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Healing Grafted Seedlings
Healing is the most critical process of grafted seedling production. Propagators should not overlook this process and should try to achieve the conditions as close to that recommended here.
Relative humidity: 95% or greater. Gradually decrease toward the end.
Air temperature: 28-29C (82-84F) INSIDE the healing chamber. The optimum healing temperature (28-29C) is slightly warmer than growing temperature (~25C). Under this warmer temperature, cell division will be enhanced to develop callus to bridge the cut surfaces of scion and rootstock.
Light intensity: Darkness for the first 24-48 hours and then provide light. Target light intensity is 100 micro-mol/m2/s PPF [or 5400 to 7400 lux (540 to 740 foot candle) depending on the light source]. For healing chamber with artificial lighting, two fluorescent lamps of the same length as the shelf will create a similar level of light intensity over the plant canopy surface. This light intensity is many times lower than mid-day natural light intensity (1,000 to 2,000 micro-mol/m2/s) outside and is closer to the light compensation point, which provides the minimum energy needed to maintain plant metabolism during the healing process. It is highly recommended that propagators obtain a light sensor to actually measure the light intensity, as human eyes are not a reliable sensor, as our eyes quickly adapt to the surrounding brightness/darkness, making difficult to determine the actual light intensity.
Duration: 4-6 days for tomato, 7 days for cucurbits
Other important note: When you graft tomato plants, we recommend not saturating the substrate with water during the healing process to avoid excess turgidity of rootstock plants (due to the resulting root pressure) that could create a thin film of water at the grafted union, preventing the complete healing of the union. Relative humidity must be in the recommended range but wetting the substrate too much should be avoided.
There are generally two types of healing systems: the first one is growth room designed for healing. The second one is greenhouse based healing system widely used in many countries. The former system can create a uniform climate independent from outside climate conditions and is more suitable for grafts that are difficult to heal.
1. Healing chambers with artificial lighting
A commonly used design of healing chamber with artificial lighting uses fluorescent lamps installed in multi-layered shelving units. The chamber needs to have good humidity control to create near-saturating humidity and shelving units and lamps in such a chamber need to be compatible with wet conditions. Alternatively small tunnels or compartments can be fabricated to create highly humid microclimates directly around the grafted plants. Such a set-up may be suitable for healing small numbers of grafted plants.
The first picture on the left is a commercial healing chamber with fluorescent lamps in four-layered shelving units. The high humidity is maintained with a fogging system.
The second picture is one used in the University of Arizona research facility. The chamber does not have capability of humidity control and therefore grafted plants are placed inside tightly sealed boxes with a shallow layer of water at the bottom for humidification. See our video clip.
The third picture is a healing chamber made of surplus refrigerated trailers in which multi-layered shelving units and fogging system are utilized.
2. Healing chambers in greenhouse with natural light
Developing healing units inside a greenhouse or high tunnel has been practiced for many years. However, maintaining air temperature, light intensity and humidity at optimum levels is more challenging especially in warm climate regions or in summer. Key equipment includes shade cloth to reduce light intensity, plastic cover to maintain the high humidity, fogging or under-bench misting system, and heating system (during night time or cloudy conditions).
The picture shows a set-up used in Spain. Chambers are created over individual benches. Black cloth (in background) maintains darkness for the first day or two after the grafting. Overhead shade cloth is for reducing the light intensity. In this particular system, a misting system is installed under each bench to humidify the air, yet avoid wetting the plants. It is also has a hot water heating system underneath the seedling trays.