Sites and Control Mechanisms of Biosynthesis

While heartwood and outer bark fractions are certainly the most interesting parts of the tree from the standpoint of variety and content of many types of organic compounds, we certainly need to develop a much better understanding of the physiologically active zones of the tree. More emphasis needs to be placed on the water-soluble constituents transported down the phloem from the site of biosynthesis and centripetally from the cambium to the rhytidome and to the heart-wood via the rays. We need more information on resin canal or axial parenchyma strand constituents as opposed to contents of ray parenchyma. Serial section analysis from the center of the tree to the outer bark will be helpful. Finally, this work must be buttressed by in vivo labeled compound experiments to determine if our biosynthetic speculations follow the same course in nature as they do in vitro. 

The latter hypothesis may need to be altered or at least augmented by new studies being conducted by Terazawa and Miyake (57) on cambial constituents. 

Another technique that will be of increasing help in our understanding of extractives biosynthesis are detailed studies of the location of various enzymes within the tree. A recent example of such work is that of Stich and Ebermann (55) who showed that isoenzymes of peroxidase and polyphenol oxidase were identical in the sapwood and heartwood of oak. This work shows that there is a potential for polymerization of monomers to lignin in unlignified parenchyma cells at the sapwood/heartwood boundary as well as in the zone of active lignification close to the cambium. It could also help to explain the formation of the heartwood lignin-like polymers mentioned in Section 11.6.5. 

Regardless of the technique used, we believe that a much better understanding of the function and the potential for control of extractive formation will be forthcoming when we have detailed information of the precise location of various types of extractives in the tree. 

2 Baes C F, McLaughlin S B 1984 Trace elements in tree rings (as) evidence of recent and historical air pollution. Science 224 494-497 

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