Photosynthetic properties of juvenile Prumnopitys taxifolia (Podocarpaceae), a divaricate and heteroblastic conifer

Abstract

The crowns of juvenile Prumnopitys taxifolia (D. Don) de Laub. are comprised of thin, interlaced, wiry stems branching at wide angles and bearing sparse, scale‐like, clasping leaves (‘divaricating’ shoots) that generally surround stems bearing oblong, compressed leaves (‘leafy’ shoots). As development proceeds, non‐divaricating branches of the mature tree form exclusively; thus, P. taxifolia is profoundly heteroblastic. Divaricating stems of juvenile P. taxifolia are brown in appearance and possess very low leaf areas. As such, one might assume that they perform little photosynthesis, returning very little fixed carbon in return for the cost of their construction, and thus represent a significant cost to the overall carbon economy of the whole plant. We examined the hypothesis that divaricating P. taxifolia stems are photosynthetically active, thus mitigating this perceived cost. We found that divaricating shoots, though brown in colouration, possessed chlorophyll contents and capacities for photosynthetic O 2 evolution on par with those of leafy shoots. Interestingly, measured rates of CO 2 assimilation in divaricating shoots were only ∼ half those of leafy shoots. This difference may be explained by the assimilation of internal CO 2 (i.e. CO 2 generated by or transported through divaricating stems), which is not measured by traditional gas exchange methods. We conclude that analyses of the costs associated with the evolution of divarication or heteroblasty in P. taxifolia should take into account the photosynthetic activity of divaricating stems. A survey of the photosynthetic properties of divaricating stems of other species might contribute to our understanding of the origins and functional significance of this fascinating phenomenon.

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