Photosynthetic parameters and stomatal conductance in attached and detached balsam fir foliage

Abstract

Leaf level gas-exchange measurements can be made on detached foliage to address the challenge of access to the crown of tall trees. However, detachment may impact leaf gas exchange. This necessitates the study of gas-exchange characteristics of foliage on detached branches to assess the feasibility of using detached branches for gas-exchange analysis. We compared photosynthetic parameters and stomatal conductance in foliage of attached and detached branches of balsam fir [Abies balsamea (L.) Mill.] during the growing season. Data were analyzed using a linear mixed-effect model, with fixed and random effects (branch status and measurement month, and tree number, respectively). Branch detachment had no significant effects on: (i) photosynthesis at the current ambient CO₂ concentration (400 µmol mol^-1, A ₄₀₀); (ii) maximum rates of Ribulose-1,5-bisphosphate (RuBP) carboxylation (V _cmax) and regeneration (J _max); (iii) the ratio of J _max to V _cmax (i.e., J _max:V _cmax), and (iv) stomatal conductance (g _s) during the study period (p = 0.120-0.335). There was a strong seasonal effect on all gas-exchange variables (p ≤ 0.001-0.015). Gas-exchange measurements made on detached foliage during the warm summer months should be performed with care. Reliable gas-exchange measurements can be obtained using balsam fir foliage on detached branches 50-80 cm in length, in cooler growing-season months, up to 30 min after detachment.

Keywords: balsam fir physiology; carbon dioxide; detached foliage; photosynthetic response; stomatal conductance.