A Tale of Two Tomato Bushes

by Deb on May 21, 2010

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A natural experiment

A fabulous gardener I know has accidentally run a natural experiment.  (It definitely wasn’t me – if it was, these would be sad, sickly little shoots.)  He planted two tomato plants, same variety, same day, same pot, soil and everything and seeing they are only about 5m apart you would hope the same weather.  But look at them now, with the same gorgeous toddler to give you a scale:

Tomato plant comparison

On the surface they’re both growing well, far better than I usually manage.  But one is short, bushy, flowering and fruiting, the other is tall and spindly with nothing but leaves.  Why is it so?

Lets look at a couple of closeups:

tomato bush fruiting

tall tomato bush

See where the difference comes from – the first one is bushy because there are so many branches, the come off the main trunk close together and there are branchlets shooting off them.  It is dark green.  The trunk is relatively thick to support it all and there are two different sections flowering and fruiting.  The second is tall because it goes straight up, there are long gaps between the branchs and they only have leaves, there are no little branchlets shooting off, no flowers and definitely no fruit.  It is slightly paler, although that could be because of the photos.

What do you think is different about the plants to cause this?  The lovely healthy bushy one is growing at the edge of a covered area, but the tall one is in a corner.  Walking from one to the other we would barely notice the change in light, but it’s enough to prompt all these changes.  One plant is growing as tall and as quickly as it can to search for light, and doesn’t have the energy to spare for growing large leaves or developing fruit.

Etiolation

This is a standard response in plants, it’s called etiolation.  You can see an extreme example here with some beans that were sprouted in the dark.

etiolation of beans

The hormones in the plant change so it grows longer before making a leaf node, the pale colour is because it doesn’t have any chlorophyll – that’s only produced in response to light.  If it’s grown in complete darkness the plant is depending on the original energy stores in the seed and will die when they are used up.  The point is to get the leaves up to the light so they can get some energy through photosynthesis, however it’s a payoff as the stems are weak.  Hopefully for the plant the leaves will get there and allow it to slow down and strengthen the stem before it breaks.  But there’s a second option – the whole point of a plant is to set seeds and produce more plants like it.  So if it is getting enough light to live but not as much as it really needs, the plant will actually speed up its time of flowering and produce seeds quickly.  This is a calculated risk – many plants can’t self-pollinate.  If they can, they are fine and will produce seeds.  If they can’t then they are depending on other plants near them also flowering early, which isn’t that much of a gamble because they’re probably light starved too!

Do it yourself:

This is a really satisfying experiment to do with plants because it’s pretty much guaranteed to give you an obvious result.  Even I can do it!  Just get some beans or other seeds and plant two pots, leave one in a cupboard and the other on a windowsill.  In fact many of us already do a version of this experiment with potatoes – if you have some sprouting in the bottom of your cupboard leave them for a while and you will notice the shoots are very long and the seeds small and pale.  Then get them out and plant them and watch them change.  Another variation for older kids is to cover several pots with different colours of cellophane, make sure you cover one with clear cellophane.  This gives you some miniature greenhouses and really interesting results.

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