Sieve-tube aphids

FIG. 2 Measurements of intracellular potentials in sieve tubes of maize via severed aphid stylets. Stimulation by ice water (above) and electric shock (below) evoked action potentials which were propagated with a velocity of 3-5cms in a basipetal direction. (From Ref. 36.)... 

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). 

Morris 1980). Auxin is apparently not available for sieve-tube-feeding aphids although Lepp and Peel (1971) have reported auxin activity in aphid honey dew. 

Fig. 222. The aphid technique. On the left a sucJdng aphid, on the right the anaesthetized aphid has been cut off Sieve tube sap streams out of the proboscis, which has punctured only one sieve tube, and can be collected.

Fig. 226. Apparent two-directional transport in one sieve tube. A Vida faba plant was laid horizontal and fluorescein was supplied to a lower leaf and C " -urea to an upper. Aphids were applied between the two leaves and their honey dew (cf. Fig. 222) was collected in a collector rotating beneath them. Both substances could be detected in the honey dew of one aphid. Since one aphid punctures only one sieve tube this result at first suggested a two-directional transport in one sieve tube (from Eschrich 1967).

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