Leaf Growth Development in Relation to Gas Exchange in Quercus marilandica Muenchh.

Summary Photosynthesis, transpiration rates and stomatal conductances were measured using a portable infra-red gas analyzer and then correlated with structural changes occurring during the development of Quercus marilandica Muenchh. leaves. Q. marilandica was found to synthesize high levels of leaf anthocyanins (0.79 ± 0.13gkg −1 fresh weight) during the period immediately following bud break. Carbon assimilation rates showed net respiration (−1.3 ± 1.6μmolm −2 s −1 ) when measured in anthocyanin-containing leaves seven days after bud break (DAB), but to be near the compensation point at 17 DAB (1.1 ± 1.4 μmol m −2 s −1 ) when most of the anthocyanins were metabolized away, but the leaves not yet fully expanded. The maximum rate (8.3 ± 2.6 μmol m −2 s −1 ) was observed in fully expanded leaves at 37 DAB and was eight-fold higher than at 17 DAB. Transpiration rates and stomatal conductances were low at 7 DAB, but then increased 250 and 160 %, respectively, at 17 DAB, but only 22 and 43 % from 17 to 37 DAB. Ultrastructural analysis showed the leaves had small intercellular air spaces and underdeveloped chloroplasts at both 7 and 17 DAB; the leaves not appearing fully mature until expansion was complete at 37 DAB. SEM images showed 7 DAB leaves to be extensively covered with trichomes on both abaxial and adaxial surfaces. The trichomes were mostly shed by 17 DAB revealing the extensive development of sto- mates. It is concluded that transpiration and stomatal conductances were controlled primarily by the boundary layer resistance associated with the trichome layer at 7 DAB and the low carbon assimilation rates seen at 17 DAB were likely a consequence of sub-optimal chloroplast function and/or limitations in CO2 uptake associated with the lack of intercellular air spaces.