Netting trees

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Israeli researchers have boosted the yield and altered the colour of apples growing in commercial orchards by stretching plastic nets with specific optical qualities over the canopy. Their ‘smart nets’ could provide an alternative to genetic modification, they say. Yosepha Shahak of the Volcani Center, part of Israel’s Agricultural Research Organization in Bet-Dagan and colleagues collaborated with plastics experts to develop a series of nets, each of which combines a filter to screen out all but a specific portion of the radiation spectrum – including ultraviolet and infrared as well as visible wavelengths – with different shading factors. The altered light reaching the trees activates photo-receptors and light-dependent pathways in different ways, and these then interact with other physiological processes that depend on climatic factors such as temperature and humidity to affect the size, colour and possibly even flavour of the fruit – though flavour has yet to be tested. The physiological pathways are so complex that it will be some time before they are able to unravel them, says Shahak, but one and a half harvests into the experiment they are already identifying trends.

The most dramatic effect is on the size of the fruit. “The ratio of the largest apples, those with a diameter of 75 millimetres or more, is six times higher with the best performing nets than in the unnetted control condition,” she said. With a black net of the same shading factor – that is, a net that screens out every portion of the spectrum evenly – the ratio of the largest fruit was doubled, indicating that a part of the effect is due merely to the intensity of the light and possibly protection from sunburn. Sunburn is a major problem for fruit-growers in hot countries, with certain varieties being particularly susceptible. “With Granny Smiths, for example, you can get up to 50% of the yield lost to sunburn,” said Shahak but the major part of the effect on fruit size must be due to another factor, she says – the filtering through of only a selected part of the spectrum.

As well as apples, Shahak’s team are testing the technique with grapes, strawberries, and peaches. Their experiments were inspired by the “very dramatic” effects they observed in previous studies using the nets on crops of flowers and plants to be sold in pots. Using red and yellow nets, they grew flowers with longer stems – a major factor determining the price of cut flowers. Whereas, says Shahak, “A blue [net] causes dwarfing, which is also beneficial for certain purposes.”

Eric Curry, a plant physiologist at the US Department of Agriculture’s Tree Fruit Research Laboratory in Wenatchee, Washington describes the new approach as “innovative” and likely to be very useful. He warns, however, that the studies will have to be carried out over several years to show the effect is sustainable, pointing out that fruit growers have to manage their trees in order to be able to rely on a consistently good harvest year after year. That means removing a proportion of the flowers so that only some develop into fruit – thereby preserving the tree’s resources. “Practices which modify physiological processes like flowering and yield in the first year, often impact the same and/or other processes in subsequent years,” he said. So he would like to know how future harvests will be affected by the nets.

Shahak’s findings were presented at the beginning of October at a meeting of the American Society for Horticultural Science in Providence, Rhode Island.

Article contributed by Ivan Gee.