This explains why both control and iaa treated plants

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This explains why both control and IAA treated plants had an upward shoot growth and downward root growth. Plant parts such as shoot tips, young leaves, developing fruits and seeds are able to synthesize auxin. Therefore, the plant is a natural source for the hormone and will exhibit positively geotropic roots and negatively geotropic shoots in the absence of any treatment. Cutting the plant’s tip would lead to decreased amounts of auxin and promote the growth of lateral buds. However, treatment with IAA mixed with lanolin will compensate for the cut and provide IAA for the plant. The plant will be as if its tip was not cut. The effect of IAA treated plants with cut shoot tips is the same as that of the control, that is, the same with the IAA the plant naturally produces. 1-Naphthaleneacetic acid or NAA is a synthetic auxin that is usually applied to plants to stimulate root growth. Since it is a kind of auxin, the plant is expected to show similar growth (towards gravity for roots and against gravity for shoots) with that discussed above only that root growth and elongation is more pronounced compared to the previous set-ups. The plant that was cut then treated with NAA is consistent with these expected results. TIBA or 2,3,5-triiodobenzoic acid is an inhibitor of the auxin efflux. Auxin will be able to enter cells since it will exist in its protonated outside the cell, allowing it to diffuse through the lipid bilayer easily. However the basic environment inside the cell causes it to lose its proton and to exist in its ionic form which will prevent its diffusion out of the cell. Membrane transport proteins then aid in exit of auxin from a cell. In TIBA treated plants, auxin efflux in inhibited. As a consequence, auxin will not be able to leave cells and will then be localized in its area of production such as shoot tips, young leaves and developing fruits and seeds. Auxin transport to target organs will also be impaired. Auxin will not be able to accumulate in roots and root elongation will then be minimal is
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not absent and the geotropic nature of the root will not be observed. However, the shoots will continue to elongate since transport is not needed for auxin accumulation in the shoot with shoot tips as sites of auxin synthesis. But auxin will not accumulate on the lower side of the shoot as usual since it cannot leave the cells from which they are produced. Therefore, growth of the lower side of the shoot will not particularly be stimulated and the negatively geotropic shoots would not be observed. Accordingly, the shoots grew downward in the experiment. Elongation of the shoot was evident and geotropism as well as root elongation was not observed. Finally, for the last set-up with cut tip that was applied both NAA and TIBA, the leaves wilted and the plant did not grow. NAA is still expected to promote root elongation, given that it is transported to the roots. On the other hand, TIBA would inhibit root growth. Applying both to a cut plant will
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  • Spring '11
  • MiriamDeVera
  • Plants, Plant morphology, IAA, NAA

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