Interactions between temperature and intercellular CO2 concentration in controlling leaf isoprene emission rates

Plant isoprene emissions have been linked to several reaction pathways involved in atmospheric photochemistry. Evidence exists from a limited set of past observations that isoprene emission rate (I-s) decreases as a function of increasing atmospheric CO2 concentration, and that increased temperature suppresses the CO2 effect. We studied interactions between intercellular CO2 concentration (C-i) and temperature as they affect I-s in field-grown hybrid poplar trees in one of the warmest climates on earth - the Sonoran Desert of the southwestern United States. We observed an unexpected midsummer downregulation of I-s despite the persistence of relatively high temperatures. High temperature suppression of the I-s:C-i relation occurred at all times during the growing season, but sensitivity of I-s to increased C-i was greatest during the midsummer period when I-s was lowest. We interpret the seasonal downregulation of I-s and increased sensitivity of I-s to C-i as being caused by weather changes associated with the onset of a regional monsoon system. Our observations on the temperature suppression of the I-s:C-i relation are best explained by the existence of a small pool of chloroplastic inorganic phosphate, balanced by several large, connected metabolic fluxes, which together, determine the C-i and temperature dependencies of phosphoenolpyruvate import into the chloroplast. Isoprene emissions from leaves are known to influence the oxidative capacity of the lower atmosphere and contribute to the formation of organic aerosol particles. Our research shows that isoprene emissions are inhibited by elevated atmospheric CO2 concentration, and that warmer leaf temperatures reduce the CO2 inhibition. The influence of warmer leaf temperatures is proposed to be because of modifications of the chloroplast inorganic phosphate balance and concomitant potential to import phosphoenolpyruvate into the chloroplast from the cytosol, which is required for isoprene biosynthesis.