GROWTH AND MAINTENANCE RESPIRATION IN LEAVES OF LIRIODENDRON-TULIPIFERA L EXPOSED TO LONG-TERM CARBON-DIOXIDE ENRICHMENT IN THE FIELD

Specific respiration rate (SRR) was mathematically partitioned into its growth and maintenance components for leaves of yellow-poplar (Liriodendron tulipifera L.) after 3 vr of CO2 enrichment in open-top field chambers. Despite the absence of a CO2 effect on individual leaf expansion or specific growth rate (SGR), increasing the CO2 concentration to ambient + 150 or + 300 cm3 m-3 decreased SRR by 28 to 45 % compared with ambient-grown controls. These lower leaf respiration rates were correlated with reduced leaf nitrogen concentrations. As described by the two-component model of growth and maintenance respiration, SRR was a linear function of SGR. Ambient-grown leaves had a growth respiration coefficient of 704 mg CO2 g-1 dry mass compared with 572 and 570 mg CO2 g-1 for leaves grown at the two higher CO2 concentrations. Leaves from the elevated CO2 treatments had an average maintenance respiration coefficient of 88 mg CO2 g-1 dry mass d-1 compared with 135 mg CO2 g-1 d-1 for leaves from the ambient treatment. Incorporating these growth and maintenance coefficients into a leaf growth simulation model indicated that total respiration would be reduced by 21 to 26 % for a leaf exposed to + 150 or + 300 cm3 m-3 CO2 throughout its 50-d lifespan compared with one grown at ambient CO2 conditions. Reductions in total respiration were dominated by a lower rate of maintenance respiration, while the contribution of a lower specific rate of growth respiration was largely offset by a greater dry mass for leaves grown at elevated CO2 concentrations. Although reductions in the respiratory loss of carbon could be beneficial, respiration is unlikely to decrease without a concomitant decrease in other metabolic processes. Whether these reductions are beneficial or detrimental to the long-term growth of plants exposed to elevated CO2 remains unresolved.