The tracer15 N2 was used to investigate sites of N2 fixation and the possible translocation of the fixed N. Young sugarcane plants (Saccharum officinarum L.) from a stem cutting were exposed to 15 N2-labeled air in a 500 mL plastic cylinder. Plants fed 15 N2 for 7 days were grown in normal air for a further chase period. After 21 days, about half of the N originating in the stem cutting had been transported to the shoot and roots, suggesting that the cutting played a role in supplying N for growth. After 3 days of feeding, the percentage of N derived from 15 N2 was higher in the roots (2.22%) and stem cutting (0.271%) than the shoot (0.027%). Most of the fixed N was distributed in the 80% ethanolinsoluble fractions in each plant part, and the 15 N fixed either in the roots or in the stem cutting remained there and was not appreciably transported to the shoot. The results were quite different from the fate of fixed N in soybean nodules, which is rapidly transported from nodules to roots and shoots.
Key words:15 N2, nitrogen fixation, Saccharum officinarum L., sugarcane roots, translocation Sugarcane (Saccharum officinarum L.) is a tall, perennial grass (family Poaceae, subfamily Panicoide), and is cultivated in tropical and warm-temperate regions between 35°N and 35°S and from sea level to altitudes of 1,000 m in a wide variety of soil types (27). For many years, sugarcane has been used mainly for sugar and for alcohol production. Recently, the use of sugarcane alcohol (ethanol) as an automotive fuel to replace gasoline has rapidly increased (5, 17). In 2006, world production of sugarcane was 1,392 million tons. This is much greater than production levels for the other major crops such as maize (695 million tons), paddy rice (635 million tons), wheat (606 million tons) and potatoes (315 million tons) (http://faostat.fao.org/site/567/default. aspx). Sugarcane production is highest in Brazil (455 million tons), followed by India (281 million tons). In 2006, sugarcane was cropped over an area of 20 million hectares, and the average yield was 68 tons per hectare. Sugarcane is a C4 plant, which has an efficient photosynthetic system, and it can convert up to 2% of incident solar energy into biomass. It grows up to 4 m in height and the thick stem stores a high concentration of sucrose which is present in the expressed juice at between 12 and 20% (w/v).In Brazil, sugarcane crops accumulate between 100 and 200 kg N per hectare per year, while N fertilization rates are relatively low-usually less than 60 kg N per hectare (27). Also, the response of sugarcane crops to N fertilizers is usually very weak or non-existant (5, 27). In some areas of Brazil, sugarcane has been grown continuously for more than 100 years without any N fertilizer being applied at all (10). This circumstantial evidence suggests a high potential for biological nitrogen fixation (BNF) in sugarcane. Using a 15 N dilution technique involving the supply of a 15 N-labeled fertilizer, Urquiaga et al. (32) calculated the contribution of BNF in several cult...
