Differential response of leaf conductance, carbon isotope discrimination and water‐use efficiency to nitrogen deficiency in maritime pine and pedunculate oak plants

SUMMARY Growth, CO2 assimilation rate (A), leaf conductance (g), transpiration efficiency (W= ratio biomass production/plant water use) and carbon isotope discrimination (Δ) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand–peat mixture with three levels of fertilization: F100, optimal complete fertilization; F25, 25% of the optimal fertilizer supply; F0, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water-use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time-integrated (A/g derived from Δ measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between F100 and F25, has not been reported before. In oak, both A and g were decreased in F25 and F0 as compared with F100. W was not affected – and instantaneous as well as time-integrated A/g values were only slightly decreased – in relation to decreasing plant N concentration. For F100, no difference in W was noticed between pine and oak though the Δ values were 2·6‰ lower in oak. We speculate that this discrepancy was linked with higher plant-carbon losses through processes like respiration, fine-root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W, but has to be used with caution when comparing different species.