Gas exchange acclimation to elevated CO2 in upper-sunlit and lower-shaded canopy leaves in relation to nitrogen acquisition and partitioning in wheat grown in field chambers.

Abstract

Growth at elevated CO2 often decreases photosynthetic capacity (acclimation) and leaf N concentrations. Lower-shaded canopy leaves may undergo both CO2 and shade acclimation. The relationship of acclimatory responses of flag and lower-shaded canopy leaves of wheat (Triticum aestivum L.) to the N content, and possible factors affecting N gain and distribution within the plant were investigated in a wheat crop growing in field chambers set at ambient (360 μmol mol-1) and elevated (700 μmol mol-1) CO2, and with two amounts of N fertilizer (none and 70 kg ha-1 applied on 30 April). Photosynthesis, stomatal conductance and transpiration at a common measurement CO2, chlorophyll and Rubisco levels of upper-sunlit (flag) and lower-shaded canopy leaves were significantly lower in elevated relative to ambient CO2-grown plants. Both whole shoot N and leaf N per unit area decreased at elevated CO2, and leaf N declined with canopy position. Acclimatory responses to elevated CO2 were enhanced in N-deficient plants. With N supply, the acclimatory responses were less pronounced in lower canopy leaves relative to the flag leaf. Additional N did not increase the fraction of shoot N allocated to the flag and penultimate leaves. The decrease in photosynthetic capacity in both upper-sunlit and lower-shaded leaves in elevated CO2 was associated with a decrease in N contents in above-ground organs and with lower N partitioning to leaves. A single relationship of N per unit leaf area to the transpiration rate accounted for a significant fraction of the variation among sun-lit and shaded leaves, growth CO2 level and N supply. We conclude that reduced stomatal conductance and transpiration can decrease plant N, leading to acclimation to CO2 enrichment.This work was funded by the Spanish ‘Plan Nacional de Investigación y Desarrollo’ (grant N° BFI2000-0871). A. Del Pozo was the recipient of a fellowship from the Spanish Ministry of Education for a sabbatical leave. R. Morcuende had a Ramón y Cajal research contract from the Spanish Ministry of Education. The technical cooperation of A.L. Verdejo in gas exchange measurements, chlorophyll and Rubisco activity determination is acknowledged. We thank the staff of the experimental farm of IRNASA for assistance in crop husbandry.Peer reviewe