Translocation, Distribution, and Accumulation

The translocation of the root alkaloids in the xylem can be observed by direct microchemical examination (251) but is best observed by bleeding of the sap. The quantity of the sap and its alkaloid concentration show daily periodicity (283), there being a maximum (based on crude fiber) in the morning (284) in Datura stramonium. 

Warren-Wilson (285) has shown that the downward migration of the Atropa alkaloids is via the phloem, and experiments with lupines confirm this (271). Here the root content is determined lai ely by that of the aerial portion, as was shown by grafting bitter lupines on sweet lupines and on non-alkaloid bearing legume stock (271, 273). A similar migration to the seeds of a Pisum has been reported by others (272). 

The speed of the migration from the leaves is probably dependent not only on the concentration gradient but also upon the regulating mechanism. Alkaloids have not been known to occur in sieve tubes, but aphids which suck only sieve tubes avoid tobacco leaves except those which are alkaloid-free growing on tomato stock. Why the parasitic plants Orobanche and Cuscuta are alkaloid-free even though growing on alkaloid-bearing plants is not clear (287, 288). 

Nicotine is mainly located in the intercostal areas of the leaf (247, 289, 290). Chojecki (291), by microchemical technique, has arrived at the following depots of nicotine in the tobacco plant given in the order of decreasing concentration leaf epidermis, particularly at the base of the hairs, spongy mesophyll tissue of the leaves, primary cortex of roots, epidermis and parenchymatous tissue of the stem and palisade tissue of the leaves small amounts in the phloem and xylem of the stem, in the veins of the leaves, and in the medulla of the stem and traces in flowers and axial cylinders of the roots. 

The tropane alkaloids in Datura and Atropa are largely concentrated in the veins of the leaves (247, 292), but when Cyphormndra is grafted on Datura the greater amount of alkaloids is outside the veins. When Atropa is grafted on tobacco the nicotine is distributed as it is in tobacco leaves (247). 

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