Photosynthesis patterns during the establishment year within two Populus clones with contrasting morphology and phenology.

Diurnal and seasonal photosynthesis patterns were studied in poplar clones Populus tristis Fisch. x P. balsamfera L. cv. Tristis #1 (NC 5260) and Populus x euramericana (Dode) Guiner cv. Eugenei (NC 5326, Carolina poplar) during their first season in the field in a short rotation, intensive culture plantation. Photosynthetic rates were low in immature leaves; increased basipetally on the shoot and peaked in leaves that had recently reached full expansion; and thereafter declined in lower-crown leaves in both clones. Photosynthesis was associated with leaf age and stomatal conductance in immature leaves; adaxial photosynthetic photon flux density (PPFD) and leaf temperature in recently mature leaves; and leaf age and adaxial PPFD in lower-crown leaves. Diurnal photosynthesis patterns within trees were highly variable due to differential light interception among leaves. Results of clonal comparisons of photosynthetic rates were dependent on which leaves were pooled for comparison and how photosynthesis was expressed. Compared to Eugenei, Tristis produced smaller leaves which had higher unit-area photosynthesis rates. The more indeterminate Eugenei outgrew Tristis principally because it more fully utilized the growing season for leaf area production. Photosynthetic production integrated over the growing season was closely related to dry matter production in both clones.