Fitters, Tamara F.J.
(2019)
Understanding water uptake in sugar beet.
PhD thesis, University of Nottingham.
Abstract
Sugar beet is grown worldwide for sugar production. In the UK the main growing area is East Anglia, which has an annual rainfall of c.600 mm and the crop is predominantly growth on sandy loam soils. Previous work has reported that sugar beet shows an average annual yield loss of 10% due to water deficits and during dry years this loss can increase up to 25%. It was also found that sugar beet can grow roots up to 1.5m deep but there is hardly any water uptake from depth which might contribute to yield loss due to drought stress.
Drought stress is common among many crops and studies into rooting patterns and water uptake have shown that within species. Not much work had been done on recent sugar beet varieties in terms of rooting patterns and root responses to variable water availabilities. During this PhD several experiments were designed to investigate possible limitations to water uptake and ways to mitigate potential issues.
The first experiment, conducted in the glasshouse, established what sugar beet roots were capable of in terms of growth and water uptake, in unlimited conditions. It was found that deep roots were formed, especially under drought stress, however there was a delay between roots arriving at a certain depth and water actually being taken up from that depth. A follow up experiment focussing on root anatomy showed that this delay could be explained by a lack of secondary xylem immediately after root formation. Once the xylem has matured and enough secondary xylem was formed, water could be taken up efficiently. Another glasshouse experiment looking at the effect of compaction below 50 cm depth showed that root growth and water uptake are limited when the bulk density is higher than 1.6 g cm-3.
A box experiment, held over two years, allowed for closer monitoring of sugar beet root growth using minirhizotrons. Again, a delay between roots reaching a particular depth and there being water uptake from that depth was seen. It was also found that under prolonged drought, large yield losses would arise while shorter periods of drought followed by periods of rewatering did not result in strong yield penalties. Stomatal conductance measurements did show differences between the treatments, indicating that there were short term drought effects, regardless of the length of the drought period. Stomatal conductance dropped before water was taken up from depth, despite roots having been found in soil layers with sufficient water.
Lastly, two years of field experiments gave an indication of sugar beet growth under ‘normal’ conditions. Multiple varieties were grown alongside each other and during dry periods half of the plots could be irrigated. In both years only very mild drought was recorded, and no strong yield losses were observed. Soil cores were taken to look at the root growth at two moments during each year and enabled us to identify differences in rooting patterns between the varieties, some varieties had a greater root length density at depth. Under mild drought conditions these differences were more pronounced than under optimum conditions.
In summary, when soil conditions were non-limiting, sugar beet did produce deep roots and take up water from depth to mitigate drought. Delays in roots reaching deeper layers and there actually being water uptake from that depth was associated with a lack of mature xylem shortly after roots were formed. The sugar beet varieties grown in the field showed different rooting patterns, which could mean that it may be possible to breed for varieties that are able to extract more water from depth in periods of water shortage.
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