Host Plant Influence on Haustorial Growth and Development of Indian Sandalwood (Santalum album)

Abstract

AbstractSelection and management of hosts have remained one of the most important silvicultural research topics, since the discovery of the parasitic nature of sandalwood (Santalum album) in 1871. It is well recognized that the selection and planting of appropriate host trees for sandal plantations are desirable to improve the returns. Sandalwood, a hemiparasite, parasitizes roots of almost all plants, and a significantly higher rate of carbon assimilation rate is observed in sandalwood growing with the host than the parasite growing without a host. Host plants with the nitrogen-fixing ability, light shade and fast-growing species appear to be the most suitable hosts. Considerable differences in xylem solute transfers between the sandalwood and different hosts have been documented, and such differences have been used together to discriminate host species that are distinctly superior to other hosts in terms of parasitic overall benefit. The most important host quality index for sandalwood is the production of biomass per haustoria and overall haustorial biomass production. Sandalwood exhibits host specificity at different developmental stages of the plant, and the hosts are classified into the pot (primary), intermediate and long-term host. Sandalwood seedlings with a suitable pot host increase the field survival rate. Intermediate hosts are generally a fast-growing short-lived perennial that serves between the pot host and field host. The main role of an intermediate host is in stimulating the early growth rate of sandalwood plantation. The intermediate host will wither away or become insignificant when sandalwood parasitizes on the field host. Field host is expected to last throughout the life of sandalwood and provide stable support for the growth of the tree. Morphological, anatomical and physiological characters of sandalwood haustoria are well studied. Haustorium is composed of the hyaline body, penetration peg and the ellipsoidal disc. The haustoria help the plant to access host resources through either direct vascular continuity or interfacial parenchymatic cells. Sandalwood haustoria lack direct phloem or xylem connections with its hosts. The flow of water and nutrients from the host occurs primarily via host xylem element pits to the interfacial parenchyma of the parasite. Substantially high amounts or endogenous phytohormones are found in the haustoria compared to either the host of sandalwood roots indicate their significant role in haustorial development and function. Application of plant growth regulators in an in vitro environment has been observed to increase the number of haustoria. The sandalwood is a debilitating parasite, and the host damage increases with good hosts; hence, sandalwood may not be a suitable agroforestry partner for ‘suitable’ hosts.KeywordsHostHemiparasitismRoot parasitismHaustoria