Development of Sulfate Uptake Capacity and ATP-Sulfurylase Activity during Root Elongation in Maize

Sulfate uptake capacity and ATP-sulfurylase activity were determined in maize roots (Zea mays L. var. XL 363 and mutant XL 363 o2) at increasing root length. The pattern of uptake showed a close similarity to that of ATP-sulfurylase, both activities reaching the maximum level at 9 and 10 cm root length in the XL 363 and XL 363 o2 hybrids, respectively. In addition to the shift of the maximum, opaque-2 mutation caused an enhancement of the two activities at root length below and above the activity peak. The kinetic parameter of uptake, Km, showed a maximum at 3 to 4 and a minimum at 7 to 8 cm. The isoenzyme pattern of ATP-sulfurylase was the same in the two hybrids and did not change with root elongation. A common regulatory mechanism is postulated for uptake and activation of sulfate. The kinetic behavior is interpreted as an index of flexibility of the transport system toward different nutrient status of the environment.