Photosynthetic capacity of field-grown durum wheat under different N availabilities: a comparative study from leaf to canopy

The effect of N availability on photosynthetic capacity, growth parameters and yield was studied in fieldgrown durum-wheat plants at both the leaf and canopy levels. Two contrasting nitrogen levels (120 and 0kgha-1) were assayed in a randomised block design with nine replicates each. Total biomass was measured at anthesis and yield and its agronomical components at maturity. Photosynthetic measurements were performed 2weeks after anthesis in two plots of eachNtreatment. Flag leaveswere measured, usinga LI-COR 6400 combined with the chlorophyll fluorescence meter, and the whole canopy by measuring CO2 and H2O fluxes in an innovative canopy-chamber system. We showed a clear increase in photosynthetic gas exchange and chlorophyll contents with N fertilisation at both canopy and leaf levels. As a consequence the increase in yield as response to N fertilisation seems the result of a larger green leaf area combined with a higher photosynthetic capacity of the leaves attributable to an increase in the maximum carboxylation velocity of Rubisco. Moreover gas-exchange measurements of the flag leaf during grain filling seem to provide a realistic characterisation, not just of the photosynthetic performance of the crop, but also about the impact of N availability on yield. Thus, measurements performed on the flag leafmatched those at the canopy level, with proportional increases in terms of gas exchange and chlorophyll content, providing a fast, cheap and reliable estimation of canopy photosynthesis and the grain yield attained by the crop.