WUE, and to select for increased WUE without compromising yield, even in nitrogen deficient environments. Nitrogen stressed seedlings had the lowest needle δ13C in both irrigated and dry land conditions. There was a positive correlation between needle nitrogen content and δ13C that was likely associated with increased photosynthetic capacity. There was some indication that decreased nitrogen supply led to increased stomatal conductance and hence lower WUE. There was a negative correlation between intrinsic water use efficiency and photosynthetic nitrogen use efficiency (NUE). This suggests that white spruce seedlings have the ability to maximize NUE when water becomes limited. There was significant genetic variation in NUE that was maintained across treatments. Our results suggest that in white spruce, there is no detectable effect of anaplerotic carbon fixation and that it is more appropriate to use a value of 29‰ (‘Rubisco only’) for the net discrimination against 13C during CO2 fixation. This leads to excellent correspondence between values of Ci/Ca derived from gas exchange measurements or from δ13C.