Auxin Transport in Roots

Abstract

The effects of temperature on the polar movement of IAA through 6-mm and 12-mm segments of Zea mays roots have been investigated over the range from 1 to 50°C. At all temperatures an acropetal polar movement of IAA predominated, although at low temperatures and at 50°C the 6-mm segments showed a transient basipetal polarity, before the persistent acropetal polarity developed. At 1°C the differences between acropetal and basipetal movement of IAA were less distinct than at the other temperatures. There is, however, a marked metabolically-dependent acropetal movement of IAA through the tissues at 1°C, because when the segments were deprived of oxygen the acropetal movement was severely reduced while the basipetal movement was reduced to a smaller extent. At 1°C and at 5°C there was always a persistent basipetal polarity of IAA movement through 6-mm and 12-mm segments under anaerobic conditions. The velocity of acropetal movement (mm h−1) was the same through the 6-mm and the 12-mm segments and was markedly affected by temperature. It increased from 1°C to a maximum value of 8 mm h−1 at 31°C and then decreased again at 40 and 50°C. The velocity of basipetal movement could be assessed only at 1 and 5°C at which temperatures it was greater than the velocity of acropetal movement, and virtually independent of segment length. The acropetal flux of IAA (cpm h−1) was much less through 12-mm segments than through 6-mm segments. For both lengths of segment, however, the flux showed a complex relationship with ambient temperature, increasing from 1°C to a maximum at 10–15°C, declining to a minimum value at 31°C and then rising again at 40 and 50°C. The basipetal flux of IAA could be astimated only at 1 and 5°C at which it was very much smaller than the acropetal flux. The amount of IAA in the receiver blocks increased linearly with time at the lower temperatures. At temperatures within the range 15°C to about 31°C, however, the amount of IAA in the receiver blocks began to decline if the transport periods exceeded a certain length. The time at which this decline in the IAA in the receiver block began was related to the ambient temperature. Chromatographic analysis indicated one radioactive substance in receiver blocks at the apical end of segments supplied with IAA-1-14C at the basal end after transport periods of 6 h at 25°C, and 72 h at 5°C. The Rf of this substance was closely similar to that of the radioactive IAA supplied in the donor blocks.