ELEVATED ATMOSPHERIC CO2 CONCENTRATIONS INCREASE WHEAT ROOT PHOSPHATASE-ACTIVITY WHEN GROWTH IS LIMITED BY PHOSPHORUS

Wheat seedlings were grown in solution culture under adequate and limi ted phosphorus treatments at current ambient and elevated (approximate ly 2X ambient) CO2 concentrations. Acid phosphomonoesterase ('phosphat ase') activity of root segments was measured using p-nitrophenyl phosp hate as substrate. When plant growth was P-limited, elevated CO2 conce ntrations increased phosphatase activity more than at ambient CO2. Thi s result (1) was evident when expressed on a unit root dry weight or r oot length basis, indicating that increased root enzyme activity was u nlikely to be associated with CO2-induced changes in root morphology; (2) occurred when plants were grown aseptically, indicating that the i ncrease in phosphatase activity originated from root cells rather than root-associated microorganisms; (3) was associated with shoot P conce ntrations below 0.18%; (4) occurred only when wheat roots were grown u nder P deficiency but not when a transient P deficiency was imposed; a nd (5) suggest that a previously reported increase in phosphatase acti vity at elevated CO2 by an Australian native pasture grass (Gifford, L utze and Barrett 1996; Plant and Soil 187, 369-387) was also a root me diated response. The observed increase in phosphatase activity by plan t roots at elevated CO2, if confirmed for a wide range of field pastur e and crop species, is one factor which may increase mineralisation of soil organic P as the anthropogenic increase of atmospheric CO2 conce ntrations continues. But, whether a concomitant increase in plant upta ke of P occurs will depend on the relative influence of root and micro bial phosphatases, and soil geochemistry in determining the rate of mi neralisation of soil organic P for any given soil.