SINK STRENGTH MAY BE THE KEY TO GROWTH AND NITROGEN RESPONSES IN N-DEFICIENT WHEAT AT ELEVATED CO2

The influence of elevated CO2 (350, 550 and 900 mu L L(-1)) and N supp lies ranging from deficient to excess (0-133 mg N kg(-1) soil week(-1) ) on the leaf N concentration and shoot growth of wheat (Triticum aest ivum L.), cultivar Hartog, was investigated. Shoot growth was 30 % gre ater at 550 mu L L(-1) compared to ambient CO2 at all levels of N supp ly. When the CO2 concentration was increased to 900 mu L L(-1), there was no increase in shoot growth at low N supply but it more than doubl ed at high N supply (67 mg N kg(-1) soil week(-1)). Growth effects wer e closely matched by changes in sink development, suggesting that sink strength, mediated through N supply controlled the shoot growth respo nse to elevated CO2 The shoot N concentration was lower at each level of CO2 enrichment and the greatest effect (30% reduction) occurred at 900 mu L CO2 L(-1), 33 mg N kg(-1) soil week(-1). The effect of high C O2 on shoot N concentration diminished as N supply increased and, at t he highest N addition rate, there was only a 7% reduction. Changes in foliar N concentration due to CO2 enrichment were closely correlated w ith lower soluble protein concentration, accounting for 58 % of the to tal leaf N reduction. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) levels we:re also reduced at high CO2 and N was allocated aw ay from Rubisco and into other soluble proteins at high CO2 when N sup ply was low. Nonstructural carbohydrate concentration (dry weight basi s) was greatest at 900 mu L CO2 L(-1) and low N supply and may have re duced Rubisco concentration via a feed-back response. Critical foliar N concentrations (N concentration at 90 % of maximum shoot growth) wer e reduced from 43 mg g(-1) at ambient CO2 to 39 and 38 mg g(-1) at 550 and 900 mu L CO2 L(-1), respectively. Elevated CO2, at N supplies of 0-17 mg N kg(-1) soil week(-1), reduced flour protein concentration by 9-13 %.