© 2004 Heron Publishing—Victoria, Canada
Variation in nitrogen supply changes water-use efficiency of Pseudotsuga menziesii and Populus × euroamericana; a comparison of three approaches to determine water-use efficiency
Francesco Ripullone (1, 2), Marco Lauteri (3), Giacomo Grassi (4), Mariana Amato (1) and Marco Borghetti (1)
1. Dipartimento di Produzione Vegetale, Università della Basilicata, 85100 Potenza, Italy / 2. Corresponding author (rf464agr@unibas.it) / 3. Istituto di Biologia Agro-ambientale e Forestale, CNR, Porano, Italy / 4. Dipartimento di Colture Arboree, Università di Bologna, Bologna, Italy / Received May 27, 2003; accepted October 26, 2003; published online April 1, 2004
Summary
We studied the effects of three nitrogen (N) supply rates (low, intermediate and high) on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings and poplar clone “I-214” (Populus × euroamericana (Dole) Guinier) cuttings growing in mini-stands. Our specific objectives were to: (1) evaluate the effects of N supply on
water-use efficiency (WUE) and biomass production; (2) determine if N affects WUE through control of carbon assimilation rates
or through stomatal control of water loss; and (3) compare three methods of estimating WUE: one short-term method (WUEi, based on gas exchange measurements) and two long-term methods (WUET, based on the ratio between biomass production and transpired water, and Δ, based on leaf carbon isotope discrimination tested
as a proxy of WUE). In both species, biomass production, WUEi and WUET increased with increasing N supply, but there was no effect of N supply on either transpiration or stomatal conductance and
Δ was negatively related to leaf N concentration. Plots of Δ versus both WUEi and WUET revealed negative trends, but the regression between WUEi and Δ was significant only for Douglas-fir, and the regression between WUET and Δ was significant only for poplar. Thus, the mechanisms underlying the response of WUE to N supply were mainly related
to a positive effect of N supply on photosynthetic rates. The data confirm that carbon isotope discrimination may be a useful
proxy of WUE. The finding that N availability enhances both biomass production and WUE may have practical implications in
regions where these factors impose constraints on forest productivity.
Keywords:
biomass production, carbon assimilation, carbon isotope discrimination, Douglas-fir, poplar, stomatal conductance.
