An Analysis of Auxin-gibberellin Interaction in Pea Stem Tissue.

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Abstract
When GA and IAA are applied jointly to excised 5-mm-long epicotyl sections of etiolated peas, the growth effects produced are no greater than additive, and are frequently less than additive. When GA is applied basally to an excised 100-mm-long epicotyl, and the IAA subsequently applied to the excised apical section, the combined growth effects are much more than additive (synergistic). The epicotyls react to as little as 10-7 [image] GA, half maximally to 10-6 [image] GA and optimally to 10-5 and 10-4 [image] GA. They can thus be used to assay GA in this range. The volume of solution to be assayed per epicotyl need not exceed 0.08 ml, and can thus be used to detect down to 0.03 [mu]g of GA. Appreciable effects of basally-applied Ga are evident at the apex, 100 mm away, within 3 to 5 minutes after application, indicating a transport rate as rapid as 20 to 33 mm/min. The effects of GA in such an etiolated epicotyl are directly proportional to the length of epicotyl between the base, which received the GA, and the apex, which received the IAA. In green stems, where GA-IAA synergism is clearly shown in excised 5-mm sections, GA effects are dependent on the length of stem traversed, even if no auxin is supplied to the excised section. These results can be interpreted in terms of a "third factor" required for GA-IAA interaction. Such a factor, apparently limiting in etiolated pea stem tissue and abundant in green tissue, may operate through an auxin-sparing mechanism.
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