Orobanchaceae: Characters, Distribution and Affinities

In this article we will discuss about:- 1. Characters of Orobanchaceae 2. Distribution of Orobanchaceae 3. Economic Importance 4. Affinities.

Characters of Orobanchaceae:

Lack of green colouration; obligate root parasites; stem usually in shades of red-brown-tan- purple, sometimes white; flowers moderately zygomorphic; corolla bilabiate; stamens 4, didynamous, epipetalous; carpels 2, ovary unilocular, parietal placentation.

A. Vegetative characters:

Habit:

Plants annual or perennial, fleshy, obligate root parasites.

Root:

Root suckers.

Stem:

Erect, unbranched and its lower part covered by scale leaves; lacking chlorophyll or seeming so.

Leaf:

Atternate, scale like.

B. Floral characters:

Inflorescence:

Solitary in a leaf or bract axil, or apparent raceme or spikes.

Flower:

Hermaphrodite, zygomorphic medianly, hypogynous, usually subtended by a pair of bracteoles, arising from the pedicel.

Calyx:

Sepals 2-5, united in 2-5 lobed, irregular calyx persistent, lobes valvate or open in few.

Corolla:

Petals 5, united in a bilabiate curved corolla, imbricate, the two adaxial petals inner most, straight or arcuate.

Androecium:

Stamens 4; the posteior 5^th wanting or reduced to a staminode, didynamous, epipetalous, inserted below the middle of corolla; anthers bithecous, more or less connate in pairs, occasionally one half anther undeveloped and sterile, dehiscing longitudinally.

Gynoecium:

Carpels 2, median (rarely 3), syncarpous, ovary unilocular, superior, parietal placentation, branched, i.e., placenta as many as carpels by connation of ventral margins or seemingly twice as many by the branches of each introduced placenta having reflexed and folded back against the ovary wall; Ovules numerous, anatropous; style 1 and slender, stignia terminal usually 2-4 lobed.

Fruit:

Bivalved, leathery, loculicidal capsule, often enveloped by persistent calyx.

Seed:

Minute, pitted or rough testa; endosperm soft, fleshy and oily.

Pollination:

Entomophilous due to protogyny.

Floral Formula:

Distribution of Orobanchaceae:

Oborbanchacceae with 13 genera and 140 species has the main centre of distribution in warm temperate parts of the old world. Orobanche with about 100 species is more or less cosmopolition. Orobanche indica is a common root parasite which attacks tobacco and brinjal plants.

Economic Importance of Orobanchaceae:

The members of Orobanchaceae are not of significant economic importance.

Orobache is a dangerous pest of the cultivated plants attacking and destroying them:

The following are the Indian species with their respective hosts:

Parasitic behaviour of Orobanche:

The seed of Orobanche when not in touch with the rost of a suitable hoot fail, to germinate. The plants are obligatory root parasites, phloem is more strongly developed than xylem. The plants derive their nutrition from host plants by means of haustoria which arise exogenously from root-hairless roots (Chemin 1920).

These haustoria penetrate the roots of the host with the help of enzyme action; in other cases (Tate 1925), e.g., Orbanche hederae Duby. Definite haustoria is absent, the intimate close union between the parasite and the host is brought about through close contact and appliance of the respective vascular system and cortex of the parasite and the host.

According to Chemin the scale leaves are of biological significance, the scale leaves function as secretory organs and storage of reserved food.

Affinities of Orobanchaceae:

The family is allied to Scorphulariaceae and Gensariaceae but is more related to Scrophulariaceae, probably directly derived from Scrophulariaceae or its ancestor by becoming parasitic in habit and consequent loss of chlorophyll. This scrophulariaceous affinity is indicated by some of the tribes of Scrophulariaceae, e.g., Rhinantheae and Gerardieae are parasites with almost the same floral construction.

Boeshore has recently traced the direct line of similar parasitism in both Scrophulariaceae and Orobanchaceae, e.g., of south African genera Harveya, Hyobanche and Indian Alectra thomsoni (Scrophulariaceae) are obligate root parasites with scale leaves and similar floral construction as members of Orobanchaceae. The family is also allied to Gesnariaceae to some extent, particularly in parietal placentation.

Common plants of the family:

1. Christisonia – a parasite on bamboos with bilocular ovary.

2. Orobanche indica – a parasite of Tabacco and Brinjals.