Growth Substances in the Roots ofVicia fabaII. THE EFFECTS OF AGEING AND EXCISION OF THE MAIN TAP-ROOT MERISTEM

Abstract

SUMMARY A quantitative chromatographic study has been made of the changes in the activity and distribution of the main ether-soluble acid auxins of Vicia faba seedling root systems during development and resulting from excision of the main tap-root meristem. An IAA-like auxin (AP (ii)) is apparently synthesized predominantly in apical and at a lower rate in lateral meristems. Production seems to stop when meri stematic growth stops. Its concentration in mature extended cells is much lower and may fall to zero in old cells, suggesting active degradation by an auxin-oxidase. Excision of the main tap-root tip gradually results in a greatly augmented produc tion of AP (ii) in lateral meristems, conceivably the result of correlative growth promotion. A second auxin and root-growth inhibitor (AP (Hi)) is present at higher activity levels than AP (ii) in tap-root meristems and at the same level in lateral meristems. In mature cells its activity is much lower than that of AP (ii). In contrast to AP (ii) it accumulates in both tap-root meristems and mature tissue as the root system ages. It could also be produced during meristematic growth but is not subse quently degraded. As with AP (ii), excision of the tap-root tip brings about a great increase in its concentration in lateral tips. A third auxin and root-growth accelerator (AP (?)) (accelerator a ?) is present in lower concentrations in both meristem and mature tissue. Its concentration tends to decrease with ageing and, in lateral meristems, is not affected by tap-root tip excision. It is suggested that AP (ii) produced by the meristem is normally at suboptimal levels in the extending cells and may be the principal hormone controlling exten sion growth. AP (iii) accumulation may account for growth deceleration on ageing. The role of AP (f) remains obscure. It is unlikely that correlative effects of the tap root tip on lateral root growth are exercised directly via these auxins.