Grafting and clonal micropropagation technologies for fruit crops

Physiology of graft union and optimal conditions for grafting

Grafting is the primary method for propagation of pome fruits, stone fruits, nut crops, citrus, and subtropical crops in a nursery. The process combines a rootstock, which forms the root system, and a scion, from which the aerial part of the plant develops. Their firm physiological union occurs through the division of living cells in the cambial zone. First, they form loose callus tissue, then form a common secondary cambium, and finally restore the conducting system — phloem and xylem.

• Union time of components — from 30–50 days
• Relative humidity — 100 %
• Temperature range for cell division — 7–32 °C
• Optimal temperature for apple — 12–20 °C
• Optimal temperature for walnut — 26–28 °C

In modern nursery management, about 400 grafting methods are used, which are divided into two main groups:

• Approach grafting (inarching) — the scion and rootstock are joined without separation from the parent plants until they are fully united.
• Grafting with a detached part — transferring a scion or a single bud (budding).

For the components to successfully unite, three technological rules must be observed. The scion must be in a dormant state with well-matured tissues, while the rootstock must be in a phase of active sap flow and cambial activity. Also, it is important to ensure optimal aeration at the connection point.

Clonal micropropagation of fruit crops

Clonal micropropagation allows for the mass production of sanitized nursery plants under laboratory conditions. The method is based on the ability of the meristematic tissue of a bud or shoot tip to regenerate. The process is carried out in an absolutely sterile environment (in vitro) using special nutrient media and growth regulators. The technology has been successfully implemented by the largest nursery companies in the USA, UK, Germany, and Italy.

1. Isolation of the apical meristem from a healthy donor plant and its sterilization in laminar flow cabinets.
2. Placement of the transplant into test tubes on a nutrient medium under regulated conditions of light, temperature, and humidity.
3. Regeneration, occurring in three stages: proliferation (multiplication), shoot elongation, and rooting.
4. Planting of rooted microplants into trays with soil and keeping them in a chamber for 15 days.
5. Transfer to greenhouses with artificial fog for further hardening off with a gradual decrease in air humidity.
6. Use of the finished super-elite material for establishing virus-free mother plantations.

Currently, the technology of clonal micropropagation has been optimized for apple rootstocks, cherry, sweet cherry, plum, as well as for berry crops, especially garden strawberry. In industrial production, hundreds of plant species are propagated using tissue culture.

Specifics of using in vitro mother plants

Plants obtained by the method of clonal micropropagation possess a physiological feature important for a nursery grower. Green or hardwood cuttings taken from them have an increased rooting capacity. In practical gardening, this trait allows for a significant increase in the efficiency of propagating valuable cultivars and rootstocks.

The high capacity for root formation has been experimentally confirmed on cherry rootstocks P-3, P-7, the Molodezhnaya cultivar, and the Pixy plum rootstock. Such increased activity of cuttings can be maintained in a mother plantation for up to 10 years. For this, a special plant spacing scheme and a specific pruning system are used in the plantations.

• Duration of high rooting capacity — up to 10 years
• Tested cherry rootstocks — P-3, P-7
• Tested cherry cultivar — Molodezhnaya
• Tested plum rootstock — Pixy