N

No mutants lacking cytokinins have ever been found.

Thus, like auxin, cytokinins seem to be required throughout the life of a plant. Cytokinins form primarily in the roots and move to other parts of the plant. They have a number of different effects:

► Adding an appropriate combination of auxin and cytokinins to a growth medium induces rapid cell proliferation in cultured plant tissues.

► Cytokinins can cause certain light-requiring seeds to germinate even when kept in constant darkness.

► Cytokinins usually inhibit the elongation of stems, but they cause lateral swelling of stems and roots (the fleshy roots of radishes are an extreme example).

► Cytokinins stimulate lateral buds to grow into branches; thus the balance between auxin and cytokinin levels controls the extent of branching (bushiness) of a plant.

► Cytokinins increase the expansion of cut pieces of leaf tissue in culture and may regulate normal leaf expansion.

► Cytokinins delay the senescence of leaves. If leaf blades are detached from a plant and floated on water or a nutrient solution, they quickly turn yellow and show other signs of senescence. If instead they are floated on a solution containing a cytokinin, they remain green and senesce much more slowly.

Ethylene

(the "senescence hormone")

Back when streets were lit by gas rather than by electricity, leaves on trees near street lamps abscised earlier than those on trees farther from the lamps. We now know that eth-ylene, a combustion product of the illuminating gas, is what caused the early abscission. Auxin delays leaf abscission, but ethylene strongly promotes it; thus a balance of auxin and ethylene controls abscission.

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