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Effect of free air carbon dioxide enrichment combined with two nitrogen levels on growth, yield and yield quality of sugar beet: Evidence for a sink limitation of beet growth under elevated CO2
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The increase in atmospheric CO concentration [CO] has been demonstrated to stimulate the growth of C crops. However, little information exists about the effect of elevated [CO] on biomass production of sugar beet, and data from field experiments are lacking. In this study, sugar beet was grown within a crop rotation over two rotation cycles (2001, 2004) at present and elevated [CO] (375ollp# and 550ollp#) in a free air CO enrichment (FACE) system and at two levels of nitrogen supply [high (N2), and 50% of high (N1)], in Braunschweig, Germany. The objective of the present study was to determine the CO effect on seasonal changes of leaf growth and on final biomass and sugar yield. Shading treatment was included to test whether sugar beet growth is sink limited under elevated [CO]. CO elevation did not affect leaf number but increased individual leaf size in early summer resulting in a faster row closure under both N levels. In late summer CO enrichment increased the fraction of senescent leaves under high but not low N supply, which contributed to a negative CO effect on leaf area index and canopy chlorophyll content under high N at final harvest. Petioles contained up to 40% water-soluble carbohydrates, which were hardly affected by CO but increased by N supply. More N increased biomass production by 21% and 12% in 2001 and 2004, respectively, while beet and sugar yield was not influenced. Concentration of l-amino N in the beet fresh weight was increased under low N and decreased under high N by CO enrichment. The CO response of total biomass, beet yield and white sugar yield was unaffected by N supply. Averaged over both N levels elevated [CO] increased total biomass by 7% and 12% in 2001 and 2004, respectively, and white sugar yield by 12% and 13%. The shading treatment in 2004 prevented the decrease in leaf area index under elevated [CO] and high N in September. Moreover, the CO effect on total biomass (24%) and white sugar yield (28%) was doubled as compared to the unshaded conditions. It is concluded that the growth of the storage root of sugar beet is not source but sink limited under elevated [CO], which minimizes the potential CO effect on photosynthesis and beet yield.
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 Record created 2010-04-06, last modified 2019-07-02

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