Influence of clonal rootstocks on the productivity of apple cultivars in southern regions

The role of the rootstock in optimizing the vital activity of a grafted fruit tree is quite difficult to overestimate. A targeted selection of this grafting component, provided it is compatible with the scion, allows for effective management of growth vigor, precocity, yield of the cultivar used, and even the formation of fruit quality within a wide range. Moreover, by applying rootstocks of different natures, one can achieve the desired degree of adaptation of fruit plants to various soil and climatic conditions and, accordingly, the necessary level of their productivity.

In other words, the most important condition upon which the degree of realization of the biological potential of a specific cultivar depends is the correct choice of rootstock.

Therefore, under the conditions of vegetative and field experiments (in 2001–2006), studies were conducted to evaluate the clonal rootstocks SK2, M26, MM106, MM102 widely used in the southern regions of the Russian Federation and the potential capabilities of the new apple cultivars Persikovoye, Florina, and Granny Smith grafted onto these rootstocks.

To study this, a complex of physiological and biochemical parameters that most fully reflect the functional state of the plant was used: leaf surface area, carbohydrate content in leaves, and the absorption activity of the root system.

The potential productivity of plants is inextricably linked to their photosynthetic activity, which is carried out mainly by leaves. Therefore, the results of determining the leaf area of various scion-rootstock combinations of apple, from the point of view of potential capabilities, in the first year of life were of significant interest.

The provided data show that the leaf area of plants depends not only on the varietal characteristics of the scion but also on the functional activity of the rootstock of the scion-rootstock combination. Table 5 – Leaf area of apple scion-rootstock combinations (vegetative experiment, August 2002)

Persikovoye Florina Granny Smith blade, cm2 blade, cm2 m2/ plant m2/plant blade,

Leaf plant Rootstock m2/ cm2 SK2 18.1 0.05 19.1 0.06 17.5 0.01 M26 19.7 0.04 21.2 0.05 18.2 0.02 MM102 20.5 0.03 22.1 0.05 17.7 0.01 MM106 22.8 0.05 26.4 0.08 17.7 0.01 LSD05 1.2 – 1.9 – 0.7 –

Based on the presented data, differences in the average area of the leaf blade and the total leaf area of the studied apple cultivars clearly manifest depending on the rootstock used. Thus, for the Persikovoye cultivar, the maximum leaf area was noted when grafted onto the SK2 or MM106 rootstock. For the Florina cultivar, such a rootstock is MM106, and for Granny Smith, it is M26.

As is known, the main product of photosynthesis is carbohydrates, which occupy a central place in the metabolism of all plants. They play the role of primary energy-storing compounds and starting organic substances from which most other compounds (proteins, lipids, phytohormones, etc.) are synthesized.

In the leaves of woody plants, in particular the apple tree, monosaccharides prevail: glucose and fructose, as well as the disaccharide sucrose. Moreover, the former largely determine the size of the harvest, and the latter serves as the transport form of carbohydrates in plants (Kramer, P.J., 1983).

Taking into account the above, we attempted to identify the characteristics of carbohydrate accumulation in apple grafting combinations with different potential capabilities.

As can be seen from the provided data, a higher glucose content is noted in the leaves of the Florina and Granny Smith cultivars grafted onto the SK2 rootstock. For the Persikovoye cultivar, a similar effect was recorded when using the M26 rootstock.

The obtained experimental data indicate that the tested cultivars accumulate sucrose and soluble carbohydrates as a whole most actively when grafted onto M26 and MM106 rootstocks.

Thus, differences in the total sugar content in leaves are detected between grafted apple plants with different levels of potential capabilities, starting from the first year of life. This indicator is significantly lower in low-productivity grafting combinations. Table 6 – Soluble carbohydrate content in leaves of apple scion-rootstock combinations, % of dry matter (vegetative experiment, July 2002)

Glu- Monosac- Sum of

Cultivar Rootstock Sucrose cose charides sugars Persikovoye SK2 3.5 4.8 2.4 7.2

M26 6.5 8.1 7.9 16.0

MM106 5.4 7.1 9.9 17.0 Florina SK2 3.9 4.9 3.1 8.0

M26 2.2 3.9 8.5 12.4

MM106 2.9 3.3 9.1 12.4 Granny SK2 5.4 6.4 3.4 9.8 Smith M26 3.5 4.5 5.6 10.1

MM106 4.9 5.8 4.6 10.4

Meanwhile, the assessment of the potential capabilities of scion-rootstock combinations based on the set of indicators of photosynthetic organs will be quite one-sided and insufficiently objective if the physiological characteristics of the root system are not taken into account. Therefore, it was necessary to compare the absorption capacity of roots in the tested apple combinations.

The absorption capacity of the roots of scion-rootstock combinations was judged by the size of the working absorbing surface of the root system. Based on the results presented in Table 7, the studied apple cultivars will be most productive when grafted onto the semi-dwarfing clonal rootstock MM106. The potential productivity of these cultivars is also quite high on the SK2 rootstock. Table 7 – Working absorbing surface of roots of apple scion-rootstock combinations (vegetative experiment, July 2002), m2

SK 2 M26 MM106 LSD 05 Persikovoye 2.73 2.26 4.67 0.60 Florina 2.42 2.25 2.45 0.19 Granny Smith 2.42 1.19 4.45 0.55

As can be seen from the data presented, for each apple cultivar there is a specific group of rootstocks that will ensure a sufficiently high yield of plantations in specific natural conditions.

However, the leading place in this list belongs to the MM106 rootstock, which, in the opinion of researchers, also possesses the unique property of delaying the absorption of heavy metals from the soil. This fact testifies to the prospect of its use in orchards focused on the production of ecologically safe fruit products, including raw materials for baby food.

Analysis of the results of early (in the first year of the plants' life) diagnostics allowed us to conclude that the best among those studied in terms of forming high fruit yields on leached chernozems of the Kuban horticulture zone are the following apple cultivar-rootstock combinations: Persikovoye and Florina on MM106 and SK 2 rootstocks; Granny Smith on MM106 and M26 rootstocks. Table 8 – Best rootstocks for apple cultivars based on early diagnostic results* (vegetation experiment, July-August 2003)

Best parameters rootstocks (by aggregate Cultivar Sugar content Adsorbing Leaf area of parameters) surface

Persikovoye MM106 MM106; MM106; MM106;

For optimal plantation formation, the following combinations are recommended:

• SK 2, M26, SK2, SK2 Florina, MM106
• M26
• MM106
• MM106
• SK2, MM106, SK 2, SK 2 Granny, M26, MM106
• MM106
• MM106
• Smith, M26, SK 2, M26

* Rootstocks are listed in order of their importance.

Thus, the selection of optimal combinations of cultivars and rootstocks will ensure the receipt of sufficiently high and regular fruit yields in the corresponding natural conditions. It should be noted that the use of the best scion-rootstock combinations is a huge reserve for increasing the productivity of future plantations that does not require any significant costs.

As is known, the main abiotic stress factors in the southern regions of the Russian Federation are recurring frosts after winter thaws, droughts, and high air temperatures in the summer period, which were clearly manifested in the years of the research as well. Taking this into account, the realization of the biological potential of apple plants largely depends on their adaptive mechanisms. To determine the resistance of the studied plants to limiting factors, we used physiological and biochemical parameters.

The most important property of scion-rootstock combinations of fruit crops, which determines the feasibility of their introduction into production, is frost resistance at different stages of overwintering. Its manifestation largely depends on the degree of resistance of the rootstock used. Considering this, we conducted an experiment to determine the resistance of some clonal apple rootstocks to recurring frosts.

It should be noted that the soil temperature in the root zone in February 2003 after a thaw dropped below –12°C. The foreign M26 rootstock had damage to the fine roots in the upper part of the root zone (in the 0–20 cm layer). In all likelihood, this apple rootstockclonal apple rootstock, while differing in significant frost resistance in the hardened state by mid-winter, is unable to restore resistance to the adverse factor upon re-hardening after thaws. In turn, no signs of root freezing were found in MM102, SK2, and MM106 rootstocks after recurring frosts.

Table 9 – Degree of damage to the root system of clonal apple rootstocks by recurring frosts (nursery KubSAU established in 1996, March 2003)

* rootstock bred by SKZNIISiV.

It is known (Doroshenko, 2000, 2002) that as a diagnostic criterion for the resistance of fruit plants to any type of drought (atmospheric or soil), it is more reliable to use a universal parameter – the genotype activity index (the RNA/DNA ratio in the apical meristems of shoots), which determines the functional activity of the organism as a whole.

Taking this circumstance into account, we attempted to evaluate the genotypic capabilities of dwarfing apple rootstocks to tolerate water deficit. In this case, the drought resistance coefficient (Kd) for the MM102 rootstock was 14%, and for the SK2 rootstock it was 22%. In other words, they should be classified into the group of drought-resistant (Kd < 25–30%). The M9 rootstock is less drought-resistant (Kd > 30%). In all likelihood, in arid conditions, it requires additional moisture supply.

It has been established that under conditions of water deficit, the length and total adsorbing surface of the roots of the MM102 rootstock are 1.3–1.4 times greater compared to similar parameters in the control. However, a slight decrease in their physiological activity was noted. Under the same conditions, some increase in the absorption capacity of the root system was found in the SK2 rootstock. Table 10 – Change in the physiological activity of roots of semi-dwarf apple rootstocks with a decrease in soil moisture (vegetation experiment, July)

75–80% FC 50–55% FC

Absorbing Absorbing surface* surface*

Length, cm Rootstock active Working total, m2

Total, total total m2

% of m2 m2 MM102 5.0 4.0 2.0 46 21.0 5.2 2.3 44 SK2 5.0 4.3 2.0 44 17.0 4.2 2.0 47 * Sx % < 3.5.

The obtained data indicate various adaptive changes in the physiological functioning of rootstocks MM102 and SK2 under the influence of a stress factor. Thus, under conditions of soil moisture deficiency, rootstock SK2 shows an increase in root absorption activity, while rootstock MM102 exhibits an increase in root system size, providing the capacity for greater water consumption in an extreme situation.

Thus, the prospects of using these rootstocks in organic orchards of the southern region are evident, even in the absence of irrigation.

The yield and resilience of an organic orchard to weather fluctuations, pests, and diseases depend not only on the innate properties of fruit trees. The quality of orchard design and the technologies applied by the agronomist in practice play a huge role. Without competent design of the agro-ecosystem, even the most promising cultivar will not reach its potential. Under the conditions of organic farming in southern regions, this balance of technology and genetics becomes critical for business survival.

Criteria for selecting cultivars and rootstocks for an organic orchard

For the successful establishment of an organic apple orchard, it is necessary to carefully select nursery plant. The plants must possess natural hardiness, as the possibilities for chemical protection and artificial stimulation in organic production are minimized. When evaluating cultivars, the agronomist must analyze the complex of their defensive reactions to environmental stress factors.

When selecting cultivars for southern regions, the following indicators are of key importance:

• high resistance to spring late frosts, which protects flowers and fruit set;
• drought and heat tolerance, confirmed by laboratory or field evaluation methods;
• the presence of genetic immunity to major pathogens combined with the ability to withstand summer heat.