Root pressure can be demonstrated by means of
Root exudates function in below ground plant defence. Plant roots are constantly exposed to a variety of natural enemies, including pathogenic microorganisms and root-feeding arthropods. These organisms inhabit the soil surrounding the root system, which is designated the rhizosphere.
Chemicals secreted into the soil by roots are referred to as root exudates. By doing this root changes the chemical and physical property of soil and inhibit the growth of other plant species. Root pressure can be demonstrated by means of exudation or bleeding caused due to root pathogens.
so the correct answer is 'Exudation'.