Comparison of the contents of sucrose and amino acids in the leaves, phloem sap and taproots of high and low sugar-producing hybrids of sugar beet (Beta vulgarisL.)

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Abstract
The contents of sucrose and amino acids in the leaves, phloem sap and taproots have been analysed in three experimental hybrids of sugar beet and compared with earlier analysed leaf and phloem sap contents in spinach and barley. The three hybrids accumulated sucrose and amino acids to various extents in the mature roots as well as in the young taproots (9–12 weeks). The differences in the sucrose-to-amino acid ratios in the taproots were reflected in the corresponding ratios in the phloem sap. The leaf contents of sucrose and amino acids in the three hybrids were found to be very similar to each other and also to those in spinach and barley. In contrast, the phloem concentration of sucrose (1.3 M) was much higher, and that of amino acids much lower than in spinach and barley. In the taproots, the overall concentration of sucrose was about half that in the phloem sap. From these findings it is con cluded that the decisive factor in the high sucrose accumulation in sugar beet roots is the very efficient process of phloem loading in the leaves. The patterns of the amino acids in the phloem sap and in the taproots resembled those in the leaves, indicating that there is no special transport form for a-amino nitrogen from the leaves to the roots, but all amino acids which are present in the cytosol are translocated.

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