Interactions between temperature and intercellular CO 2 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 ) decreases as a function of increasing atmospheric CO concentration, and that increased temperature suppresses the CO effect. We studied interactions between intercellular CO concentration (C ) and temperature as they affect I 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 despite the persistence of relatively high temperatures. High temperature suppression of the I :C relation occurred at all times during the growing season, but sensitivity of I to increased C was greatest during the midsummer period when I was lowest. We interpret the seasonal downregulation of I and increased sensitivity of I to C as being caused by weather changes associated with the onset of a regional monsoon system. Our observations on the temperature suppression of the I :C 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 and temperature dependencies of phosphoenolpyruvate import into the chloroplast.