INCREASING GROWTH TEMPERATURE REDUCES THE STIMULATORY EFFECT OF ELEVATED CO2 ON PHOTOSYNTHESIS OR BIOMASS IN 2 PERENNIAL SPECIES

We examined how anticipated changes in CO2 concentration and temperatu re interacted to alter plant growth, harvest characteristics and photo synthesis in two cold-adapted herbaceous perennials, alfalfa (Medicago sativa L. cv. Are) and orchard grass (Dactylis glomerata L. cv. Potom ac). Plants were grown at two CO2 concentrations (362 [ambient] and 71 7 [elevated] mu mol mol(-1) CO2) and four constant day/night temperatu res of 15, 20, 25 and 30 degrees C in controlled environmental chamber s. Elevated CO2 significantly increased total plant biomass and protei n over a wide range of temperatures in both species. Stimulation of ph otosynthetic rare, however, was eliminated at the highest growth tempe rature in M. sativa and relative stimulation of plant biomass and prot ein at high CO2 declined as temperature increased in both species. Lac k of a synergistic effect between temperature and CO2 was unexpected s ince elevated CO2 reduces the amount of carbon lost via photorespirati on and photorespiration increases with temperature. Differences betwee n anticipated stimulatory effects of CO2 and temperature and whole pla nt single and leaf measurements are discussed. Data from this study su ggest that stimulatory effects of atmospheric CO2 on growth and photos ynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.