**3. Tree growth and vigour**

Prunus

conditions [13].

**2. Nature of paclobutrazol**

has also been reported [22].

growth. Besides this, it also increases fruit set the years following application as a carryover effect. Paclobutrazol treatments have also shown to increase Delicious fruit firmness at harvest, [3]. Paclobutrazol, a gibberellins inhibitor, has been effectively used in reducing canopy volume and increasing flower intensity in peach [4], plum [5], almond [6], grapes [7] and mango [8]. Paclobutrazol is effective not only in flower induction but also in early and off season flower induction in mango [9, 10]. Paclobutrazol application in McIntosh apple trees shortly after full bloom affected fruit quality characteristics with respect to accelerated colour development, delayed and synchronised fruit maturation, control of preharvest fruit drop and maintenance of better fruit quality during storage [11]. Sebastian et al. [12] also reported that the foliar application of plant growth regulators improves the yield and quality of fruit crops. However, the action of plant growth regulators (PGRs) is highly specific to plant species, cultivar and stage of development, and strongly dependent on its rate of application and environmental

The plant growth regulator, [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2- (1,2,4-triazol-1-yl)-pentan-3-ol] (paclobutrazol; PP333) with chemical formula C15H20ClN3O, is a triazole derivative and has been shown to inhibit shoot growth on apple trees [14]. The ancymidol blocks the oxidative steps with high specificity leading from ent-kaurene to ent-kaurenoic acid in the pathway of GA biosynthesis. The same oxidative steps are thought to be inhibited by the active triazol derivatives. Paclobutrazol has been reported to inhibit GA biosynthesis in plants by inhibiting kaurene oxidase, a Cyt P-450 oxidase, thus blocking the oxidation of kaurene to kaurenoic acid [15]. The inhibitory activity of paclobutrazol can be reversed by GA [14]. Besides reducing gibberellins level, PBZ increases cytokinin contents, root activity and C:N ratio, whereas its influence on nutrient uptake lacks consistency [16]. Paclobutrazol was also shown to shift assimilate partitioning from leaves to roots, increase carbohydrates in all parts of apple seedlings, increase chlorophyll content, soluble protein and mineral element concentration in leaf tissue, increase root respiration and reduce water use [17]. Browning et al. [18] investigated the effect of PBZ on the translocation of endogenous IAA (indol-3-acetic acid) in Doyenne du Comice pear cultivar and found that PBZ caused a slower movement of IAA in shoot tips. The usual application of paclobutrazol has been either by foliar spray or soil drenching. With foliar spray applications, absorption through mature leaves was limited and PBZ may be taken up through stem absorption [19] or from excess dripping onto the soil. Elfving and Proctor [20] have reported that protecting the soil from foliar drip reduced the PBZinduced inhibition of extension growth in apples. When applied to the soil, a continuous supply of PBZ taken up by the roots is translocated acropetally via the xylem, thus maintaining the concentration of PBZ above the threshold required for the inhibition of gibberellins biosynthesis [21], although phloem translocation

Paclobutrazol (PBZ), a non-polar broad spectrum growth regulator, has been characterised as an environmentally stable compound in soil and water environments with a long half-life under both aerobic and anaerobic conditions. Moreover, PBZ is unlikely to volatilise to any significant extent owing to a low estimated vapour pressure (1.9 × 10–6 Pa). Paclobutrazol has been registered in 1985 (cultar, ICI Americas, Goldsboro, NC), however it has now been permitted for use on food crops in Australia, New Zealand, South Africa, India, Philippines,

**46**

Paclobutrazol when applied during early summer has been observed as effective suppressant of stem growth in sweet cherry [25]. Young [26] reported that paclobutrazol when sprayed on 'Redhaven' cultivar of peach reduced terminal growth and advanced leaf fall. Webster and his associates [27] observed that application of 1.6 g a.i. tree<sup>−</sup><sup>1</sup> paclobutrazol to cherry trees followed by 0.8 g a. i. paclobutrazol in next year inhibited extension growth of young trees on either colt or FB22 rootstocks. Gaash [28] stated that 1000–4000 ppm of paclobutrazol on 'Canino' apricot cultivar decreased the lateral shoot length. Foliar spray of 1000 ppm paclobutrazol to sweet cherry trees suppressed shoot growth and delay in fruit colour [29]. Blanco [30] stated that paclobutrazol decreased the shoot development of 'Crimson Gold' nectarines. Kaska et al. [31] observed that application of paclobutrazol decreased shoot elongation in cherries when applied on the vegetative and reproductive parts. Shoot extension growth was reduced by 57% at stone hardening and by 47.6% at harvest following soil and collar drench of cultar (2.0 g a.i. PP333/tree) applied in autumn in peach cv. Flordaprince [32]. Kuden et al. [33] observed that 250 ppm of paclobutrazol decreased the shoot growth by about 34.1–42.4% in apricot. Leaf chlorophyll of almonds was increased with the application of 50 and 100 ppm paclobutrazol [34]. Arzani and Roosta [35] reported that paclobutrazol significantly reduced vegetative growth in apricot. They also reported that the total pruning dry weight, shoot growth and trunk cross sectional area (TCSA) of treated trees were lower than those of the control. Pant and Kumar [36] reported that the application of different concentrations of paclobutrazol and chloromequat at 250, 500, 1000 and 5000 ppm to 'Red Delicious' apple decreased the extension growth and leaf area of trees. Wani [37] applied paclobutrazol through soil to the basin of tree on trunk diameter basis. The investigation was carried out under two independent experiments. In one experiment, treatments were applied for two years consecutively and in another during first year only in order to assess carryover effect during second year. Application of paclobutrazol decreased yearly increment of tree trunk cross-sectional area, tree spread, volume and internodal length in sweet cherry. Gupta and Bist [38] noticed that soil application of paclobutrazol @ 10 ml/lit on high density pear plantation effectively controlled the excessive and vigorous growth. They also observed that the vegetative growth was inhibited by paclobutrazol. Asin et al. [39] observed that paclobutrazol resulted in shortest shoot length in pear. Sharma and Joolka [40] recorded reduced extension growth, plant height and plant spread with paclobutrazol in nonpareil almond plants. Abdollahi et al. [41] reported that paclobutrazol reduced vegetative growth by reducing both fresh and dry weights of shoots and the leaf area was also significantly decreased in strawberry cv. Selva. Mir et al. [42] reported that paclobutrazol significantly retarded the shoot growth, shoot diameter and trunk cross-sectional of 'Roundel' apricot trees growing under low density planting system. Ashraf et al. [43, 44] revealed that apple trees cv. Red Delicious treated with paclobutrazol @ 750 ppm and summer pruning resulted in minimum vegetative growth and vigour in terms of tree height, trunk diameter, annual shoot extension growth, tree spread and leaf area in comparison to control and other treatments. Reduction in growth is

attributed to paclobutrazol, which is a triazol that inhibits gibberellin biosynthesis especially three steps in the oxidation of the GA precursors ent-kaurene to entkaurenoic acid [15, 45].
