Instant Notes: Plant Biology

Section M – Interactions between plants and other organisms

M6 Parasites and saprophytes

The great majority of plants use only sunlight and inorganic molecules for all

their energy and nutritional needs, but there are some exceptions. There are

around 3000 species of plants that are at least partially parasitic on other plants,

i.e. a little over 1% of all flowering plants. Only about 20% of these are entirely

dependent on their hosts, the remainder having some green parts so potentially

making some of their own sugars (known as hemiparasites). A few of these can

live without a host plant, although they rarely do. At least eight plant families

have parasitic members, some of which also have non-parasitic species, so the

habit has probably evolved several times within the flowering plants. Among

land plants only angiosperms have become parasitic (one unusual conifer of

Parasites

Key Notes

There are 3000 species of parasitic plants, 20% entirely dependent on

their hosts; 60% of plant parasites are root parasites, 40% parasites of

stems. Most are herbaceous ephemerals or perennials of grasslands and

disturbed areas, but a few are woody and sandalwoods are trees. Many

have numerous small seeds. Stem parasites have fleshy animal-dispersed

fruits.

In root parasites, root exudates from a potential host stimulate

germination and a radicle to grow to the host root. Once attached, most

parasites establish a haustorium connecting the vascular systems to

absorb some of the host’s nutrients. Stem parasites attach in a similar way

to above-ground parts.

Parasites can reduce crop yields by 30%, the most important pests being

Strigaspecies on cereal crops in Africa and Asia, Alectraspecies on

groundnuts and others and Orobanchespecies mainly on beans. These are

all members of the Scrophulariaceae family group. One unrelated pest is

Cuscutaspecies, stem parasites of low host specificity.

Saprophytes obtain nutrients from decaying plant parts. Plants can only

do this using mycorrhizal fungi and all mycorrhizal plants are partially

saprophytic. A few specialized members of the two ectendomycorrhizal

family groups, the heathers and orchids, have no green parts and depend

entirely on their fungi. The gametophytes of some spore-bearing plants

are saprophytic.

Related topics Roots (C2) Mycorrhiza (M1)

Ecology of different growth

forms (K3)

Parasites

Growth of parasites

Economic importance

of parasites

Saprophytes