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

Abstract

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.