From Vanuatu to Australia, and then Yishun: The yellow plant giving Singapore fall colours year-round

This depends on the type of genetic mutation that causes the plant’s leaves to be yellow.

In the plant world, there are two main types of mutations.

Germline mutations occur in the reproductive tissue of the plant, and can be passed on to its offspring. In comparison, somatic mutations happen in non-reproductive cells, such as in a leaf or flower, and will not be passed on.

Mr van der Schans said that while comprehensive studies on the cultivar have yet to be done, the mutation that gives the cultivar yellow leaves instead of green ones is likely a germline mutation. The mutation also possibly involves a dominant, rather than recessive, gene, he added.

Traits associated with a recessive gene are expressed only when both parents contribute the gene to the offspring, whereas traits associated with a dominant gene show up even when paired with a recessive copy.

Around early 2011, about 10 saplings of the plant – the first Dendrolobium umbellatum cv “Vanuatu” specimens in Singapore, to Mr van der Schans’ knowledge – were planted in Gardens by the Bay.

It was about six to 12 months later that he noticed that their offspring – all bearing yellow leaves – sprouting beneath their canopies from seeds that the plants had dropped. Since then, the “grandchildren” of the original saplings have also had yellow leaves.

That successive generations of the plants have turned out yellow 100 per cent of the time came as a surprise to Mr van der Schans, largely because the odds of most mutations appearing between successive generations are low.

“That was when I knew it was worth propagating by seed. Otherwise, I’d assumed that yellow or variegated plants don’t come true from seed, and you usually have to grow them vegetatively by cuttings or even layering,” he said. Variegation in plants refers to irregular colours or streaks in foliage.

Plants grown by cuttings or layering are clones of their parents, whereas plants grown from seeds contain genetic material from two parents and may have traits that are harder to predict.

“But this one coming true to type from seed makes it extremely fast and cheap to propagate,” said Mr van der Schans, who gave seeds of the cultivar to National Parks Board (NParks) officers.

“True to type from seed” means that the seedling takes exactly after its parent – in this case, the yellow parent plants bear yellow offspring.

NParks has gone on to use the cultivar in streetscape planting.

In just over a decade since the first Dendrolobium umbellatum cv “Vanuatu” saplings were planted in Singapore in Gardens by the Bay, plants from the cultivar – many possibly the descendants of the original 10 saplings – can be found all over the country, giving streets a touch of fall foliage year round.

NParks group director for streetscape Oh Cheow Sheng told The Straits Times that the Dendrolobium umbellatum cv “Vanuatu” thrives in coastal areas and has been planted on coastal roads such as Tanah Merah Coast Road.

It is also well-suited to urban areas that have growing conditions similar to coastal areas, said Mr Oh, citing conditions such as high light incidence, strong winds and dry environment.

Mr Oh said such urban areas include Dunearn and Napier roads, adding that the yellow foliage “adds a touch of colour and vibrancy to the landscape”.

Mr van der Schans said that while there also are yellow pigments in the green-leaved Dendrolobium umbellatum, these are typically masked by chlorophyll – the green pigment that plants use to make their own food through photosynthesis, a process that requires sunlight.

The genetic mutation in the Dendrolobium umbellatum cv “Vanuatu”, he said, causes less chlorophyll to be present in the leaves, and the yellow pigment to show through.

He noted that the plant shows its yellow colour most when exposed to sunlight regularly, and that portions of the plant that are often under shade could have greener leaves.

This is because more chlorophyll is present in these leaves to compensate for the lack of light exposure and produce sufficient food for the plant.

As far as Mr van der Schans knows, the Dendrolobium umbellatum cv “Vanuatu” was first brought into Australia from Vanuatu in the early 2000s by Australian landscape architect and horticulturist Arno King.

Mr King had collected seed of the plant on Efate, the island on which Vanuatu’s capital, Port Vila, is located.

Mr van der Schans, who practised landscape architecture in Australia before moving to Singapore, said Mr King told him that locals in Vanuatu said the Dendrolobium umbellatum cv “Vanuatu” was brought to Efate around the 1990s from Tanna, another of Vanuatu’s islands.

The Gardens by the Bay principal horticulturist and Mr King believe that the variant could have first been spotted by English landscape gardener and botanist William Guilfoyle on Tanna in 1868.

Guilfoyle in an 1869 journal described a tree about 4.5m tall that was likely fully grown and had leaves that were “of so intense a golden yellow”.

He said that the tree had beauty and magnificence “unequalled in the flora of Polynesia, and perhaps not surpassed for elegance and splendour in any part of the world”, adding that he could not find seeds, seedlings or suckers from the tree and had to make do with cuttings, which died not long after he left Tanna.

Mr van der Schans notes that Guilfoyle had in the journal identified this tree as coming from the Inocarpus genus – something he believes was a mistake as trees from this genus can be up to 30m tall when fully grown, with much larger leaves, and also seed prolifically.

“So either Guilfoyle made a botanical boo-boo, and what he did see was actually the Dendrolobium, or somewhere on Tanna is possibly the world’s most beautiful tree, unseen by human beings,” said Mr van der Schans.