Structural controls on photosynthetic capacity through juvenile-to-adult transition and needle aging in Mediterranean pines

1. Needle photosynthetic potentials strongly vary among primary (juvenile) and secondary (adult) needles (heteroblasty) in Pinus species, but there is limited understanding of the underlying structural, diffusional and chemical controls.
2. We studied differences in needle photosynthetic characteristics among current-year juvenile and adult needles and among different-aged adult needles in Mediterranean pines Pinus halepensis Mill., P. pinea L. and P. nigra J. F. Arnold subsp. salzmannii (Dunal) Franco, hypothesizing that needle anatomical modifications upon juvenile-to-adult transition lead to reduced photosynthetic capacity due to greater limitation of photosynthesis by mesophyll conductance and due to an increase in the share of support tissues at the expense of photosynthetic tissues. We also hypothesized that such alterations occur with needle aging, but to a lower degree.
3. Photosynthetic capacity per dry mass was 2.4-2.7-fold higher in juvenile needles, and this was associated...