Unflooded Plants Research Articles

Root-shoot Hormone Relations I. The Importance of an Aerated Root System in the Regulation of Growth Hormone Levels in the Shoot of Helianthus annuus

Using mature, vegetative, sunflower (Helianthus annuus) plants, a study was made of the interactions of flooding of the root system with several other simultaneous treatments. Leaf epinasty was induced in intact plants by flooding, but this effect of waterlogging of the root system was removed when the shoot was decapitated. The addition of indole-3-acetic acid (IAA) in lanolin to the cut surface of decapitated plants restored the epinasty-inducing effect of flooding. This is taken as evidence that flooding brings about an increased auxin content of the shoot. In unflooded plants decapitation of the shoot resulted in a hyponastic movement by the leaves, and applications of IAA to the stem tip antagonized this. Girdling of flooded plants resulted in a great reduction of the tendency to form adventitious roots in the region below the girdle. Thickening and various morphological changes were produced in the hypocotyl as a result of waterlogging the root system. The effects of flooding could be simulated in the upper region of the shoot of unflooded plants by girdling the more basal region of the stem. These various results are interpreted as indicating that the root system may normally serve as a centre for the oxidative inactivation of excess shoot-synthesized auxin, so regulating shoot auxin level. Certain of the results obtained, however, show that an adequately aerated root system supplies some growthpromoting, and hyponasty-inducing, principle to the shoot. The effects of this hypothetical principle were found to be similar to those of gibberellic acid. It is envisaged, therefore, that an early effect of flooding on plants is to raise shoot auxin levels (either by preventing entry of auxin from the stem, or by inhibiting the oxidative catabolism of shoot auxin in the roots), and also to stop the synthesis of a non-auxin shoot growth-hormone in the roots. The induction of wilting of the shoot by flooding was found to be correlated with low root-temperatures.