Paris: An unusual vine discovered by a 90-year-old volunteer nature guide in Japan has a "unique" way of using its leaves to curl around its fruits to envelop them in a protective microclimate, scientists said on Wednesday.

The cucurbitaceous vine, a type found in East Asia, is an oddity because while leaves come in all shapes and sizes and perform a crucial role in photosynthesis, they are rarely associated with reproduction.

But a new study published in the journal Proceedings of the Royal Society B: Biological Sciences found that the vine had specialised leaves able to enclose fruit and enhance seed production in colder conditions.

The research was led by Nobuyuki Nagaoka, the 90-year-old guide at Yamagata Prefectural Natural Museum Park, who first spotted the leaf behaviour in 2008 and has observed it every year since.

"Here, we report, for the first time, a unique function of leaves that enclose immature fruits in an annual vine," said the authors, who also include researchers from Kyoto University and Japan's Forestry and Forest Products Research Institute.

They describe the vine as a slender, annual plant that often inhabits the edges of deciduous forests with disturbances like roads, rivers or mountains.

It can either be hermaphrodite or male and produces small, white flowers pollinated from August to September and later develop into fruits, each with a single seed.

The researchers looked at plants at different altitudes at the foot of Mount Gassan, in the southern part of the Dewa Mountains, in an area partly within Yamagata park.

They noted some leaves on hermaphrodite plants that were undeveloped in summer "expanded and overlapped with each other" to create a sort of cocoon around immature fruit.

The study found that these specialised "enclosure leaves" are produced towards the end of the growing season and produced a microclimate of up to 4.6 degrees Celsius warmer than was recorded around fruit where the leaves had been plucked off.

Removing the leaf enclosures negatively affected the survival and growth of the vine's fruit, although they were unable to identify the mechanism, said the authors.

They also found that the leaves grew thicker protective layers in colder areas.

"The results suggested that enclosures allow the plant to produce seeds under the cold weather the plant encounters at the end of its life," the study said.

Reproductive role

These enclosure leaves were found to have less photosynthetic ability and were different in greenness and structure from others.

Previous research has described some functions of leaves that aid reproduction, such as the plant Saururus chinensis, whose leaves can temporarily turn white to attract pollinators.

But the study said such traits were likely "in conflict with traits that promote photosynthesis, the primary function of leaves".

"Leaves that play a major role in reproduction rather than in photosynthesis have not been reported so far to the best of our knowledge," the authors said.

They added that researchers usually look at plant reproduction and vegetative traits separately, but said the new findings suggest how these functions "can dynamically interact".