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

Abstract

A , assimilation rate a , fractionation against 13 C for CO 2 diffusion through air b, net fractionation against 13 C during CO 2 fixation C a , ambient CO 2 concentration C c , CO 2 concentration at the chloroplast C i , intercellular CO 2 concentration D , vapour pressure deficit E n , needle transpiration rate E p , whole plant water use g w , leaf internal transfer conductance to CO 2 g s , 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 δ 13 C, carbon isotope abundance parameter δ 13 C a , carbon isotopic composition of atmospheric CO 2 θ , volumetric soil water content The effect of nitrogen stress on needle δ 13 C, 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 δ 13 C 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 δ 13 C, WUE and dry matter production demonstrate that it should be possible to use δ 13 C 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 δ 13 C in both irrigated and dry land conditions. There was a positive correlation between needle nitrogen content and δ 13 C 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 13 C during CO 2 fixation. This leads to excellent correspondence between values of C i / C a derived from gas exchange measurements or from δ 13 C.