Patterns of cytokinins and leaf gas exchange among canopy layers of a mature sugar maple (Acer saccharum) stand

Leaf cytokinins (CKs) were profiled within four locations throughout the inner and outer layers of a mature sugar maple (Acer saccharum) canopy. Leaf CK was associated with leaf gas exchange activity and some corresponding microclimate variables. Both inner and outer layers in the upper canopy had higher concentrations of leaf CKs than the lower canopy layers and the difference was comprised primarily by riboside forms of CK. Transpiration (E) showed a similar pattern to leaf CK content, with significantly higher rates in the upper canopy. There was, however, no clear pattern discernable in stomatal conductance (gs), other than it tended to be higher in the outer canopy layers. The upper/outer canopy showed a significantly different environment than all other canopy positions with higher photosynthetically active radiation (PAR), ultra-violet light (UV-B) and leaf temperature. Simple linear regression analysis showed that the nucleotide CK group (including iPNT, cis- and trans-[9RMP]Z, [9RMP]DZ) was positively related to PAR. Exogenous applications of benzylaminopurine (BAP), showed that low concentrations of BAP reduced E and gs, and indicated that CK may help regulate stomatal aperture. The similar patterns in E and CK content suggest that CKs and leaf gas exchange are functionally connected.