Tuesday, 5 June 2012

Wednesday weed: velvety nightshade.

This one's velvety nightshade, Solanum chenopodioides.  Both its names are descriptive, because the leaves feel velvety due to fine dense hairs all over them and they are similar in shape and colour to Chenopodium album, hence chenopodioides.
Solanum chenopodioides.  A, leaves, underside on left; B, flowers; C, a flower cluster; D, leaf edges and surfaces, lower on left; E, stem; F, a fruit cluster (unripe).
Velvety nightshade and black nightshade, S. nigrum, are the two commonest nightshades around Wellington, often growing together in waste land and roadsides.  Velvety nightshade is a South American plant, like so many of the genus, whereas black nightshade is from Eurasia.

Solanum and its family, Solanaceae, have a lot of poisonous plants.  Many contain solanine and chaconine, which are inhibitors of the chemical reaction that stops a nerve impulse from endlessly firing along the nerve.  When that reaction is blocked, the nerve doesn't switch off, with disastrous effects.  These toxins are advantageous to the plant, because they can inhibit the activities of insects and other predators.  Potatoes (Solanum tuberosum) may have small amounts of solanine and chaconine, but not enough to affect us because our livers break down that small amount pretty effectively.

Crops like potatoes have been selected over centuries of farming for reduced amounts of the natural pesticides that many plants have.  Much of early crop selection was inadvertent, with growers cultivating next year's crop from the survivors of this year's.  (And of course if the crops contained some toxins, it was only the surviving growers—those who had eaten less-toxic strains—that were around to plant the crop again).  So the crops became less toxic, but often retained some toxicity that prevented them being lost to pests.

Crops' wild relatives sometimes still contain toxic amounts of these natural and organic pesticides. So here's a bit of admittedly rank speculation: I wonder if pollen from wild relatives could ever contaminate a crop, making some plants in the next generation more toxic (i.e., naturally pest resistant).  Then, hybrid plants with higher levels of natural toxins should compete better with the non-hybrid plants, especially if pesticide sprays weren't being used, as on organic farms.  Organic growers might even prefer to grow them for their higher natural resistance to pests, and inadvertently select strains that are more toxic to humans.  I emphasize I have no evidence that this happens, but I wonder if anyone's looked?  And I'm certainly not suggesting these wild nightshades hybridize with potatoes; Solanum is a big genus and most species aren't closely related or genetically similar enough for hybrids to form.

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