This year saw one of the driest Aprils on record in East Anglia and much of the East Midlands – the UK’s main sugar beet growing area. It is yet another indication of more extreme climatic conditions that may affect crop performance and returns.
Drought tolerance is one of several traits on the radar of plant breeders. It is particularly important for sugar beet because the crop uses increasing amounts of water throughout the season – especially once crops have achieved full cover.
Water demand from sugar beet peaks in July and August when yields will suffer if it becomes a limiting factor. On average, annual water demand is between 600-700mm, which can be met without irrigation only if sufficient water is available.
Sugar beet requires only half the amount of water required by sugar cane. But prolonged drought can still reduce sugar yields by up to 50%, says SESVanderHave research and development manager Richard Robinson.
The key to drought tolerance is to develop physiological and metabolic properties that allow a plant to maintain a high degree of tissue hydration, even when water supply is limited.
There are four potential scenarios when it comes to developing more resilient sugar beet varieties:
1) Just carry on. Here plants develop normally in normal conditions. This is fine when crops come under stress for short periods. But the risk of significant underperformance increases as stress periods lengthen.
2) Reduce drought damage. Varieties that achieve this are able to reduce the energy and sugar that is used for regrowth or to repair cellular damage which can occur when drought is severe during the growing season.
3) Improve water efficiency.Varieties that require less water for growth. They are adapted to reduced water transpiration during carbon dioxide exchange. They are able to regulate temperature in a number of ways including curling the leaves, wilting or by closing the stomata in times of drought to reduce transpiration.
4) Increased water uptake. Varieties capable of this are capable of developing deep, highly branched roots to ‘dig deeper’ for better absorption of soil water.
Which of these four scenarios is the best to adopt depends on the severity and duration of drought events.
Alternatively, the growing season for beet could be adapted to take account of more frequent droughts. In the hot climates of Italy and Spain, there is a shift to autumn sowing. But that is not an option for the UK because bolting would increase.
The means a different approach is needed. And Mr Robinson believes the most durable solution will be one that combines a number of these strategies rather than simply hoping we can target a single gene.
“Drought tolerance is a complex trait. It impacts multiple functions within the plant and therefore potentially the impacts on many genes including those that modify rooting, leaf surface, stomatal opening, and more.”
It is a compromise between limiting water loss and the need to absorb carbon dioxide for photosynthesis to continue, which in turn is the driver for yield. High yields are only achieved with high rates of transpiration.
For some time, however, SESVanderHave has seen promising varieties in its trials.
“What is clear, is that our breed- ing targets must routinely create varieties that use water more efficiently and withstand heat and drought. Conversely it needs to exploit the rain when it comes to maximise yield,” says Mr Robinson.
Results to date are promising with drought tolerant lines losing no more than 10% of yield, where those without any tolerance recording losses of 30% or more.