Can plants affect one another in the garden?

We put this question to Tony Hooper:You certainly wouldn't want to try to grow anything under a pine tree, as they drop resins which are poisonous to other plants, as do walnut trees.With regards arable crops, rather than gardens, wheat produces chemicals in the soil that are generally toxic both to other plants and to insects that may attempt to colonise them. We're working on getting this respondse 'on demand', to protect crops from attack.