Sink Strength May Be the Key to Growth and Nitrogen Responses in N-Deficient Wheat at Elevated CO₂

Abstract

The influence of elevated CO₂ (350, 550 and 900 μL L^-1) and N supplies 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 aestivum L.), cultivar Hartog, was investigated. Shoot growth was 30 % greater at 550 μL L^-1 compared to ambient CO₂ at all levels of N supply. When the CO₂ concentration was increased to 900 μL L^-1, there was no increase in shoot growth at low N supply but it more than doubled at high N supply (67 mg N kg^-1 soil week^-1). Growth effects were closely matched by changes in sink development, suggesting that sink strength, mediated through N supply controlled the shoot growth response to elevated CO₂. The shoot N concentration was lower at each level of CO₂ enrichment and the greatest effect (30% reduction) occurred at 900 μL CO₂ L^-1, 33 mg N kg^-1 soil week^-1. The effect of high CO₂ on shoot N concentration diminished as N supply increased and, at the highest N addition rate, there was only a 7% reduction. Changes in foliar N concentration due to CO₂ enrichment were closely correlated with lower soluble protein concentration, accounting for 58 % of the total leaf N reduction. Ribulose- IS-bisphosphate carboxylase/oxygenase (Rubisco) levels were also reduced at high CO₂ and N was allocated away from Rubisco and into other soluble proteins at high CO₂ when N supply was low. Non- structural carbohydrate concentration (dry weight basis) was greatest at 900 μL CO2 L^-1 and low N supply and may have reduced Rubisco concentration via a feed-back response. Critical foliar N concentrations (N concentration at 90 % of maximum shoot growth) were reduced from 43 mg g^-1 at ambient CO₂ to 39 and 38 mg g^-1 at 550 and 900 μL 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 %.