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Articles

Comparison of Endogenous Gibberellins and of the Fate of Applied Radioactive Gibberellin A₁ in a Normal and a Dwarf Strain of Japanese Morning Glory ¹

^a Department of Botany, The University of Nijmegen, Nijemgen, The Netherlands, ^b Michigan State University-Atomic Energy Commission Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48823

The effect of application of GA₃ on hypocotyl growth, the endogenous GAs, and the metabolism of applied ³H-GA₁ were investigated in relation to dwarfism and light-mediated growth inhibition in the normal (tall) strain Violet and the dwarf strain Kidachi of Japanese morning glory (Pharbitis nil). GA₃ applied in a wide concentration range (10^–9 to 10^–3M) to 4-day-old seedlings caused great extension of the hypocotyls in light-grown plants of both the normal and the dwarf strain. However, the dwarf strain did not attain the same length as the normal one at any given GA₃ concentration, even when saturation was reached. Dark-grown plants of the dwarf strain responded to GA₃, although relatively much less than light-grown ones; dark-grown plants of the normal strain showed no GA₃ response at all.

The levels of free GAs in both strains remained more or less constant in both dark- and light-grown plants up to 18 days after germination. The levels of bound GA in dark- as well as light-grown plants of both strains increased after germination, reached a maximum at the 9th day after germination, and then rapidly declined again. The period of increase coincided with rapid elongation of the hypocotyl and the expansion of the cotyledons.

The dwarf strain, Kidachi, contained less endogenous gibberellins, particularly bound gibberellins, than the normal strain, Violet. Dark-grown plants of both strains contained less bound GAs than light-grown plants.

Applied ³H-GA₁ was metabolized to the same extent in both dwarfs and normals, on the one hand, and in both dark- and light-grown plants, on the other. This metabolism involved binding as well as breakdown of the ³H-GA₁.