| dc.description.abstract | While most of the plants are autotrophic and are the main sustenance of life on this planet, a few plants have acquired the parasitic habit and thrive on other plants. The extent of this dependence varies, and so also the organs that have been developed for purposes of nutrition, etc. Almost all of these parasitic plants are disadvantageous to man, for they live on his field and orchard crops and on his forests. In contrast, Santalum album (Sandalwood), a hemi-root parasite, is a source of great revenue, by virtue of its fragrant heartwood and the oil contained therein. Paradoxically, the biology of sandal seems to have been investigated rather poorly except for the contributions on the "spike" disease. Some aspects of the interaction of the sandal plant with the environment, particularly in relation to the microflora on its surfaces and its capacity to take up mineral nutrients in comparison with its host plants, were investigated. Acacia farnesiana and Dolichos lablab were used as experimental hosts. The salient features of the observations made are summarized below:
The sandal roots, like other plants, exhibit the rhizosphere effect. The microbial population increases with age and on parasitization. There is a further stimulation of microbial activity on the roots. A stimulatory effect is noticed on the young haustorial surfaces also. But soon after the formation of parasitic unions, the microbial load on the haustorial surfaces decreases. Different groups of microorganisms are stimulated to varying extents.
A number of amino acids are exuded by the roots of sandal and its host plants.
Sandal seeds harbor thousands of bacteria and molds, and these seem to play a protective role. Surface-sterilized seeds generally display lower emergence, and if sown in non-sterile soils are completely destroyed by soil organisms. A Fusarium species isolated from the seed surface improves germination of sandal seeds. This beneficial effect is apparent whether the seeds are treated with culture filtrates or with a dilute spore suspension of the organism. Gibberellic acid could be isolated from the culture filtrates of the isolate. Soaking the seeds in gibberellic acid solution also increased emergence enormously. Sandal seeds do not require any host factor for germination. The plant has no specific host requirement and can parasitize any plant that it comes across (more than 500 species have so far been listed as its hosts). Thus, the non-requirement for any host factor for germination is an adaptation suited for its habits.
Just as the roots do, the leaf surfaces (the phyllosphere) of Dolichos and sandal harbor a selective microflora. The leaves of parasitized plants appear to support a denser population than free-living or unparasitized plants. Nitrogen-fixing organisms are highly prevalent in the phyllosphere. The microbial incidence on the dorsal and ventral sides of the leaves are also distinct. When Dolichos plants were grown semi-axenically and the leaves were inoculated with Beijerinckia, the plants exhibited better growth and weight gain. Even the pattern of amino acid exudation by the roots was altered.
The dependence of hemiparasites is frequently believed to be restricted to minerals and water. Sandal is a perennial (in contrast to hemi-root parasites that have so far been experimentally investigated, which are all annuals) and can survive independently at least for a few months. Hence, the cation exchange capacity (CEC) of the roots and the ability to take up Ca² and PO ³ ions of sandal and Dolichos were studied. The CEC of sandal is very low (7 meq/100g) when compared to Dolichos (44 meq/100g). In order to verify that low CEC may not be a general property of perennials, the CEC of four other tree species was tested and was observed to be significantly high.
The Ca² uptake of sandal is correspondingly poor when compared to Dolichos. In contrast, PO ³ is absorbed as efficiently by sandal as by Dolichos. Interestingly, while Ca² uptake by roots is low, sandal seems to be able to transport rapidly the absorbed ions to the shoots. One-fourth to one-half of absorbed ions are transported by sandal; Dolichos plants seem to transport only about 10 percent to the shoot system.
In order to study the movement of organic substances between the plants, host-parasite associations were developed in pots. Then either the host or the parasite was selectively exposed to ¹ CO atmosphere. When Dolichos plants were exposed to ¹ CO , radioactivity could be recovered in sandal within a few hours. A number of amino acids and sugars were found to be labeled both in the host and the parasite. On the other hand, when sandal plants were exposed to ¹ CO , no radioactivity could be recovered in the host. This shows there is a mechanism to regulate flow of materials. When ¹ C-labeled glucose and ³²P-labeled sodium phosphate were applied on the foliage of Dolichos, these were easily transported to sandal.
The photosynthetic ability of sandal is less efficient than that of Dolichos. The holoparasites are extremely deficient in photosynthetic mechanism. But partial ability in the hemiparasite is again an evolutionary adaptation. The sandal seeds germinate without any host stimulus, and so it is incumbent on the part of the seedlings to possess machinery for independent survival. Thus, the seedlings are green and thrive well in the early days. This ability persists even in the mature stage of the plant, long after successful parasitization. But the capacity to fix all the CO needed does not seem to be prevalent, and hence a good amount of organic substances are drawn from the host. Similarly, high transportation rates help to alleviate, to an extent, poor uptake ability. This high transport efficiency also aids the parasite to draw nutrients from the host. Thus, we see several interesting parasitic adaptations in sandal. | |
