The relationship between sugar and amino acid export to the phloem in young wheat plants

The relationship between amino acid and sugar export to the phloem was studied in young wheat plants (Triticum aestivum L. 'Pro-INTA, Isla Verde') using the EDTA-phloem collection technique. Plants grown with a 16 h photoperiod showed a rapid decrease in the concentration of sugars and amino acids in the phloem exudate from the beginning of the dark period. When plants grown with a 16 h photoperiod were kept in the dark for longer than 8 h the free amino acid content in leaves and exudate (on a dry weight basis) increased continually throughout the 72 h of darkness. During the first 24 h of darkness the sugars in the phloem exudate decreased to 30% of the initial value, and returned to the control level when plants were returned to light. When plants grown under low light intensity for 10 d were transferred to high light intensity, they showed an increase in leaf sugar content (dry weight basis) after 3 d but there were no differences in leaf free amino acid content (dry weight basis) compared to low-light plants. The sugar concentration in the phloem exudate was increased by higher light intensities, but there was no difference in the amino acid concentration of the phloem exudate, and thus the amino acid:sugar ratio in the phloem decreased in the high-light plants. The present results suggest that amino acids can be exported to the phloem independently of the export of sugars. (C) 1999 Annals of Botany Company.