The absorption of L-α-alanine and L-α-amino-iso-butyric acid during their movement through the xylem vessels of tomato stem segments

Two labelled amino acids (1 ml, 0.005 M), L-a-alanine and L-a-amino-iso-butyric acid were allowed to perfuse separately through isolated stem segments of about 12 cm length. After a rapidly increasing initial absorption a steady-state absorption was established, from which for each amino acid the fractional rate of escape per unit stem length could be calculated (*K in mm" ‘). The values found could be confirmed by measuring the radioactivity in one-cm pieces of the stem segment used. *K-alanine was found to be about 8 x *K-a-amino-isobutyric acid. Influences of transpiration and flow rate through the xylem vessels on the *K-values could be demonstrated. A provisional model for amino acid absorption from the xylem vessel is presented.