We tested if fumigation with exogenous monoterpenes might induce thermotolerance in leaves of an oak species
(Quercus suber) which does not form and emit isoprenoids. To understand if exogenous monoterpene fumigation
results in internal accumulation of monoterpenes, a physical method of monoterpene extraction was used. The
internal content of monoterpenes increased in concert with increasing fumigation doses. This unambiguously
demonstrated acquisition of exogenous monoterpenes. We exposed fumigated Q. suber leaves to two cycles of
increasing temperatures from 35 to 55°C at 5°C steps. When leaves were exposed to a low dose of exogenous
monoterpenes, yielding an internal content similar to that endogenously formed in the leaves of the monoterpene-emitter Q. ilex, no clear improvement in thermotolerance was found. When leaves were exposed to a high dose of
exogenous monoterpenes, yielding an internal content of about five fold the endogenous pool of Q. ilex, but
comparable with the expected content following stress-induced stomatal closure, photosynthesis inhibition at high
temperatures was attenuated. This effect was observed only at temperatures <45°C during the first cycle, but at
all temperatures between 35 and 55°C when plants were exposed to two cycles of high temperatures.
Monoterpenes were still found in the leaves of Q. suber 12 h after ending the fumigation. Monoterpenes were also
found in non-fumigated leaves distant up to 45 cm from the fumigated leaves. If monoterpenes make the
photosynthetic apparatus more resistant to high temperatures, the effect might not be limited to the fumigated
leaves and might be persistent after fumigation.