Interactions Between Rising CO₂ Concentration and Nitrogen Supply in Cotton. I. Growth and Leaf Nitrogen Concentration

Abstract

The influence of sink development on the response of shoot growth in cotton (Gossypium hirsutum L. cv. Siokra BT1-4) was investigated by growing plants at three levels of CO2=₂ concentration: 350 (ambient), 550 and 900 μL L^-1 and six levels of nitrogen (N) supply ranging from deficient to excess (0-133 mg N kg^-1 soil week^-1). Changes in leaf N concentration were also investigated. At 59 days after sowing, there was an average 63% increase in shoot growth at 550 μL CO₂ L^-1 compared with ambient CO₂-grown plants, with no significant growth increase at 900 μL CO₂ L^-1 and, this response was closely matched by sink development (flower number and stem weight). Low N supply restricted the responses of both sink development and shoot growth to high CO₂. At elevated CO₂, leaf N concentration was reduced by an average 27% at low to adequate N supply. The high CO₂-induced reduction in leaf N concentration, however, disappeared when the N supply was increased to a high level of 133 mg N kg^-1 soil week-1. These CO₂ effects on leaf N concentration were smaller when N was expressed per unit leaf area, apparently due to a combination of the effects of elevated CO₂ or high N supply reducing specific leaf area and, to an N uptake limitation at low to moderate levels of N supply. The critical foliar N concentrations (leaf N concentration at 90% of maximum shoot growth) were reduced from 42 to 38 and 36 mg g^-1 when CO₂ concentrations were increased from 350 to 550 and 900 μL L^-1 respectively, indicating that changes in fertiliser management may be required under changing CO₂ concentrations.