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Articles

Growth and Gibberellin A₁ Metabolism in Excised Lettuce Hypocotyls ¹

John L. Stoddart^b

^a Department of Botany, University of California, Berkeley, California 94720, ^b Welsh Plant Breeding Station, Plas Gogerddan, Aberystwyth, Wales United Kingdom

Excised lettuce (Lactuca sativa L. cv. Arctic) hypocotyls retain the ability to elongate in response to exogenously supplied gibberellic acid and gibberellin A₁ (GA₁). We have studied the relationship between metabolism of GA₁ and elongation in this tissue. In 24 hours at 28 C, hypocotyls treated with 3 µM GA₁ double in length while controls elongate less than 45%. After an exogenous hormone supply is removed, hypocotyls continue to grow faster than untreated controls, although as the hormone application time is decreased, the GA₁ concentration required to effect a given length change increases. [³H]GA₁ was used to determine rates of hormone uptake, efflux, and metabolism. In the presence of [³H]GA₁, hypocotyls accumulate and metabolize lable for at least 24 hours. When the exogenous label is removed, the amount of acidic GA in the hypocotyl declines rapidly to a constant level while ethyl acetate-insoluble metabolites increase rapidly to a constant level. Lable accumulation and metabolism at any time are proportional to the external GA₁ concentration below 50 µM GA₁. Chromatographic analysis of radioactive compounds present in tissue extracts suggests that unaltered GA₁ is the major component of the acidic ethyl acetate-soluble fraction, and gibberellin A₈ is a minor component. The ethyl acetate-insoluble fraction appears to contain an unidentified GA₁ metabolite with chromatographic properties similar to those of GA₁. The strong retention of accumulated GA₁ confirms the possibility of a continuing requirement for GA₁ during the sustained response to a GA₁ "pulse" but raises the question of accessibility of the stored hormone for growth promotion.

¹ This research was supported by Grants, NSF BMS 71-00711 and NSF BMS 73-06846 from the National Science Foundation and NATO No. 970 from the North Atlantic Treaty Organization.