In Germany, from March to September precipitation in 2003 was only 57 % of that in 2002. Despite visible symptoms of drought stress, beet yield and white sugar yield were similar in both years. Similar growth under conditions of reduced precipitation pointed to an adaptation by accumulation of solutes. We therefore investigated changes in the concentration of major solutes in different organs (beet, crown, young and mature leaf petioles and blades) of sugar beet in response to low precipitation as a possible explanation for sustained plant growth. In three field trials, sugar beet was harvested at three time points from 100 to 170 days after sowing. The response to low precipitation differed in the different organs, both in magnitude and in the type of solutes involved. Despite considerable changes in the concentrations of single solutes in all organs, the sum of solutes was significantly affected by the year only in beet, crown and mature petioles. In the beet, the concentration increased in the dry year whereas in crown and mature petioles it decreased. The beet accumulated 156 % more glutamine, 72 % more amino N, and 57 % more betaine in 2003 than 2002, concentrations of fructose and glucose were increased by 44 % and 42 %, and of sucrose by 5 %. The accumulation of glutamine, glucose and fructose was specific to the beet, in the other organs their concentrations were decreased. Specific activity of glutamine synthetase was 68 % higher in 2003 than 2002 in the mature blades, thus being a possible source of glutamine in the beet. Another source may have been protein degradation since a net loss of protein occurred in all organs. This accumulation of solutes in the beet may have contributed to achieve osmotic adaptation and to sustain beet growth under conditions of low precipitation, however, it led to a decrease in the technical quality of sugar beet.
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