Saprophytic Orchids

A very small minority of orchids have developed the peculiar habit of depending upon the mycorrhizal fungus in their roots , for their entire supply of nourishment . These plants do not possess any chlorophyll and their plant-body is reduced to a knob –like stem and a bunch of coralloid roots. They emerge above ground only during the flowering time , when a shoot is sent up which bears the flowers . From South India, Fischer has reported three species which are entirely saprophytic –Didymoplexis pallens , Epipogum nutans and Aphylorchis Montana . Varjrvelu and Radhakrishnan have reported a fourth saprophytic species , Aphyllorchis prainii ,from the forests of Attapadi.

All orchid seeds growing in the wild ,require the association of a fungus for purposes of germination. For a long time the exact nature of this association was not understood clearly . It was Noel Bernard , the French physiologist who first discovered that a fungus was necessary for the germination of the orchid seed. In 1899 he came across a bunch of seedlings of Neottia nidus-avis growing from a decaying fruit. In 1903 he was able to prove that by infecting seeds with pure cultures of fungi they could be made to germinate and develop into seedlings . But his deduction that the increased osmotic pressure produced by the conversation of starch into sugar , was responsible for the necessary stimulation for germination of the seed was proved wrong by Knudson . The latter was able to show that it was the sugar itself which started the process of germination . The mycorrhizal fungus was responsible for the supply of this sugar to the orchid seed.

Most terrestrial orchids may be able to dispense with the services of the fungus and become autotrophic after 4-10 years of existence . Not so the saprophytic species . They are incapable of developing root hairs , which hampers the further growth of the seedling . Burgeff (1959) has reported that under cultural conditions the seedlings of saprophytic species do not develop beyond the first leaf-primordia and the first root.

Saprophytic orchids possess a greater amount of mycorrhiza in their roots than the non-saprophytic species . Considerable amount of research has been done on the exact nature of the relationship between the host plant and the mycorrhizal fungus, the pioneers of the work being Noel Bernard (1904, 1909), Hans Burgeff (1909, 1936, 1959) and Louis Knudson (1922, 1924, 1925). The oldest theory is that the relationship is one of symbiosis , where both the participants are benefited. This is valid only in the case of the non-saprophytic species , whereas in the case of saprophytes it is entirely a matter of the orchid living off the fungus.

The orchid plant can obtain only part of its nourishment from the atmosphere, washed down by rain and absorbed via the velamen roots . For the major part of its mineral requirements, it depends on the mycorrhiza. In return ,the fungus gets its supply of carbohydrates from the host plant. Burgeff (1959) has recorded that as soon as the fungal hypae enters the host tissue ,the starch grains in the amyloplasts of the host cells disappear .

The Mycorrhizal fungus which resides partly inside the roots and partly in the surrounding humus, is supposed to peg down the plant in the soil , absorbing minerals and other nutrients from the surrounding medium which is the utilized by the host by a process of digestion of the fungal cells within the host tissue . Fungal hyphae which have been fed on sugars with labeled radioactive carbon has been allowed to infect the roots of orchid seedlings , which later showed distinct radioactivity in its stem and leaves . In the meristamatic regions of the infected root , there are three layers of cells beneath the piliferous layer- the fungal host cells , the digestion cells, and the storage cells . When the hyphae are replete with storage materials , the digestion process sets in , no doubt , initiated by the host cell nucleus (Burgeff, 1959). The process goes on repeating .

Instead of being the peaceful co-existence as depicted above ,the hostfungus relationship has also been interpreted as a process where the two participants are constantly at war with each other . Depending upon the comparative strength of the participants ,either the host of the fungus may have the upper hand . When the host has the advantage , as for example , during the time when the plant is growing actively under favourable climatic conditions ,there is extensive digestion of fungal cells . On the other hand , when the fungus is the more aggressive of the two , large tracts of the host tissue are invaded by the fungus . The state of symbiosis is reached when the destructive powers of the fungus and the host are each perfectly balanced by the other , so that to all outward appearances there is a peaceful co-existence between the two.

Production of phytoalexins in orchid bulbs immediately after infection by a fungus is pointed out as a sure proof of the antagonistic nature of the relationship . Phytoalexins are similar to antibodies produced in animals in response to infection , and resists further attack by the fungus (Arditti , 1972).

The fungus most commonly found in chlorophyllous orchids belongs to the imperfect genus Rhizoctonia . Hymenomycetes are confined to the saprophytic ., are also occasionally seen in some orchids (Burgeff, 1959).