Sprouting a sandalwood sapling

Passage through the digestive system of birds is good for the seeds as they germinate quickly and have better chances of maturing into trees

Sandalwood hardly needs an introduction to the readers of The Hindu. It has been valued for many centuries, for its fragrant oil, its prized wood, and the many medicinal uses it has been put to. The tree that all this comes from, however, is not all that familiar. Growing in deciduous forests, it is a partial, or hemiparasite that needs four or five other trees growing around it. Under the ground, sandalwood roots form a haustorium that forms an octopus-like hold on the host tree’s roots, from where water and nutrients are taken.

The sandalwood fruit is probably even more unfamiliar. About 1.5 cm in diameter, the fleshy fruit is a shiny purplish black when ripe. The one seed inside is a hard, dry kernel, not the usual tough seed coat protecting a fleshy interior. This makes it difficult for the seed to survive beyond one season.

Both the above properties – the need for other trees in the early growth phase, and the seeds, which are short-lived and cannot be stored, have added to the overexploited tree’s difficulties. This has led to a drastic fall in the number of sandalwood trees in the forests of South India. The IUCN has classified sandalwood as a vulnerable species. It is not surprising that Australia is now the world’s largest supplier of sandalwood and its oil.

Dispersal by birds

The fruit is bitter, and not to human tastes. But it is loved by birds. About 10 species, such as the Asian Koel, and the Gray Hornbill swallow the fruit whole, and over time drop the seeds at great distances from the tree they feasted on. These birds are among India’s larger frugivores, or eaters of fruit. The sandalwood tree’s fruit is just right for the koels and hornbills. It has been established that sandalwood trees that produce larger seeds usually end up with the seeds close by. Although the large seeds are better equipped for germination, birds cannot swallow those large seeds, and drop them off after pecking away at the flesh.

The passage through the digestive system is good for the seeds. The seeds now germinate very quickly and have better chances of maturing into trees. This is the reason why forests, and not plantations, are where we get to see a few mature sandalwood trees. Sadly, the thinning of forests has reduced bird populations, and therefore the chances of proper seed dispersal.

Can humans try to emulate birds? Researchers at the Kerala Agricultural University in Thrissur, working with European colleagues have tried various ways of priming sandalwood seeds for germination (Forests, 14:1076, 2023). Best results were obtained when they soaked freshly collected sandalwood tree seeds in a 5% solution of polyethylene glycol-6000 for two days. This interesting synthetic substance induces osmotic pressure on the cells of the seed and pushes the germination process forward. This is called osmopriming, and when done correctly is more effective than soaking in just water. The sprouting rate was 79% compared to 45% when just planting the seed.

(The article was written in collaboration with Sushil Chandani, who works in molecular modelling.