Twiglet: Abscission; abscisic acid

Topical for this time of year (in the northern hemisphere at least), abscission referes to the rejection of plant organs, such as leaves in the autumn.

This occurs at an abscission zone, where hydrolytic enzymes reduce cell adhension. The process can be promoted by abscisic acid and inhibited by respiratory poisons, and is controlled in nature by the proportions and gradients of auxin and ethylene. Other hormones may be involved.

Abscisic acid is a terpenoid compound that is one of the five major plant horones. Althought it is synthesised in the chloroplasts, it occurs throughout the plant body and is particularly concentrated in the leaves, fruits and seeds.

It has a powerful grown inhibiting properties generally and also promotes leaf abscission and the senescence of plants and/or their organs, and induces the closing of the stomata and dormancy in seeds and buds.

Its effect is antagonistic to the plant growth hormones, and it is thought to act by inhibiting the synthesis of protein and nucleic acids.

Source: Oxford Dictionary of Plant Sciences

Apples’ autumn colour change clue

The long-standing debate about why autumn leaves change colour has new impetus from the humble apple tree, the BBC News website reports.

Domesticated apples – selectively bred for fruit size and taste rather than insect defence – tend to have less red leaves than their wild cousins.

Researchers suggest that fact supports one theory for the change: that autumn’s red colours ward off insects, indicating a plant’s chemical defences.

The research is published in Proceedings of the Royal Society B. However, other experts remain sceptical of this “co-evolution” theory.

The idea, taking into account the full range of autumn colours, was first suggested in 2001 by the late biologist Bill Hamilton.

More recent research, however, has shown that autumn’s oranges and yellows are caused by carotenoids.

These are pigment molecules present year-round, normally serving to protect chlorophyll – the green-coloured molecule at the heart of photosynthesis – from damage caused by sunlight.

In the autumn, as chlorophyll is actively broken down in the leaves, the carotenoids become visible.

Autumn’s brilliant reds and purples, however, are caused by molecules called anthocyanins that are produced during the same period.

It is a costly job of molecule building for the plant and an enigma to scientists, since the leaves will at that point soon be dropped entirely.

“If you wanted to prove this hypothesis that the autumn colours are necessary to repel insects, what you would do is take two populations of trees and let them evolve – one with and one without the insects – expecting the one without insects would lose its colours,” said author of the research Marco Archetti, of the University of Oxford’s zoology department.

“That’s exactly what’s been done starting 2,000 years ago when they started to domesticate apple trees, because they’ve been sheltered from the influence of insects and parasites,” Dr Archetti explained.

“There is no longer natural selection; what is going on is artificial selection by the farmer for fruit size and flavour, not resistance against insects.”

David Wilkinson, an environmental scientist at Liverpool John Moores University who has published on the leaf colour debate, says that the work is not proof positive of the co-evolution theory.

“There’s a difference between a ‘signal’ and a ‘cue’,” he explained.

“A cue is something that hasn’t evolved for a signalling function but is used by something else as information that affects its behaviour.

“But that doesn’t mean that the autumn colour has evolved for that purpose.”

Dr Wilkinson points out that a competing theory holds that anthocyanins are performing a different task altogether.

Plants are known to break down components of their leaves and harvest a number of precious compounds – particularly those containing nitrogen – before cutting the leaves loose entirely.

“I think the most likely explanation is that these [anthocyanins] are effectively sunscreens that allow the photosynthesis to continue as the machinery of photosynthesis is broken apart in the autumn.

“The idea of, as it were, ‘the trees are talking to the insects’, is wild and wacky and it would be rather nice if it were true.

“But I still have not seen anything that convinces me of the signalling.”

Dr Archetti believes that the loss of red leaves among domesticated plants cannot be explained by the “photoprotection” theory favoured by Dr Wilkinson.

He now intends to study apricot and walnut trees, the domesticated varieties of which he says have also experienced the loss of autumn’s reds.

Source: BBC News website

Date: 15/04/2009

UK weather sparks autumn spectacular

Parks and forests in the UK are making up for the miserable summer by providing visitors with spectacular autumn leaf displays, experts say.

A report on the BBC News website said that public gardens have been carpeted in an array of deep red and yellow leaves, thanks to the year’s unusual weather.

Experts at the Royal Botanic Gardens, Kew, London, say a wet summer, followed by mild frost and some warm September days, were perfect for the display.

American oaks, ash, and sweet gum trees provide some of the best colours.

Tony Kirkham, Kew’s head of arboretum, said the wet summer provided good growing conditions for the trees.

An unusual mild frost on 28 September then helped the leaves turn to gold.

He said: “In September, we had some cold temperatures in the evening and warm, sunny days.

“The mild frost acts as an early warning to the trees to shut down for winter, so they can take some of the goodness out of the leaves and you get the good colours.

“If you get a long frost, they don’t get a chance to do that and the leaves fall quickly.”

“We have a real variety of trees in Kew, so you get a rainbow of colours but we seem to be getting good autumn colour all over the country.”

The National Trust says its gardens such as Sheffield Park in East Sussex and Stourhead in Wiltshire are attracting many visitors, keen to see the displays.

Meanwhile, the Woodland Trust says the wet summer has been good for nature, providing an abundance of fungi and berries for birds to feed on ahead of the winter.

Source: BBC News website

Date: 20/10/2008

Autumn watch on both sides of the Atlantic

Tree lovers on both sides of the Atlantic are able to make sure that they do not miss out on the colourful delights of autumn, thanks to the websites of the US Forest Service and the UK Forestry Commission.

The US Forest Service is offering people a free “hotline”,  which is an automated phone service that will inform callers about the colour of the leaves in the national forests.

Meanwhile in the UK, the Forestry Commission has set up a website that offers a colour-coded website of the commission’s plantations. The website shows what colour the leaves of the various woodlands have reached (ranging from “still green” to “turned golden”).

Although the UK experienced a much wetter than average August, an official for the Forestry Commission said that the woodlands were still “on course” for a colourful autumn.

Source: US Forest Service and UK Forestry Commission websites

Date: 24/09/2008

Why leaves fall from trees

Us researchers have published a paper in the Proceedings of the National Academy of Sciences (PNAS) in which they explain the sequence of events that cause plants to shed their leaves.

Writing in the UK’s Telegraph newspaper, science editor Roger Highfield explained that trees use an elaborate cellular mechanism to part company from their leaves, which act as “solar cells” in the summer but become superfluous in the darker winter months.

Reporting the researchers’ findings, he said that at the base of each leaf is a special layer called the abscission zone.

When the time comes in autumn to shed a leaf, cells in this layer begin to swell, slowing the transport of nutrients between the tree and leaf.

Once the abscission zone has been blocked, a tear line forms and moves downwards, until eventually the leaf is blown away or falls off. A protective layer seals the wound, preventing water evaporating and bugs getting in.

The discovery into how trees take on their winter aspect follows a study explaining the bright colours of autumn foliage.

Source: UK Telegraph newspaper

Date: 22/09/2008

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