Interactive effects of free-air CO ₂enrichment and drought stress on maize growth

The increase in atmospheric CO2 concentration [CO2] has been demonstrated to stimulate the growth of C3 crops. However, little information exists about the effect of elevated [CO2] 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 [CO2] (375 µl l-1 and 550 µl l-1) in a free air CO2 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 CO2 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 [CO2]. CO2 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 CO2 enrichment increased the fraction of senescent leaves under high but not low N supply, which contributed to a negative CO2 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 CO2 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 a-amino N in the beet fresh weight was increased under low N and decreased under high N by CO2 enrichment. The CO2 response of total biomass, beet yield and white sugar yield was unaffected by N supply. Averaged over both N levels elevated [CO2] 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 [CO2] and high N in September. Moreover, the CO2 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 [CO2], which minimizes the potential CO2 effect on photosynthesis and beet yield.