Tuesday, 29 May 2012

Wednesday weed: black nightshade


Every so often there's a bit of a fuss over some nightshade berries found in peas, and it's made worse by people confusing their nightshades.  The culprit is black nightshade, not deadly nightshade.  Deadly nightshade, Atropa bella-donna, is very rare in New Zealand.  It's in the same family as black nightshade, but its berries are a bit larger than a pea (12–18 mm) and glossy black (Sykes in Webb et al. 1988).
Black nightshade, Solanum nigrum, Northland, Wellington, New Zealand.
If you do think you've found nightshade berries in among your peas, they're easy to distinguish from peas, even though they're the same size, shape, and, if unripe, colour.  A pea is a seed; it has a thin seed coat and two large hemispherical cotyledons (seed leaves) inside.  It doesn't have a calyx of small leaf-like sepals attached with a stalk.  A nightshade is a berry; it has a thin fruit wall and several small seeds inside it.  It has a stalk and a calyx at one end (see E in the figure below).  If you slice one in half, you'll see its structure is just like a tiny tomato, hardly a surprise because they're in the same genus.  A nightshade berry has two compartments with many seeds attached to the partition that divides it in two.  Cultivated tomatoes often have three or even four compartments; probably they've been bred that way to increase the flesh making a firmer fruit.

Solanum nigrum, from Northland, Wellington. A, leaves, upper surface on left, lower on right; B. leaf hairs; C, flowers, the youngest with a receptive stigma but anthers haven't opened yet, the middle one presenting pollen; D, the smooth stem, E, a cluster of young berries.
Black nightshade and its close kin are common weeds in New Zealand.  Their unripe fruit is just the right size and colour to be very hard to sort from a harvest of fresh peas, although they'd be easy to sort from pea pods before the peas are shelled.

Black nightshade, Northland, Wellington, New Zealand.
The leaves and stems of black nightshade are usually almost hairless, although they have a few scattered minute hairs along the leaf edges.  Sometimes the leaves have a purplish red midrib, and sometimes the whole leaf is reddish.

Black nightshade leaves, underside on left.
So are they poisonous?  There seem to be no documented poisoning cases in New Zealand, and at least some forms of the species are edible.  But the best botanical advice is to treat them with caution anyway, knowing the reputation of the family for some pretty nasty toxins.  However the genus Solanum contains many edible plants—potato, tomato, egg plant, tamarillo, pepino—as well as nasties like Jerusalem cherry, bittersweet, and apple of Sodom.

The native S. americanum is similar to S. nigrum, but it has smaller flowers, shorter anthers, and the sepals are reflexed at the fruiting stage.  I'll watch out for it and post on it when I find it.
Reference
Webb, C.J.; Sykes, W.R.; Garnock-Jones, P.J. 1988:  Flora of New Zealand, Vol. 4.  DSIR, Christchurch.

3 comments:

  1. Gosh, I'd never heard of that, but I can imagine it happening. Has it happened in NZ recently?

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  2. Nightshade in peas? Yes, the last one I was involved with was maybe 3 or 4 years ago.

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  3. Purplish leaves probably a bioindicator of low phosphorus availability.

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