Why do plants like warm soil?

This question from listener Grant has really grown on us. Plant biologist Nadia Radzman answered it...

Nadia - Warmer soil is important for two things. One is for growth and nutrient uptake. So low soil temperature would actually increase water viscosity, and also reduces the root permeability to uptake the water. This would then affect nutrient uptake, because that is like part of the root-to-shoot transport, through xylem, that needs water. So this would pretty much affect the plant growth. Another one is for germination, but this is a little bit tricky because the seeds actually need to have low temperature periods, to break dormancy. And then when it's in a warmer temperature, when you can actually tell, from winter to spring, it starts to germinate in warmer soil.

Phil - So is it part that water and nutrients, those flow more easily; and part that it just triggers the seed germination itself?

Nadia - Yeah. Actually the breaking of dormancy is because of the low temperature during wintertime. There are two different hormones that are regulating this. So one is abscisic acid, and this pretty much, it inhibits germination and the other one is gibberellic acid. And this promotes germination. And during the low temperature in winter you have increased gibberellic acid and reduced abscisic acid. And this breaks the dormancy. So then when spring comes it can grow.

Phil - Chris Smith?

Chris S - Nadia, it's an extreme of temperature, but heat from fire, and the products of burning can also stimulate some seeds to grow, can't they? Because a friend of mine in Australia actually discovered what some of these molecules are by burning bits of tissue paper, and they're getting the molecules out and showing they would make some seeds germinate, how does that work?

Nadia - So actually, there is a particle from smoke called karrikin, and this is actually a plant hormone. So this actually comes from smoke and this is also partly why some seeds will actually start to germinate after fire.

Chris S - Why would the seeds want to do that? Why is it beneficial to the plant to grow after a fire?

Nadia - It's part of the cycle of the forest itself. Some forests like for example in Australia actually need this particular cycle.