To evaluate the effect of thiamine treatments on plant defence
against aphids, barley seeds were soaked in different concentrations
of thiamine or the vitamin was added to the nutrient solutions to
plants cultivated in vermiculite/perlite. Adult mixed instar aphids
(R. padi) were added to the plant and aphid numbers were counted
after 1 week. Seed soaking or addition of thiamine to the nutrient
solution resulted in reduced aphid population growth to ca. 60% to
70% on the treated plants as compared to on control plants (Fig. 1).
The effect was at an optimum with 150 lM of thiamine, for both
types of treatments. Similar effects were obtained when barley
plants were sprayed with a solution containing thiamine (Fig. S1).
Similar reductions in aphid numbers were also obtained when seed
soaking treatments were carried out with three other barley genotypes
not near-related to Lina; Golf, CI 16145 and Barke (Fig. S2)
and we conclude that the thiamine effect is not dependent on the
cultivar. Experiments were also carried out with grain aphid
(Sitobion avenae) on barley and pea aphid (Acyrthosiphon pisum) on
pea. These two aphid species were also restricted in their population
growth when the seeds of their host plants had been soaking in thiamine
before germination (Table 1). To find out if the effect was restricted
to the first days of aphid infestation, the population growth
of R. padi on barley was recorded every day during 1 week. Fig. 2
shows that there was a somewhat stronger effect during the first
days, which was due to a lower number of nymphs being born in
the treated plants. After this, the relative reduction in aphid numbers
on thiamine-treated as compared to on control plants remained
stable during the duration of the experiment