Scion-stock interactions in Fruit Crops
Authors: Manish Srivastav, R.R. Sharma, A. Nagaraja and A.K. Goswami
The relationship and interactions between two or more components of a graft age are quite variable and interesting. Some of these interactions have major horticultural value, whereas the others may not have any significance at all. These interactions are known to alter the size, growth, productivity, fruit quality and other attributes of different horticultural crops and thus are of commercial importance. The different interactions or the relationships have been developed in the following three different categories: i) Influence of rootstock on scion, ii) influence of scion cultivar on rootstock and iii) influence of inter-stock either on stock, scion or both.
i) Effects of rootstock on the scion
The effect of rootstock on the scion cultivar is one of the most significant influences. The rootstock influences the scion cultivar in many ways. The effects are variable, depending upon on type of stock and scion.
a). Influence on size and growth habit: The effect of rootstock on the size and growth of scion cultivar is perhaps the most pronounced influence of rootstock. The rootstocks may affect the growth, and some other habits of the scion cultivar. There are many examples in the literature to quote these effects of rootstocks on scion cultivar but the most important example is the rootstocks of apple. In fact, with the proper selection of rootstocks in apple, the complete range of tree size i.e. from very dwarf to very large has been obtained in the given scion variety, grafted or budded on these rootstocks. The Malling series of rootstocks (16) were selected by R.G. Hutton at East Malling Research Station, Kent (England) from old European clones like French Paradise and Doucin were assigned the Roman numeral, prefixed with the word Malling (M). Later, MM series of rootstocks (MM101, MM 102, MM106, MM109, MM111) were developed by crossing Malling series with Northen Spy to gent nematode resistant rootstocks. Out of these, MM111 and MM106 produce trees of medium vigour under high density planting systems. At present, many more rootstocks have been developed and they influence the tree canopy in variably. Mazzard (Prunus avium) rootstock usually produces vigorous trees of any scion variety of sweet cherry, whereas Mahaleb (Prunus mahaleb) produces small trees of the scion cultivar grafted or budded on it. Similarly, Stockton Morellgo also induces dwarfness in cherries. Quince C is a known rootstock for inducing dwarfing in pears. In citrus, Dragon fly in being used universally under high density plantings because it has very high potential to induce dwarfing in scion. In India, Troyer Citrange rootstock has been recommended as a dwarfing rootstock for Kinnow, whereas KarnaKhatta and Sohsarkar are semi-vigorous and vigorous rootstocks for Kinnow mandarin respectively. Creeping and Vellaicollumban rootstocks have been reported to induce dwarfness in vigorous mango cultivars like Banganpalli or Alphonso.
b). Influence on flowering and fruiting: The effects of dwarfing apple rootstocks for inducing precocity in bearing are world known. The dwarfing rootstocks like M7, M9 and M27 have pronounced effects on precocity (earliness) in bearing in all the cultivars grafted or budded on them. These rootstocks also enhance the fruit set in the scion cultivar. Pears usually come into bearing quite early if grafted on Quince C rootstock. Similarly, different rootstocks have been reported to stagger the fruit maturity in Kinnow mandarin. With the use of three different rootstocks, the availability of Kinnow in the market can be enhanced for longer a period. Troyer Citrange hastens the maturity in Kinnow, whereas Sohsarkar delays it. Thus, Kinnow fruits on Troyer Citrange are ready for harvesting by end of November (quite early), whereas these are ready for harvesting by mid January on Sohsarkar, thereby making its availability for 21/2 to 3 months.
c). Influence on fruit size and quality: Rootstocks also influence the fruit characteristics of the scion cultivar invariably. For example, pear fruits have pronounced tart and astringent flavour, if grafted on quince rootstock, which is otherwise not present if no rootstock is used. The dwarfing apple rootstocks though do not influence fruit size but colour development in the scion cultivar is much better as compared to standard or seedling rootstocks. In citrus, the striking effects of the rootstock appear in fruit characteristics of the scion variety. Sour orange rootstock produces thin skinned, juicy and excellent quality fruits of scion varieties like sweet orange, tangerine and grapefruit. Rough lemon usually produces thick, coarse skinned fruits of inferior quality of the scion cultivar. Similarly, the bitterness of the grapefruits disappears if budded on trifoliate rootstock but it never disappears on Rough lemon rootstock. Troyer Citrange rootstock usually reduces fruit size in Kinnow but Karna Khatta and Sohsarkar improves it. Further, the fruits of Kinnow on Troyer Citrange are thin skinned with attractive bright orange colour having good TSS, compared to those obtained from Karna Khatta and Sohsarkar rootstocks. Rootstocks also affect the storage quality of the fruits. For example, the fruits produced on the dwarfing apple rootstocks have poor storage life as compared to those produced on standard rootstocks.
d). Influence on insect-pests and disease resistance: Dwarfing apple rootstocks are susceptible to wooly apple aphid and crown gall but Malling-Merton (MM) series of rootstocks (MM104, MM 106, MM109, MM111etc) impart resistance against to the scion cultivar against woolly aphids. French pear rootstock imparts resistance against fire blight disease to the scion cultivar. Old Home pear is resistant to pear decline. Further, Mahaleb rootstock of cherry imparts resistance against buckskin virus. Similarly, a considerable variability exists in citrus rootstocks in their capacity to induce resistance against many diseases. For example, Sour orange is found to be highly susceptible to tristeza virus, whereas Rough lemon and Cleopatra imparts resistance against it. Citrus macrophylla is a very good rootstock for lemons because it is known to impart resistance against gummosis, the most serious disease of lemons. Poncirustrifoliata rootstock imparts resistance against trisreza virus and root rot disease.
e). Winter hardiness : Some rootstocks impart resistance to scion cultivar for cold tolerance. Apple rootstocks like EM-IX, EM XVII and Alnarp 2 have been found to be resistant to winter injury. EM-IX is resistant to early winter frost, though susceptible to late winter frost. Among citrus rootstocks, trifoliate orange is considered as the hardiest rootstock to low temperature followed by sour orange and sweet orange.
f). Other influences : Different rootstocks have been recommended for different fruit crops and even for different cultivars of the same species. Similarly, one rootstock recommended for one cultivar in a particular locality may not be suitable for same cultivar in the different locality. Thus, selection of a proper rootstock as per scientific recommendation for different regions having different soil and climatic condition is very important, because the success or failure of a particular fruit crop in that particular locality would be decided by it. For example, Jatti khatti citrus rootstock performs well in semi-arid zones of Punjab and Rajasthan, whereas Karna khatta is suitable in Indo-Gangetic plains of Uttar Pradesh but not the others. Myrobalam plum is highly suitable rootstock in regions of excessive soil moisture, whereas almond is most susceptible to wet conditions. Sour orange, Rangpur lime and Cleopatra mandarins are found to be resistant rootstocks for higher salt concentrations. In general, Rangpur lime has shown wider soil adaptability as compared to other citrus rootstocks. In mango, Kurukkan performs well in soils having higher salt concentrations.
How does rootstock bring about dwarfing effects upon scion?
Dwarfed fruit trees have essentially the same structure and parts as do the standard trees. They carry on the same physiological processes and they respond to various stimuli similarly. The difference between a dwarfed and a standard tree lies in the manner in which various structures and, physiological processes are acted upon or manipulated. The following mechanisms have been reported to be associated with dwarfing in different fruit crops:
Anatomical features: The dwarfing rootstocks have higher bark to wood ratio for stems and this character is used for screening dwarfing rootstocks in breeding trials. Further the dwarfing rootstocks had higher phloem to xylem ratio as compared to invigorating rootstocks. The roots of dwarfing rootstocks have smaller xylem vessels and lesser xylem fibers as compared to vigorous rootstocks.
Limited ability to transport auxin: In dwarfing rootstocks, the basipetal transport of auxin from shoot tips toward the root system is slower as compared to vigorous rootstocks. Higher amount of IAA (indole acetic aid; a natural auxin) was destroyed by shoot and root bark of various dwarfing apple rootstocks. Bark of more dwarf apple rootstocks caused higher rate of auxin destruction than bark of the less dwarf rootstock. The amount of auxin degrading depends on the amount of IAA oxidase, IAA peroxidase and phenols and perhaps some other compounds present in the phloem and cambial cells of dwarfing rootstocks. The destruction of IAA in dwarfing rootstocks leads to limited transport of IAA to roots and this leads to poor vascular development since, auxin control cambial activity. This effects the transport of photosynthates, nutrients etc to and from roots. High auxin levels favour differentiation of xylem and low auxin levels favours differentiation of phloem. This suggests the presence of low auxin in stems or roots of dwarfing rootstocks as the dwarfing rootstocks have thicker bark as compared to vigorous ones. The role of IAA in carbohydrate metabolism may be important in dwarfing mechanism as citrus and apple dwarfing rootstock contain more starch than vigorous ones.
Role of phenols: Phenols inhibit plant growth by affecting the levels of auxin in plant tissue. Phenols that inhibit growth enhance oxidative decarboxylation (degradation) of IAA, whereas the phenols that promote growth suppresses decarboxylation of IAA. Apple bark contains phenols that have reported to be synergistic or to antagonize IAA. Phenols may also retard the synthesis of Tryptophan, the precursos of IAA thereby indirectly reducing the levels of IAA in plant. Therefore, the phenols are the important growth controlling compounds in the bark of as they influence synthesis and translocation of auxin in the plant.
Cytokinin supply from roots to shoots: Cytokinins are synthesized primarily in roots and are translocated through the xylem to the shoot tip where they influence shoot growth. The differences in cytokinin production play a major role in causing the effects of rootstocks on scion growth. The level of auxin that reaches the roots of the plant influences the root growth and metabolism, including the synthesis of other hormones, such as cytokinins in the roots. The amount and/ or kind of cytokinin transported up through the xylem would, therefore, reflect the amount of shoot synthesized auxin reaching the roots. The cytokinin arriving at the shoot tip then influence shoot growth, stem elongation and leaf production (i.e. plant vigour). The cytokinin arriving at the shoot tips would also influence the synthesis of auxin and the amount translocated in the roots.
Higher ABA levels: Abscisic acid is synthesized in leaves. ABA is involved in the growth inhibition and dormancy and also is known to have an adverse affects on auxin transport. ABA inhibits stem elongation. ABA is moved by more than one transport system or mechanism and it occurs in both xylem and phloem depending upon the site of synthesis. Higher amount of ABA in the roots, shoots and leaves have been found in dwarfing rootstocks of apple as compared to vigorous roostocks, but in some other fruits like Prunus ABA had no role. Hence the amount of ABA is not as important in dwarfing as compared to sensitivity of a species or cultivar to ABA.
Reduction in phloem transport : When scion is on a dwarfing rootstock, the phloem transport from the leaves to root is reduced. In apple, the movement of sorbitol (the predominant photo-assimilate in apple) from mature leaves to roots was greater in vigorous rootstocks as compared to dwarfing rootstocks. This is because the (i) dwarfing rootstocks roots are a less strong sink for assimilates (have a limited capacity to make use of assimilates) or (ii) the transport is restricted by poorer transport system (across the graft union) or abstraction enroute. Experiments have indicated restriction of assimilate movement downwards at the upper graft unioin of a dwarfing interstock.
Limited root system in dwarfing rootstocks : The root system of dwarfing rootstocks is small so they have limited growth potential and they control scion growth. It has been found that the rootstocks had larger root system with vigorous scion varieties as compared to dwarfing varieties. The root system of vigorous rootstocks of apple is greatly reduced in size if an inters tock (a piece of stem) of a dwarfing rootstock is used.
2). Influence of scion cultivar on rootstock
It is not only the stock, which influences the scion but the top (scion) can also influence the stock many ways, though, examples are very less but not the least. Influence on the vigour is the major effect of the scion on the stock. If a vigorous cultivar is grafted on a weak rootstock, the growth of the stock is also stimulated. Further, if a dwarf or a weak scion is grafted on the vigorous rootstock, the growth of the stock is also reduced. Similarly, the size, nature and form of the root system of the stock are also influenced by the scion cultivar. If the rough lemon seedlings are grafted or budded with Malta and Santra scions, a more fibrous root system is developed as compared to the non-budded plants. Similarly, Red Astrachan apple scion produces a fibrous root system with a few tap roots, while Olden Burg scion produces two or three prolonged deep tap roots but not fibrous root system. A strong scion seems also to cause the roots to be more resistant to wet, poorly aerated soil than the same roots would be if the scion were of a weaker growing variety.
3). Influence of interstock
The ability of certain dwarfing rootstocks, used as interstock between vigorous top for producing a dwarfing and early bearing trees is in practice. Paradise apple is widely used as interstock between scion cultivar and Crab apple rootstock, for inducing precocity in bearing. Similarly, use of Malling IX dwarfing rootstock as inter stock is a commercially viable technology. In mango, Anupam has been recommended as interstock for Amrapali, grafted on Mallika rootstock, to induce dwarfness, precocity in bearing and improvement in fruit quality. However, some times interstock may not result in favourable or desirable influences.
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