Influence of Growth Regulators on canopy and High Density Planting in Fruit crops

Canopy management and High density planting are the manipulation of tree canopy to optimize its production potential with excellent quality fruits. In many fruit crops, increase in production with enhanced fruit quality is obtained by managing canopies of short statured trees by using different growth retardants/ regulators in different fruit crops.

Chemical Growth Retardants:

Growth retardants are plant growth regulators that retard growth of plants. There are at least three basic mechanisms by which tree height can be restricted by chemicals.

(a) By killing the terminal buds of branches or severely inhibiting apical meristemic    activity: The most important chemicals of this group include Maleic Hydrazide (6-Hydroxy-3-(2H)-Pyidazione) and in some cases ethylene and ethelene realising compounds (Luckwill, 1978). These chemicals, however, are not of much use in controlling tree size in commercial fruit production as they also inhibit normal leaf and flower initiation and development at the concentrations required to be bring about growth inhibition of shoots. 

(b) By reduced apical control: Reduction of tree height may also result from simultaneous growth of many shoot axes, which owing to competition for limited nutrients reduce the elongation of one or more axes on the same root system (Sachs and Hackett, 1972). Promising chemicals for inducing auxiliary bud growth include TIBA (2,3,5-Triiodobenzoic acid), which is an inhibitor of auxin synthesis and translocation, cytokinins, particularly benzyl adenine, which promotes the growth of auxiliary buds. Unlike those of the previous group, these chemicals are not translocated to the meristem of the new lateral branches and hence do not interfere with leaf and flower development.

(c) By inhibiting internodes elongation without disrupting apical meristamatic functions: Near specific inhibition of internodes elongation without concomitant disruption of apical meristamatic function and loss of apical controls caused by certain growth retardants like, SADH (Succinic acid-2, 2-dimethyl hydrazide), CCC, Amo-1618 and Paclobutrazol. These growth retardants reduce gibberellins’s biosynthesis, though different retardants act at different steps in biosynthesis pathway. Amo-1618 and SADH prevent the conversion of trans-geranyl geranyl pyrophosphate to Kaurene and CCC blocks Kaurenoic acid from getting converted to GA, whereas, Paclobutrazol retards the formation of Kaurene-19-ol from Kaurene (Anon, 1984). The chemical when applied to the leaves is readily translocated through phloem to the apical meristem where it inhibit DNA synthesis resulting in a temporary check in apical growth, thereby, promoting sprouting of auxiliary buds and reducing intermodal elongation
(Arzee et al, 1977). Among tropical fruit trees, Cycocel at 6000 ppm inhibit vegetative growth leading to reduction in plant height, intermodal length and scion girth and promotion of flowering while CCC at 4000 ppm resulted in 50% stunted growth in case of mango. More recently PP₃₃₃ or Cultar has been found more effective and persistent than these chemicals in tree size regulation during the bearing phase of fruit like apple and mango (Unrath, 1999).

About the Author :
Dr. Sable P.  A. Assistant Professor, Deptt. Horticulture, Sardarkrishinagar Dantiwada Agriculture University, Gujarat                                                     

References:

Anonymous. (1984). Technical data sheet- PP333, PGR for Fruits. ICI. England.
Arzee, T. Langeanaur, H and Gressel, J. (1977). Effect of dikegulac, a new growth
regulator on apical growth and development of three composite.
Bot. Gaz.  (138): 18-28. 
Luckwill, L.C. (1978).Chemical induction of early cropping in fruit tree.
Acta Hort.(65):139-45.
Sachs, R.M and Hackett, W.P. (19 72). Chemical inhibition of plant height.
Hort Sc.(7): 440-47.
Unrath, C.R. (1999). Prohexadione-Ca:A promossing chemical for controlling vegetative 
growth of Apples. Hort Sci.(34): 1197-1200.