11 June 1999

Search is on for more drought-resistant types

WITH lack of water the biggest single cause of sugar beet yield loss, researchers are seeking ways to make the crop less drought prone and more economically reliable.

On average 10.5-11% of the national crop is lost through lack of rainfall in June, July and August. This costs the nation about £28m a year, and adds £2m to British Sugars processing costs.

"Average yield loss ranges from nothing to 30%, but may be even more in East Anglia where there is less summer rain," says Keith Jaggard of IACR-Brooms Barn. "The last severe drought year was 1995, but a lot of sugar was lost in 1994, 1990 and 1983."

Only a tiny part of the UK crop is irrigated. Up to 17% has received water in some years, but in 1995 other crops took priority so there was only enough water and irrigation capacity for 6% of the crop.

Dr Jaggard says irrigation is not the answer and wants to make beet weatherproof.

Performance could be further compromised by global warming. "Any rise in carbon-dioxide concentrations will probably make the situation worse because evaporation rates will increase."

Brooms Barns main research started last year, but a new MAFF-funded project on global warming has increased the significance of the work.

"It seemed sensible to start by finding a way to improve the drought tolerance so the crop can cope in dry seasons without suffering a yield penalty in a "normal" wet year," says Brooms Barns Eric Ober. "No one has ever looked for drought tolerance in existing and old sugar beet types. They could be harbouring genes breeders need, or they could be lurking in an old forgotten variety. If they are out there we want to find them so they can be plugged into a modern high-yielding variety."

If unsuccessful, the researchers will look further afield and check wild beet types. Beta maritima which survives along the exposed UK coastline is the first target.

Searching out and analysing wild types is labour intensive so breeders are not keen to do the initial work. This is why Brooms Barn, with funding from SBREF, Novartis, KWS, Perry Foundation, and the Chadacre Trust, is involved.

Another part of the project involves screening wild and conventional beet and fodder types for disease resistance and drought stress. So far two of 150 lines checked have been selected for a further look.

One is a coastal wild type, the other an eastern European sugar beet line from Georgia. They are included with 34 others in a trial this summer to see which yields best when stressed.

Last year the researchers could shield only a tiny area of their crop from rain and had hoped for a dry summer. Of the 16 lines checked the Georgian type performed slightly better than the rest in what was far from a severe drought. This year 0.5ha will be covered using polythene tunnels to keep rain off.

"We hope that lines with drought tolerance will stand out more clearly when starved of water.

"The next step is to stamp them with a genetic marker so the breeders job of selecting them from lines grown in a wet part of the country on moisture-retentive soil is easier. Then it should also be possible to retrospectively review the drought tolerance of varieties currently available," says Mr Ober. &#42

Danger stages

There are three stages when beet is sensitive to lack of water – during establishment, when the canopy is developing, and in the early sugar storage phase in July.

There is seldom a problem early on except on well-bodied soils where the crop is drilled late and seedling emergence coincides with rising temperatures and heavy evaporation.

Plants need adequate moisture during canopy development because any shortage slows the growth rate. But roots have to usually get down enough for plants to cope.

The crunch comes if there is no available water in July. A dry June will have depleted soil reserves and crops on sand can run out of water. When this happens the canopy wilts, leaf activity ceases and the plant shuts down and cannot cool itself. Leaf temperatures rise and in a heatwave leaves can burn and die.

If much leaf is lost the next worthwhile rain triggers survivors to produce fresh foliage. This uses up stored energy and reduces yield potential. And because many small fibrous roots have gone the crop loses its ability to forage for moisture and becomes more prone to late season drought.