ELEVATED CO2 IMPROVES THE GROWTH OF WHEAT UNDER SALINITY

Wheat plants (Triticum aestivum cv. Matong and T. durum cv. Modoc) wer e grown at ambient and elevated CO2 (350 cm3 m-3 above ambient) in soi l with or without 150 mol m-3 NaCl for 6 weeks. The increase in dry ma tter, leaf area and tillering under high CO2 was relatively greater un der saline than non-saline conditions for both cultivars. Tillering wa s the primary component of growth affected by both salinity and high C O2. Salinity greatly reduced tillering and high CO2 partly reversed th e effects of salinity. High CO2 increased dry matter accumulation of t he salt-sensitive Modoc to a greater extent (+ 104%) than that of the more salt-tolerant Matong (+ 73 %) in the salt treatment. Transpiratio n rates were greatly reduced by salinity for both cultivars. Under hig h CO2, increased leaf areas compensated for reduced transpiration rate s per unit leaf area (i.e. greater stomatal closure), and total transp iration was little affected by CO2 level within each treatment. The mo re salt-tolerant Matong showed greater stomatal closure and higher tra nspiration efficiencies than the salt-sensitive Modoc under salinity. High CO2 reduced transpiration rate (per unit dry weight) by 40 to 50% , but did not significantly change the rate of sodium accumulation (pe r unit dry weight), indicating that salt uptake was largely independen t of water uptake, and that high CO2 did not increase growth by reduci ng the salt load. Our results suggest that high CO2 increased growth b y stimulating the development of tiller buds that would otherwise have been inhibited.