An Introduction
Canopy management and High-density planting are one of the most important production factors confronting the burgeoning fruit industry in India. Indeed, canopy management is the manipulation of tree canopy to optimize its production potential with excellent quality fruits. In many fruit crops, an increase in production with enhanced fruit quality is obtained by managing canopies of short-statured trees by using dwarf rootstocks. Small trees capture and convert sunlight into fruit production in a better way than larger ones. Fruit production involves the capturing and conversion of sunlight into the production of fruit biomass (dry matter content).
Influence of rootstock and Interstocks in Dwarfing effect
Dwarfing rootstocks are economical, effective and environmentally safe means for controlling tree vigor canopy management and for high-density orchards. Rootstocks for high-density planting must control tree size, reduce vigor and induce precocity or early fruiting. Many of the modern tree forms for intensive orchards such as slender spindle and vertical axis are achievable only with the help of dwarfing rootstocks. There are at least three categories of rootstock based on their vigor regulating features: standard, semi-dwarf, and dwarf. Trees on standard rootstock reach their full-size potential.
Standard rootstock results in a tree that grows to 5m or more in height although the mature size can be highly variable between different trees and locations depending upon vigor of the scion cultivar and the environment in which the tree is grown. A semi –dwarfing rootstock produces a tree between 3 to 5 m tall, again dependent upon the scion cultivar in which the tree is grown. Sometimes the term semi-vigorous is used to denote a rootstock that is intermediary to standard and semi-dwarfing rootstock generally restricts tree height to less than 3m and takes only two to three years to bear fruit in most cases as opposed to a standard rootstock on which the tree requires five to seven years.
However, a true dwarfing rootstock may be defined as a precocious one, which in combination with other plant parts, and independent of viral, environmental or other influences, results in a mature tree no larger than 2.5 m in height. The impact of dwarfing rootstocks upon changing characters of the fruiting industry has been impressive in the case of fruit trees, particularly apple, e.g. M.9 and M.27, in mango Vellaikulumban’ is identified as a dwarfing rootstock for Alphanso’ mango (Kurian et al, 1996) and Flying Dragon as dwarfing rootstock for citrus.
The Mechanism of Dwarfing
The mechanism by which a rootstock induces dwarfing is not well understood and little is known of its genetic control. Delayed incompatibility, nutrient supply, hormonal control, phenol content, etc. are cited as some of the probable causes of the dwarfing effect of rootstock on the scion. There are few indications that the effects of rootstock are scion-specific suggesting an interaction of the two. (Tubbs, 1967) suggested that there must be feedback mechanism from the scion to rootstock and proposed a hypothesis based on destruction or inhibition of auxin by substances in or coming from the stock rather than on production of auxin in the scion. It is possible that is entirely different from either of them individually. Some success in tree size control has been achieved, for e.g. with citrus relatives and species of Citrus even though the rootstock by itself is not a true dwarf.
A dwarfing rootstock could also be a true genetic dwarf by itself and impart this character to scion budded upon it. The extent of vegetative flushing was markedly less when Alphanso’ mango was grown on the dwarfing Vellaikulumban’ rootstock. Kurian et al, (1996). The earliest explanations of rootstock dwarfing assumed that a reduced water or nutrient supply reached the scions from the root which is inconsistent with the findings that scion leaf nutrient differ very little among different rootstocks. (Chaplin and Westwood, 1980). However, Kurian et al (1996) recorded less leaf concentrations of some nutrients, especially of nitrogen, in Alphanso’ mango grafted on Vellaikulumban’ rootstock. Excessive non-conducting phloem and Ca accumulations were found associated with the extreme dwarfing rootstocks. Very vigorous rootstock induced many more secondary phloem fibres in the narrow bark of the scion, while a dwarfing stock induced thicker bark with less phloem fibre.
The strongest evidence indicates that the bark is the key to the dwarfing mechanism by rootstocks. It has been postulated that vigour regulation depends on the destruction of auxins produced by the shoot tip of scion while translocation down the phloem, the amount arriving at the root influencing root metabolism and affecting the amount and kind of cytokinins synthesised and translocated to the shoot scion through the xylem vessels. The dwarfing effect on Alphanso’ mango rendered by Vellaikulumban’ rootstock could be attributed to the lower xylem sap yield and lower level of cytokinins and higher level of ABA produced in the root and transported to the shoot in the xylem sap (Murti et al, 2000). Higher concentrations of phenols are, however, known to lower the endogenous auxin levels (Letham, 1978) by promoting its oxidative decorboxylation by IAA oxidase enzyme (Zenk and Muller, 1963). Though dwarfing rootstock are very effective in controlling tree size, but in most cases dwarfing rootstocks have poor soil anchorage, posing problems for their use on hill slops, cyclone prone area etc.
Reference:
Chaplin, M.H and Westwood, M.N. (1980). Nutritional status of Bartalett pear on Cynodonia and Pyrus species rootstock. J. Amer.Soc.Hort.Soci.(105): 60-63.
Kurian, R.M., Reddy, V.V.P and Reddy, Y.T.N. (1996). Growth, yield, fruit quality and leaf nutrient status of thirteen year old Alphanso’ mango on eight rootstock. J.Hort.Sci. (71): 181-186.
Letham, D.S., (1978). Natural-occurring plant growth regulators other than the principle hormones of higher plants. In Letham, D.S., Goodwin, P.B and Higgins, T.J.V. (eds.). Biochmistry of Phytoharmons and Related Compounds: A comprehensive Treatise, (I), Elsevier, Amsterdam.pp 349-465.
Murthi, G.S.R,. Upreti, K.K., Kurian, R.M and Reddy, Y.T.N. (2000). Endogenous hormones and phenols of seedling trees of polyembroynic mango cultivars and their role as rootstock in scion vigour of cv. Alphanso’. J.Applied Hort. (2):6-9. orchards: a review. Annals of Hort. 4(1) : 37-44.
Tubbs, F.R. (1967). Tree growth control through dworging rootstocks. Proc. XVII Intern.Hort. Congr. (III): 43-56.
Zenk, M.H and Muller, G. (1963). In vivo destruction of exogeneously applied IAA as influenced by naturally occurring phenolic acids. Nature, (200): 761-763.
Author
Dr. Sable P. A. Assistant Professor, Deptt. Horticulture, Sardarkrishinagar Dantiwada Agriculture University,
Gujarat and Sonpure Sushma, Sr. Ph.D. Scholar, Department of Agronomy, MPKV, Rahuri, Maharashtra.
