Graviresponse in Eichhornia cressipes (water hyacinth) II. Hormone involvement in graviresponse in the peduncle

Abstract

The effects of decapitation of the inflorescence and application of hormones and related substances on the bending of the peduncle cutting and the isolated peduncle segment were observed and the following results were obtained. (1) With isolated cuttings excised at the base of the peduncle cultured in test tubes, the bending of the peduncle normally occurred at a slower rate than in intact plants. (2) Removal of whole flowers and flower buds 10 or more h before full flowering inhibited the bending but that of petals did not. Flower removal at or after full flowering did not affect the bending response. (3) The bending in intact plants was eliminated when 2,3,5-triiodobenzoic acid (TIBA), an antiauxin preventing polar transport of auxin, was applied to the upper most region of the peduncle in a lanolin ring. However, indole-3-acetic acid (IAA) at a concentration of 10 −6 to 10 −4 and gibberellin (GA) at 10 −4 M exogenously applied from the basal cut ends of the peduncle in the test tube inhibited the bending. IAA, 3 mg/g lanolin, and GA, 3 mg/g lanolin, applied to the top cut end of the peduncle also inhibited the bending. (4) Exogenously applied hormones showed no significant effect on elongation of the isolated peduncles but GA applied to intact plants promoted the elongation of petioles and inhibited the formation of floats on the petiole. (5) Aminooxy-acetic acid (AOA), 3 mg/g lanolin, an ethylene inhibitor, applied to the segments of the peduncle excised at the time of full flowering inhibited the curvature. (6) Segments excised less than 6 h after the start of flowering showed a negative response and those excised after the flowering phase showed a bending response. From the above results it appears that a subtle balance of IAA and ethylene in the peduncle is necessary for the manifestation of the bending, whereas gibberellins and relatively high concentrations (more than 10 −6 M) of auxins serve to maintain the normal negative gravitropic response in the peduncle.