A gas-exchange study of photosynthesis and isoprene emission in Quercus rubra L

We have investigated the signals which affect the rate of isoprene emission from photosynthesizing leaves of red oak (Quercus rubra L.) using analytical gas-exchange techniques, chlorophyll-fluorescence measurements, and inhibitor feeding. Isoprene emission increased with increasing photon flux density at low CO, but much less so at high CO, partial pressure. Photosynthetic CO, assimilation exhibited the opposite behavior. In C0,-free air. isoprene emission was reduced; above 500 pbar CO, partial pressure, isoprene emission was also reduced. The high-CO, effect appeared to be related to low ATP levels which can occur during feedback-limited photosynthesis. At high temperature. which can prevent feedback limitations, isoprene emission remained high as C02 partial pressure was increased. After exposing the leaves to darkness, isoprene emission declined over 15 min, while photosynthesis stopped within 2 min. Adding far-red light to stimulate cyclic photophosphorylation during the post-illumination period stimulated isoprene emission. These analyses lead us to propose that the rate of isoprene emission is regulated by ATP. Analysis of transients indicated that isoprene emission is also related to photosynthetic carbon metabolism. Inhibitor feeding indicated that 3-phosphoglyceric acid and 1,3-bisphosphoglyceric acid are possible candidates for the link between photosynthetic carbon metabolism and the regulation of isoprene emission. Given the ATP dependence, we suggest that the concentration of 1,3-bisphosphoglyceric acid may exert control over the rate of isoprene emission from oak leaves.