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 stud ied in young wheat plants (Triticum aestivum L. 'Pro-INTA, Isla Verde') usi ng the EDTA-phloem collection technique. Plants grown with a 16 h photoperi od showed a rapid decrease in the concentration of sugars and amino acids i n the phloem exudate from the beginning of the dark period. When plants gro wn with a 16 h photoperiod were kept in the dark for longer than 8 h the fr ee amino acid content in leaves and exudate (on a dry weight basis) increas ed continually throughout the 72 h of darkness. During the first 24 h of da rkness the sugars in the phloem exudate decreased to 30% of the initial val ue, and returned to the control level when plants were returned to light. W hen plants grown under low light intensity for 10 d were transferred to hig h light intensity, they showed an increase in leaf sugar content (dry weigh t basis) after 3 d but there were no differences in leaf free amino acid co ntent (dry weight basis) compared to low-light plants. The sugar concentrat ion in the phloem exudate was increased by higher light intensities, but th ere 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-l ight plants. The present results suggest that amino acids can be exported t o the phloem independently of the export of sugars. (C) 1999 Annals of Bota ny Company.