Detailed Explanation of Pecan Breeding and Pollinator Cultivar Configuration Techniques

Pecan Breeding and Pollinator Cultivar Configuration Techniques

1 Seed Warming and Germination Promotion Techniques

After pecan fruits mature, the husk splits open on its own and the seeds fall out naturally. When the outer husk turns from green to yellow-brown and begins to split slightly, harvest should be done promptly, generally from mid-October to early November. After collection, the fruits should be piled indoors promptly; once most fruits have split open, remove the seeds, spread them out to air-dry for 3-5 d, then store them indoors in damp sand or place them in cold storage. Before spring sowing, carry out germination promotion treatment on the seeds.

Method 1: After the seeds are harvested, first soak them in clean water for 12 h, drain, then soak again in clean water for 12 h, spread them out to air-dry, then store them in sand. Use granular sand of a certain moisture level (it holds together when squeezed in the hand and does not fall apart when released) and layer it alternately with the seeds. The bottom sand layer should be about 15 cm thick, the sand between the two seed layers about 5 cm thick, and the top sand layer about 15 cm thick; the total height of the germination bed should preferably be 50-60 cm. If the germination bed area is too large, insert straw bundles into the sand for ventilation. Pile them outdoors under natural conditions and let them go through the natural low-temperature dormancy process. In early February, cover with black plastic film and add a 10 cm thick layer of rice straw. After about 20 d of warming and germination promotion, most seeds have already cracked open or grown radicles, and they can be sown in the field.

Method 2: After the seeds are harvested, spread them indoors to air-dry for about 1 month, until the end of December or early January of the following year, then sow directly in the field in rows 20 cm apart and 4-5 cm between plants, water thoroughly and cover with plastic film to retain heat; if conditions permit, add a layer of rice straw on top of the plastic film. When the weather warms up and the maximum temperature reaches 25-28°C, remove the plastic film. Observe carefully; if the weather is dry with no rain, water thoroughly once, and seedlings can emerge in mid-to-late March. With this method, the seeds are easily damaged by animals, diseases, and pests, and uneven emergence may also occur.

2 Container Seedling Cultivation Techniques

Pecan container seedling cultivation has the advantages of a short seedling-raising period, uniform seedlings, no root damage, no transplant shock period after afforestation, and transplanting not being limited by season. Use containers made of white non-woven fabric, commonly known as nutrition pots; the diameter depends on the purpose, generally 28 cm in height and 25 cm in diameter. White non-woven fabric is light-transmitting, and when roots grow and touch the container wall they stop growing, so root coiling does not occur. The container substrate is configured from loam, peat, organic fertilizer (decomposed chicken manure, slow-release compound fertilizer, etc.), and rice husks at 50:25:15:10, while mixing in phoxim granules, then spraying ferrous sulfate, carbendazim, and other agents for disinfection. Fill the containers with substrate, arrange them neatly on the seedbed, place the germinated pecan seeds flat in the nutrition pots, cover with 3-5 cm of the same substrate, and water until moist. Pecan likes water and fertilizer; with frequent fertilizer and water management, seedlings emerge in about 1 month.

3 Grafting Propagation Techniques

3.1 Spring Branch Grafting

Generally from mid-March to early April each year, when the temperature is stable above 15°C; the main grafting methods are cleft grafting and cut grafting. The nursery must not be irrigated before grafting; 3-5 d before grafting, make 2-3 cuts at 8-10 cm above the ground to "release water" (to prevent bleeding at the graft interface), then graft at 30 cm above the ground.

3.2 Large Square Patch Budding in Summer and Autumn

Budding is generally carried out from mid-to-late May to early September. Use a blade to take a bud patch 3-4 cm long and 0.8-1.2 cm wide from the scion, with the bud located in the exact center of the bud patch. On the rootstock, choose a smooth spot and cut a bark layer the same length as the bud patch and 1-2 mm wider; while removing the bark layer, tear off a piece of bark 2 cm long and 1-2 mm wide on one side below the cut as a bleeding port to "release water." Place the bud patch on the rootstock cut, press the left side tightly, leave the bleeding port open on the right side, and wrap and seal with plastic film, with moderate tightness; do not seal the bleeding port tightly, and leave the bud exposed. When the new shoot from the grafted bud grows to 20 cm, cut the rootstock 2 cm above the grafted bud.

4 Large Rootstock High Grafting Techniques

Pecan seedling trees generally begin bearing fruit in 10-12 years, while grafted trees begin bearing after 4-5 years. Using large-rootstock high grafting to replace superior cultivars, they begin bearing in 2-3 years and rapidly form a canopy and produce yield. The method is to select a suitable thickness (diameter 2-10 cm is acceptable), saw it off at a smooth place on the bark, and from the end of March to early April, when the temperature is stable at 20-25°C, use the bark-prying graft method for grafting; for rootstocks with a diameter above 4 cm, multiple grafts can be made.

5 Rapid Seedling Cultivation Techniques

Pecan is dual-purpose for fruit and timber, with good economic benefits, a straight trunk, and excellent wood form and quality; it is well received by the market, especially with a large promotion area in the Jianghuai River basin. In order to provide seedlings quickly and efficiently, we experimented and explored year-round grafting techniques, achieving seedling cultivation in the same year and the release of improved, vigorous seedlings from the nursery in the same year.

In mid-October, store the harvested pecan seeds in sand; in mid-January of the following year, carry out warming and germination promotion; select seeds that have cracked open or grown radicles for container seedling cultivation, and add an arched shed over the seedbed. Observe carefully, irrigate at the proper time, and control the greenhouse temperature at 35°C and humidity at 70%. To maintain temperature and humidity, bury most of the nutrition pots in the seedbed soil, and cover the greenhouse with a shade net with 70% shading to prevent damage to young seedlings from high temperature and strong light. When the seedlings normally unfold leaves and the stems turn green, open both ends of the arched film around noon on sunny days for ventilation and cooling to enhance the seedlings' adaptability to the external environment. When the seedlings grow close to the arched shed film, remove the arched shed promptly to avoid scorching the seedlings. By August, 80% of the seedlings have reached grafting standard (at 10 cm above the ground, ground diameter 7-8 cm or more, height 80 cm or more), and large square patch budding can be carried out successively. Because this is a period of vigorous growth, draining and "releasing water" during budding is an important step. The method is to tear an opening 2 cm long and 1-2 mm wide below the interface. The survival rate of budding can reach 85%. Seedlings grafted in mid-to-late August generally do not need the rootstock cut; when released from the nursery for afforestation, cut off everything above the interface. During tending and management, pay attention to promptly removing sprouts below the grafted bud.

6 Main Trunk Film-Sleeving Moisture-Retention Cultivation Techniques

Pecan seedlings have a well-developed main root, fewer fibrous roots, and thin bark on the trunk and branches; with conventional bare-root seedling cultivation, the survival rate is not high. Using main trunk film-sleeving moisture-retention cultivation techniques can greatly increase the survival rate of afforestation. The method is: after the seedlings are planted, make a long strip-shaped cylinder from plastic film, soak the long strip-shaped cylindrical plastic film bag in water, take it out, leave some water droplets inside the film, then sleeve the long strip-shaped cylindrical plastic film bag with water droplets over the main trunk of the planted pecan, with the lower part buried in the soil. In this way, the seedling trunk and branches will receive a very good moisture-retention effect, until the trunk and branches sprout and the new shoots grow to 5-6 cm, then remove the moisture-retention plastic film cylinder.

7 Pollinator Cultivar Configuration Techniques

Pecan is monoecious with separate male and female flowers, wind-pollinated, and the male and female flowers mature at different times. Through the configuration of different cultivars and reasonable shaping and pruning, high and stable yields can be achieved. For example, among common cultivars, Pawnee is a protandrous cultivar, while Maoshan No. 1 and Weston are protogynous cultivars; the combined cultivation of Maoshan No. 1, Pawnee, and Weston is a relatively good cultivar pollination configuration, and it performs well in the Yangtze River basin.

8 Branch Bending and Branch Twisting Techniques

Branch bending: Pecan has relatively long internodes. To increase the number of shoots produced per unit area, before bud break in winter and spring, use plastic ropes to pull long branches diagonally so that the internodes receive sufficient light and short shoots (fruiting shoots) are promoted.

Branch twisting: When the front end of a long branch is semi-lignified, hold the branch with the hand and gradually move from the base toward the tip while slightly bending and injuring the xylem to promote opening of the branch angle. When twisting branches, pay attention to the amount of force in the hands to avoid breaking the branch or seriously injuring the bark. The main targets are relatively upright vigorous branches, competing branches, auxiliary branches, etc. Branch twisting can open the branch angle, increase the bud-break rate of branches, promote the formation of flower buds and medium and short branches, and cultivate fruiting branch groups.