INTERACTIONS BETWEEN RISING CO2 CONCENTRATION AND NITROGEN SUPPLY IN COTTON .1. GROWTH AND LEAF NITROGEN CONCENTRATION

The influence of sink development on the response of shoot growth in c otton (Gossypium hirsutum L. cv. Siokra BT1-4) was investigated by gro wing plants at three levels of CO2 concentration: 350 (ambient), 550 a nd 900 mu L L(-1) and six levels of nitrogen (N) supply ranging from d eficient 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 mu L CO2 L(-1) com pared with ambient CO2-grown plants, with no significant growth increa se at 900 mu L CO2 L(-1) and, this response was closely matched by sin k 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 co ncentration, 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 lea f N concentration were smaller when N was expressed per unit leaf area , apparently due to a combination of the effects of elevated CO2 or hi gh N supply reducing specific leaf area and, to an N uptake limitation at low to moderate levels of N supply. The critical foliar N concentr ations (leaf N concentration at 90% of maximum shoot growth) were redu ced from 42 to 38 and 36 mg g(-1) when CO2 concentrations were increas ed from 350 to 550 and 900 mu L L(-1) respectively, indicating that ch anges in fertiliser management may be required under changing CO2 conc entrations.