Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free-air CO2 enrichment

Poplar (Populus x euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO2] produced more leaves than those in ambient [CO2]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source-sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO2] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO2] was higher in plants growing in elevated than in ambient [CO2]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO2]. This may indicate a [CO2]-driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO2]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO2]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO2]. When integrating over the plant profile, emissions in the two [CO2] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO2] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO2]. Our results indicate that elevated [CO2] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO2].