Response of isoprene emission and carbon metabolism to drought in white poplar Populus alba) saplings

o The mechanism uncoupling isoprene emission and photosynthesis under drought
was investigated in Populus alba saplings.
o Isoprene emission, incorporation of 13 C into the isoprene molecule, isoprene
synthase (ISPS) activity, concentration and gene expression, and photosynthesis
were measured as a function of the fraction of transpirable soil water (FTSW) and
in plants recovering from drought.
o Photosynthesis sharply declined below FTSW30 (a FTSW of 30%) and its inhibition
was not caused by metabolic factors. A decline in isoprene emission was only evident
towards the FTSW endpoint. 13C incorporation into isoprene was lower when
photosynthesis was constrained by drought. ISPS activity was inhibited by mild
drought, while ISPS gene expression and concentration declined in concert with
isoprene emission at the FTSW endpoint. Following rewatering, isoprene emission
was higher than, and photosynthesis was similar to, prestress values. ISPS activity
and concentration, and 13C incorporation into isoprene, also rapidly recovered to
prestress values, while ISPS gene expression remained low in rewatered plants.
o Our experiment revealed a larger contribution of alternative carbon sources to
isoprene emission only when photosynthesis was dramatically constrained by
drought. Isoprene emission was likely controlled at the posttranscriptional level
under severe drought.