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Growth and Canopy Carbon Dioxide Exchange Rate of Spring Wheat as Affected by Nitrogen Status

USDA-ARS Northwest Region, Agric. Eng. Res. Ctr. Colorado State Univ., Fort Collins, CO 80523

^* Corresponding author.

Little information is currently available describing N nutritional effects on canopy CO₂ exchange rate (CER). This field study was conducted to investigate how fertilizer N affects growth and photosynthesis of a dryland wheat (Tritlcum aestivum L.) stand in a Nunn clay loam (Aridic Argiustoll). Two tr eatments were imposed; an unfertilized (U) treatment, in which no fertilizer was applied, and an N-fertilized (F) treatment, in which 100 kg fertilizer N ha^-1 was applied in the form of NH₄NO₃. Greater above ground growth and grain yield of fertilized wheat compared with unfertilized wheat were associated with greater CER and leaf area index throughout ontogeny and with greater crop growth rates early in development. Net assimilation rates, relative growth rates, and CER expressed on a leaf area basis either were unaffected by N or were reduced. However, greater leaf conductance to water vapor in the most recently fully expanded leaves of the F treatment during vegetative growth stages suggests that the upper leaves in the canopy may have had greater photosynthetic activity at that time compared with the U treatment. Utilization of intercepted photosynthetic photon flux density for CER by plants of the F treatment was equal to or greater than for plants of the U treatment despite more self-shading in the former. Greater leaf photosynthetic capacity conferred by N fertilization apparently results in heavily shaded canopies using intercepted light as efficiently as open, better-illuminated unfertilized canopies.

Key Words: Triticum aestivum (L.) • Growth analysis • Photosynthesis • Water potential

Received for publication February 27, 1987.