SENSITIVITY OF LEAF PHOTOSYNTHESIS TO CO2 CONCENTRATION IS AN INVARIANT FUNCTION FOR C-3 PLANTS - A TEST WITH EXPERIMENTAL-DATA AND GLOBAL APPLICATIONS

Rising atmospheric CO2 concentration (C-a) may alter two components (s ensitivity and acclimation) of global photosynthetic carbon influx int o terrestrial ecosystems (P-G). Most existing global models focus on l ong-term acclimation We have developed a leaf-level function (L) to qu antify short-term increment of P-G associated with sensitivity. The L function is the normalized response of leaf photosynthesis to a small change in C-a and has been suggested to be an invariant function for C -3 plants grown in diverse environments. This paper tests the hypothes is that L is an invariant function. We calculated values of L from 9 s ets of experimental data which incorporated photosynthetic responses o f 12 plant species to measurement conditions of light and temperature and to growth in different light, temperature, nitrogen, phosphorus, w ater stress, and CO2 concentration. Absolute rates of leaf photosynthe sis differed by more than tenfold due to species differences and envir onmental variation. However, L values derived from these data sets con verged into a narrow range defined by two equations of the L function, confirming that L was insensitive to differences in photosynthetic ca pacity among species and between plants acclimated to different growth environments. Using the L function, we predict that a yearly increase of 1.5 parts per million (ppm) in C-a will induce an increase in P-G by 0.18 to 0.34 Gt (1 Gt = 10(15) g) C yr(-1) in 1993, provided that ( 1) P-G = 120 Gt C yr(-1), (2) 85% of P-g is generated by C-3 plant ass imilation, and (3) the 1.5-ppm increase in C-a will not induce signifi cant photosynthetic acclimation.