Aust. J. Plant Physiol., 1996, 23, 119-125 Interactions Between Rising CO, Concentration and Nitrogen Supply in Cotton. I. Growth and Leaf Nitrogen Concentration G. S. ~o~ers~~, P. J. ~ilharn~, M.-C. hi baud^^ and J. P. conroyA *School of Horticulture, University of Western Sydney, Hawkesbury, Locked Bag No. 1, PO Richmond, NSW 2753, Australia. BBiological and Chemical Research Institute, NSW Agriculture, PMB I0 Rydalmere, NSW 2116, Australia. CPermanent address: Departement de Physiologie Vegetale et Ecosysternes, CEA, Centre de Cadarache, F-13108 Saint Paul Lez Durance, Cedex, France. DAuthor for correspondence, email: g.rogers@uws.edu.au 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 C02 concentration: 350 (ambient),550 and 900 y~ L-I and six levels of nitrogen (N) supply ranging from deficient to excess (0-133 mg N kg-' soil week-'). Changes in leaf N concentration were also investigated. At 59 days after sowing, there was an average 63% increase in shoot growth at 550 ILL C 0 2L-' compared with ambient C02-grown plants, with no significant growth increase at 900 yL C02L-' 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 C02. At elevated C02, leaf N concentration was reduced by an average 27% at low to adequate N supply. The high C02-induced reduction in leaf N concentration, however, disappeared when the N supply was increased to a high level of 133 mg N kg-' soil week-'. These C02effects on leaf N concentration were smaller when N was expressed per unit leaf area, apparently due to a combination of the effects of elevated C02or 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-' when C02 concentrations were increased from 350 to 550 and 900 yL L-' respectively, indicating that changes in fertiliser management may be required under changing C02 concentrations. Introduction Few CO, enrichment studies employ more than one level of CO, elevation and thus the shape of the plant growth curve in response to rising C 0 2 concentration remains largely unknown. The photosynthetic response to rising atmospheric C 0 2 concentration, however, has been more carefully studied. It appears that the capacity of plants to generate sinks in their shoots is crucial in determining the magnitude and shape of the photosynthetic response to increasing atmospheric CO, concentration (Webber et al. 1994; Xu et al. 1994; Krapp and Stitt 1995). In species where sink strength is limited genetically (e.g. clover or sunflower) or where sink limitations are imposed (Xu et al. 1994), down-regulation of the rate of CO, assimilation (A) may limit both the magnitude of the growth response and the range of C0, concentrations over which such a response will occur (Stitt 199 1; Morin et al. 1992; Woodrow 1994~). Conversely, in species which are more flexible in their sink capacity, e.g. wheat, which can produce large numbers of tillers (Gifford 1977; Hocking and Meyer 1991; Rogers et al. 1995), the down-regulation of photosynthesis through feedback inhibition in response to elevated CO, is less likely to occur (Woodrow 1994b). Thus in the wheat variety Hartog, where optimal nutrient conditions were maintained, the tiller number nearly doubled in the vegetative phase under conditions of elevated C0, and there was a matching linear increase in vegetative shoot dry weight as CO, concentrations were increased from 350 to 550 and then to 900 pL L-' (Rogers et al. 1995). The capacity to generate sinks is dependent not only upon species but is also a function of the soil nutrient supply (Radin and Mauney 1982; Wardlaw 1990). In wheat, both the tiller number and the capacity to respond to elevated C0, is greatly stimulated by increasing nitrogen (N) supply (Conroy and Hocking 1993; Rogers et al. 1995). Furthermore, the shape of the growth response to elevated C 0 2 is altered by N supply, with extra shoot growth occurring between 350 and 550 pL CO, L-' but not above when N is deficient (Rogers et al. 1995).