Wheat ( Triticum aestivum L. cv Yecora 70) plants were grown with various concentrations of nitrate nitrogen available to the roots. Sampling of flag leaves began after they had reached full expansion and continued throughout senescence. Rates of gas exchange, ribulose-1,5-bisphosphate (RuP 2 ) carboxylase activity, and the amounts of chlorophyll, soluble protein, nitrogen, and phosphorus were determined for each flag leaf. Rate of CO 2 assimilation was uniquely related to total leaf nitrogen irrespective of nutrient treatment, season, and leaf age. Assimilation rate increased with leaf nitrogen, but the slope of the relationship declined markedly when leaf nitrogen exceeded 125 millimoles nitrogen per square meter. Chlorophyll content and RuP 2 carboxylase activity were approximately proportional to leaf nitrogen content. As leaves aged, RuP 2 carboxylase activity and calculated Hill activity declined in parallel. With normal ambient partial pressure of CO 2 , the intercellular partial pressure of CO 2 was always such that rate of assimilation appeared colimited by RuP 2 carboxylation and RuP 2 regeneration capacity. The initial slope of rate of CO 2 assimilation against intercellular partial pressure of CO 2 varied nonlinearly with carboxylase activity. It is suggested that this was due to a finite conductance to CO 2 diffusion in the wall and liquid phase which causes a drop in CO 2 partial pressure between the intercellular spaces and the site of carboxylation. A double reciprocal plot was used to obtain an estimate of the transfer conductance.
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