The effects of high temperature on isoprene synthesis in oak leaves

Isoprene emission from plants is highly temperature sensitive and is common in forest canopy species that experience rapid leaf temperature fluctuatio ns. Isoprene emission declines with temperature above 35 degrees C but the temperature at which the decline begins varies between 35 and 44 degrees C. This variability is caused by the rate at which leaf temperature is increa sed during measurement with lower temperatures associated with longer measu rement cycles. To investigate this we exposed leaves of red oak (Quercus ru bra L.) to temperature regimes of 35-45 degrees C for periods of 20-60 min. Isoprene emission increased during the first 10 min of high temperature ex posure and then decreased over the next 10 min until it reached steady stat e. This phenomenon was common at temperatures above 35 degrees C but was no t noticeable at temperatures below that. The response was reversible within 30 min by lowering leaf temperature to 30 degrees C. Because there is no s torage of isoprene inside the leaf, this behaviour indicates regulation of isoprene synthesis in the leaf. We demonstrated that the variability in iso prene decline results from regulation and explains the variability in the t emperature response. This is consistent with our theory that isoprene prote cts leaves from damage caused by rapid temperature fluctuations.