Growth and Development of Horticultural Crops (1+1)

4. Paclobutrazol

Mode of action

• Suppression of growth by paclobutrazol occurs because the compound blocks three steps in the terpenoid pathway for the production of gibberellins by binding with and inhibiting the enzymes that catalyze the metabolic reactions
• When gibberellin production is inhibited, cell division still occurs, but the new cells do not elongate. The result is shoots with the same numbers of leaves and internodes compressed into a shorter length.
• Recent research has demonstrated that blocking a portion of the so-called terpenoid pathway causes shunting of the accumulated intermediary compounds above the blockage. The consequence is increased production of the hormone abscisic acid and the chlorophyll component phytyl, both beneficial to tree growth and health
• Paclobutrazol treated trees have greater tolerance to environmental stresses and resistance to fungal diseases. Morphological modifications of leaves induced by treatment with paclobutrazol such as smaller stomatal pores, thicker leaves, and increased number and size of surface appendages on leaves may provide physical barriers to some fungal, bacterial and insect infections.
Influence in fruit culture

• Treated trees of apple have more compact crowns and somewhat smaller and darker green leaves, but otherwise look normal. The amount of shoot growth reduction ranges from a low of 10% to a high of 90%, with average growth reduction being 40-60% when recommended dose rates are applied. As a consequence of the reduced growth in height, there is a parallel reduction in biomass removed when trees eventually require trimming
• Increased concentration of paclobutrazol (1000ppm) suppressed the root length and increased the root diameter of Assam lemon. Paclobutrazol-treated plants showed better survival at the nursery stage than control. (Singh et al., 2000)
• Paclobutrazol increased root:shoot ratio
• Paclobutrazol applications stimulate flowering 2 months after the application, or 2 months earlier than natural flowering. The application increases fruit production by as much as 73 -142 %.
• Trees treated with paclobutrazol generally have leaves with a rich green color suggesting high chlorophyll content. There are two possible explanations for this response. One is that the leaves of both treated and untreated trees contain the same number of cells, but because the cells in leaves of treated trees are smaller, the chlorophyll is more concentrated in the reduced cell volume. Paclobutrazol treatment, which blocks the production of gibberellins, results in a shunting of the intermediate compounds from gibberellin synthesis to the production of more leaves.
• Treatment with paclobutrazol promotes the production of abscisic acid - cause stomates to close, reducing water loss from leaves through transpiration.
Growth regulators are mainly used as foliar spray, dipping cuttings and fruit bunches (grapes) and soil application is not commonly followed.