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 (Is ) decreases as a function of increasing atmospheric CO2 concentration, and that increased temperature suppresses the CO2 effect. We studied interactions between intercellular CO2 concentration (Ci ) and temperature as they affect Is 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 Is despite the persistence of relatively high temperatures. High temperature suppression of the Is :Ci relation occurred at all times during the growing season, but sensitivity of Is to increased Ci was greatest during the midsummer period when Is was lowest. We interpret the seasonal downregulation of Is and increased sensitivity of Is to Ci as being caused by weather changes associated with the onset of a regional monsoon system. Our observations on the temperature suppression of the Is :Ci 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 Ci and temperature dependencies of phosphoenolpyruvate import into the chloroplast.