Paclobutrazol is a xylem-mobile, and therefore effective retardation relies primarily on uptake of the chemical via the roots. Unfortunately, the effects of paclobutrazol can persist within the soil, which may limit its suitability as a growth retardant in some situations.

Because triazoles chemicals are transported in the xylem, they may be absorbed by the leaves, but cannot be transported out of the leaves to other parts of the plant. Because of this fact, it is recommended that when applied as a spray, triazoles should be applied so that the solution contacts the stems.

Although the most consistent effect of Paclobutrazol is on growth retardation, there have been reports that this chemical (and also CCC) can induce additional effects such as an increase in ear or grain number or modification in canopy structure, or confer limited fungicidal protection. Indeed, other members of the triazole family have been commercially developed into fungicides, such as 'Bayleton'. The fungicidal properties of the triazoles are due to their capacity to inhibit the biosynthesis of ergosterol, which is vital component of fungal membranes. The biosynthesis pathway of sterols and GAs have features in common, and both CCC and Paclobutrazol have been reported to reduce the sterol levels of plant tissues. The functional role of sterols in membranes and the regulatory properties of the brassinosteroids may contribute to the spectrum of effects that these growth retardants can exert under certain conditions.

Paclobutrazol is being extensively used in the horticulture industry to regulate the growth of fruit trees and ornamentals. Also, there is a considerable interest in using it to regulate the growth of grass and trees in amenity areas. The 'greens' market is of considerable commercial significance since it includes such sites as grass verges, golf course fairways, cemeteries and other areas difficult to move.

Because paclobutrazol is very active at low rates, the potential for error and crop damage is much greater. Usually as a spray, is applied at the rate of 2 to 90 ppm.

Environmental Characteristics:

 Adsorption and leaching in basic soil types: Paclobutrazol could leach in sandy soils with low organic content. In other soil types, the chemical does not have a high propensity to leach.
Loss from photodegradation: Paclobutrazol does not photodegrade after exposed to 10 days of simulated sunlight.
Resultant average persistence: Paclobutrazol degrades aerobically in soil with half-lives of about 1-7 months depending upon soil type. Paclobutrazol is not expected to hydrolyze in the environment.

Paclobutrazol is a post-emergence growth regulator and is applied anytime after emergence of target plants. Effects may not be noticeable for up to eighteen months.

Growth Responses to Paclobutrazol

Shoots
Paclobutrazol reduces stem elongation in many species although the extent of growth inhibition may vary. Reduced internode length and reduced stem weight accompany paclobutrazol-induced inhibition of stem elongation. Furthermore, paclobutrazol reduces stem dry weight per unit length and reduces stem taper in at least some species.

Leaf area and dry weight are also reduced by paclobutrazol. The compound has a greater effect on leaf area than dry weight hence leaf dry weight per unit area is increased after treatment. In addition paclobutrazol increased leaf thickness. The production of new leaves after treatment is slightly decreased or relatively unaffected, thus the number of modes per plant is reduced slightly or remains unchanged. The compound can reduce petiole length on strawberry leaves. 

When compared against other growth retardants, paclobutrazol is very active. For example the recommended rate of application for CCC is approximately 1000 times that of paclobutrazol. Menhennett (1984) reported that, on a weight base, paclobutrazol was considerably more effective in controlling Chrysanthemum morifolium growth than those of ancymidol but the two compounds had similar dwarfing effects when soil-applied. 

The length of time which paclobutrazol inhibits shoot growth in various species has not been studied in great detail but preliminary studies indicate that the compound is quite persistent. For instance a single application of 2 grams paclobutrazol per square meter of soil surface controlled terminal shoot growth on apple trees for several growing seasons. 

Relatively high rates of paclobutrazol generally do not cause phytotoxicity. There are no reports of chlorosis or necrosis following application of the compound. High rates may completely stop shoot growth and may result in some leaf curling, cupping or crinkling. Plants which have stopped growing because of heave paclobutrazol application can be induced to grow by the application of gibberellins.

Generally, paclobutrazol is more effective in retarding shoot growth when applied to the soil or directly to stems compared to foliar sprays. The characteristics of the soil medium can, however, influence the effectiveness of soil-applied paclobutrazol.

Roots

Paclobutrazol generally increases root diameter but decreases root length. Stang and Weis (1984) however reported that treated strawberry plants had roots which were reduced in diameter and had more root hairs than controls. Paclobutrazol reduced surface area of apple root systems in one study but had no effect in another

Root growth is generally less affected by paclobutrazol than shoot growth. Hence, treated plants have increased root to shoot ratios compared to untreated plants. 

Paclobutrazol may slightly delay flowering in greenhouse-grown Chrysanthemum morifolium although the delay is less that caused by chlorphonium chloride and piproctanyl bromide. Flower number and pigmentation, however, are unaffected by the compound.  Menhennett and Hanks (1983) reported that the effect of paclobutrazol on flowering of Tulipa hybrida  was cultivar-dependent as flowering in one of the three cultivars was reduced, while the other two were unaffected. In Hydrangea macrophylla paclobutrazol has been reported to stimulate floral initiation. 

Physiological and Biochemical Responses to Paclobutrazol

Gibberellin biosynthesis
Paclobutrazol presumably retards growth by inhibiting oxidative reactions in the biosynthesis of gibberellins. Specifically, the microsomal oxidation of kaurene, kaurenol and kaurenal are inhibited. There are some sites of action for a number of other growth retardants with different structures such as ancymidol and tetcyclacis. Paclobutrazol-treated plants have been found to contain lower amounts of gibberellin-like substances compared to untreated plants. 

Sterol biosynthesis
Paclobutrazol is structurally related to a number of sterol biosynthesis inhibitors which have been used extensively in both agriculture and medicine. For example, triadimefon is a highly active and useful triazole fungicide which exhibits some growth regulator properties. Triazole inhibitors generally block the 14 alpha-demethylation step of sterol biosynthesis. Hence, is has been suggested that the inhibition of sterol biosynthesis may play significant role in the growth regulating activity of a number of triazole compounds including paclobutrazol. 

ABA Synthesis
Norman et al. (1986)  have reported that paclobutrazol inhibits ABA synthesis in the fungus Cercospora rosicola. At 0.1 micromolar, paclobutrazol inhibited ABA synthesis by 33%.

Photosynthesis
On a leaf area basis, paclobutrazol generally has little direct effect on rates of net photosynthesis.  It may affect photosynthesis by altering canopy structure. thereby influencing light penetration and absorption. This possibility has not been studied in any detail, however.

Chlorophyll Formation
Leaves of paclobutrazol-treated plants are darker green than controls. This response is not unique to paclobutrazol as other growth retardants also intensify the green color of foliage. It is not known however whether the increased chlorophyll content of paclobutrazol-treated leaves is a result of enhanced chlorophyll synthesis or is simply a result of a "concentrating effect" due to reduced leaf expansion. Studies on the activities of enzymes of chlorophyll formation and catabolism after treatment would be worthwhile in this respect.

Water Relations
Plants treated with paclobutrazol typically use less water than untreated plants. It is not known, however, if this is simply due to the reduced leaf area of treated plants or to altered conductances to water vapor flux. Water potential of treated plants is generally higher than that of untreated plants. It has been suggested that treated plants may be better able to withstand drought conditions. This view is supported by the observation that water-stressed apple seedlings treated with paclobutrazol evolved less ethylene than controls, perhaps indicating that the treated plants were under less stress. 

Stress Tolerance
Paclobutrazol-treated plants appear to be more tolerant to a number of other stresses. Paclobutrazol has been reported to confer considerable resistance to sulfur dioxide damage in Phaseolus vulgaris. The effect of paclobutrazol is very rapid in that within 24 hours after treatment plants are better able to tolerate damaging levels of atmospheric sulfur dioxide. The mechanism for such tolerance is unclear but does not appear to be due to changes in stomatal opening. Interestingly, paclobutrazol does not protect plants from ozone-induced damage. 

Paclobutrazol-treated plants may be less susceptible to both low and high temperature damage. Also they appear to be more tolerant of low light conditions.  

References: S.S. Purohit "Hormonal Regulation of Plant Growth and Development, Volume III".  
                   Agro Botanical Publishers (India), 1986.