• Brassica;
  • cell size;
  • gibberellins;
  • growth;
  • rapid cycling plants

Three rapid cycling Brassica rapa genotypes were grown in greenhouse conditions to investigate the possible relationships between endogenous gibberellin (GA) content and shoot growth. Endogenous GA1 GA3 and GA20 were extracted from stem samples harvested at 3 weekly intervals and analyzed by gas chromatography-mass spectrometry with selected ion monitoring, using [2H2]-GA1 and [2H2]-GA20 as quantitative internal standards. During the first 2 weeks, GA levels of the dwarf, rosette (ros), averaged 36% of levels in normal plants (on a per stem basis). Levels in the tall mutant, elongated internode (ein), were consistently higher, averaging 305% of levels in normal plants.

Differences in shoot height across the genotypes resulted from varying internode length which resulted from epidermal cell length and number being increased in ein and decreased in ros relative to the normal genotype. The exogenous application of GA3 to normal plants increased cell length while the application of paclobutrazol (PP333), a triazole plant growth retardant, reduced cell size. Thus, exogenous GA manipulations mimicked the influence of the mutant genes ros and ein. The dwarf, ros, had reduced shoot dry weights and relative growth rates compared to the other genotypes. Total dry weights were similar in ein and the normal genotype but stem weights were increased in ein, compensating for decreased leaf weights. Thus, the gibberellin-deficiency of ros resulted in generally reduced shoot growth. The overproduction of endogenous GA by ein did not result in enhanced shoot growth but rather a specific enhancement of internode elongation and stem growth at the expense of leaf size.