Auxin Relationships of the Rice Coleoptile

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Abstract
The work reported here concerns the well known fact that the rice coleoptile elongates more rapidly and more extensively under water than in air. Decapitation (removal of the apical tip) experiments carried out with plants in air or under water, as well as experiments done with the growth of floating or submerged coleoptile sections, suggest that the superior growth of rice coleoptiles under water depends on a decreased capacity of the tissue to destroy auxin under water as compared to the extensive capacity of the tissue to destroy auxin in air. Thus, submerged rice coleoptile sections grow more in response to low concns. of added auxin than do sections floated on the surface of solns. Although the maximal growth rate of sections in air is equal to that of submerged sections, this maximal rate is achieved only at an auxin concentration which is 20 to 30 times as great for sections in air as for submerged sections. Rice coleoptile sections floated on the surface of an IAA solution remove IAA from the solns. much more rapidly than do similar but submerged coleoptile sections. Similarly, IAA disappears much more rapidly from coleoptile tissue infiltrated with IAA and incubated in air than from similar coleoptiles incubated under reduced oxygen tension. The growth of the rice coleoptile depends on aerobic respiration even under conditions of complete submersion in water. The inhibition of growth by KCN as well as by DNP was found to be the same for floating and for submerged sections. It is suggested as a working hypothesis that growth of the rice coleoptile is determined by the interaction of two important opposing factors: (1) decrease of auxin destruction at lower oxygen tension, and (2) decreased aerobic respiration at low oxygen tension.
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