Different inhibitors of the gibberellin biosynthesis pathway elicit varied responses during in vitro culture of aspen (Populus tremula L.)

Several synthetic plant growth regulators (PGRs), including prohexadione-calcium (ProCa), paclobutrazol (PBZ), and chlormequat chloride (CCC), known for their ability to inhibit gibberellin (GA) biosynthesis, were investigated for their influence on Populus tremula L. (aspen) shoots grown in vitro. Changes in plant growth induced by these inhibitors were compared to the effects of exogenous gibberellins (GA(3) and GA(4/7)). All PGRs were added to the nutrient medium at concentrations of either 1 or 5 mu M. Stem segments with and without apical buds were excised from in vitro-grown shoot culture, and these explants were incubated either in test tubes or Petri dishes. In the presence of 5 mu M ProCa, shoot growth and rooting were inhibited when grown in test tubes, while shoots grown in Petri dishes exhibited strongly enhanced shoot and root growth. PBZ suppressed shoot development both in test tubes and Petri dishes, although 1 mu M PBZ promoted adventitious root formation when shoots were grown in test tubes. Five micromolars CCC suppressed shoot and root development in test tubes, but promoted shoot growth in Petri dishes.