How Are Seedless Watermelons Cultivated? A Detailed Explanation of the Whole Process
Seedless watermelons are grown from seeds, but these seeds are not the seeds found inside a seedless watermelon. They are the seeds inside a triploid watermelon, which forms after crossing a natural diploid watermelon with a tetraploid produced through mutagenesis. The cultivation techniques share some similarities with ordinary watermelons, but there are also differences. Let's take a look at the cultivation process of seedless watermelons together!
The Cultivation Principle of Seedless Watermelons
Seedless watermelons are obtained by the method of artificially inducing polyploids. For example, treating the seeds or seedlings of diploid watermelons with colchicine (a plant alkaloid) blocks the formation of spindle fibers and the primary cell wall at the metaphase of cell division, so that the already replicated chromosome sets cannot separate toward the two poles and a secondary cell wall forms in the middle. As a result, cells with doubled chromosome sets are formed, producing tetraploid watermelon plants. These are then crossed with diploid watermelon plants (used as the male parent) to obtain triploid seeds. In triploid plants, because of disordered pairing of homologous chromosomes during meiosis, normal reproductive cells cannot be formed. The mature pollen of ordinary diploid watermelons is then used to stimulate the ovary of triploid plant flowers, resulting in triploid fruit, because their ovules cannot develop into seeds.
Seed Cultivation of Seedless Watermelons
1. Artificial induction: Seedless watermelons use the method of artificially inducing polyploids. Soak the seeds of diploid ordinary watermelons in a 0.2-0.4% colchicine (C22H26O6N) solution for 12-24 hours, or drip a 0.2-0.4% colchicine solution onto the stem-tip growing points of the seedlings at 6-7 p.m. every day for four consecutive days.
2. Careful management: After treatment, wash the seeds or seedlings clean with water. During treatment and the recovery period, place the plants under diffused light to avoid direct sunlight causing the colchicine to decompose and break down. At the same time, high temperatures must also be strictly avoided, because under high-temperature conditions the toxicity of colchicine to plants increases, easily causing seedling death. Before they survive, good cultivation conditions and careful management should be provided.
3. Bagging and pollination: Use tetraploid watermelons as the female parent and pollinate them with diploid pollen grains to produce triploid watermelon seeds. For small-scale seed production, bag the flowers on time every afternoon according to the flowering habits of watermelons, and carry out artificial pollination the next morning while attaching markers. For large-scale seed production, isolation by plots is needed, and tetraploid female parent plants and diploid male parent plants are grown in a certain ratio.
Seed Treatment of Seedless Watermelons
1. Seed selection and soaking: Select seeds according to the variety's traits based on seed shape, color, size and plumpness, pick out tetraploid and diploid watermelon seeds, and remove deformed seeds as well as hybrid and inferior seeds. Then, a few days before soaking, sun-dry the seeds for 1-2 days. Next, put the triploid seeds into warm water at 55°C and stir, then let the water cool naturally and soak the seeds for 6-8 hours.
2. Artificial shell cracking: The methods for cracking the shells of seedless watermelon seeds can be divided into dry-seed cracking and wet-seed cracking, and wet-seed cracking is currently the main method. Dry-seed cracking is more efficient, but it easily damages the seed embryo, and the soaking time and soaking temperature are hard to control, affecting the improvement of the germination rate. Wet-seed cracking is slippery and less efficient, but the length of soaking time has little effect on the germination rate, it is easy to control, and the germination rate is relatively high and stable.
3. Seed germination: Place the seeds that have been soaked and shell-cracked under constant temperature conditions of 33-35°C for germination, and a certain humidity needs to be maintained. After 24 hours, more than 90% of those with germination ability can germinate. Pick out those with sprouts 0.5 cm long and sow them, while the rest continue to germinate. After 36 hours, basically all normal seeds have germinated. Seeds that still have not germinated after 48 hours have lost their viability and no longer need to be germinated.
4. Grafting and seedling raising: The low survival rate and slow healing speed of grafted seedless watermelon seedlings are common problems in current production, restricting the development of seedless watermelon production and causing relatively large economic losses to melon farmers. To improve the survival rate of grafted seedless watermelon seedlings and cultivate robust grafted seedlings, grafting and seedling raising must be carried out according to the structural characteristics and growth features of seedless watermelons.
Field Management of Seedless Watermelons
1. Land preparation and cultivation: Apply 3-5 cubic meters of high-quality farmyard manure, 20-30 kg of urea, 20-30 kg of diammonium phosphate and 20 kg of potassium sulfate per mu, prepare the land finely, and make beds at a row spacing of 1.7-2 m. Use high-ridge cultivation, with ridges 60 cm wide and about 15 cm high, covered with 80-90 cm wide plastic film. Choose a windless sunny day for transplanting, plant 500-700 plants per mu, and water as you plant. Adopt three-vine pruning, and it is best to keep the 3rd female flower on the main vine for fruit set, avoiding fruit set at low nodes.
2. Fertilizer and water management: During the seedling stage of seedless watermelons (around the rosette stage), apply 5 kg of urea per mu to promote seedling growth. During the flowering and fruit-setting stage, generally do not water or fertilize. When the watermelon is about the size of a duck egg, the fruit has basically set steadily; apply a fruit-swelling fertilizer once, with 15-20 kg of urea and 15 kg of potassium sulfate per mu. Spraying 0.2% monopotassium phosphate on the leaves in the later growth stage can increase yield and quality.
3. Artificial pollination: Seedless watermelons use artificial assisted pollination, which is highly targeted, sets fruit early and neatly, and ripens relatively uniformly. At around 8 a.m., use male flowers of the pollinating variety that open that day to pollinate the female flowers of seedless watermelons. The earlier the pollination after flowering, the better, and no later than 10 o'clock. One male flower can pollinate 3-4 female flowers. Pollination should be uniform and the movements gentle, avoiding damage to the stigma.
4. Reasonable ripening acceleration: When seedless watermelons need to be marketed early, ethephon can be used for ripening treatment. When the fruit has reached full size but is not yet ripe, apply a 200 mg ethephon solution per kg of watermelon once along the surface of the melon around evening. A small amount of neutral soap powder can be added to the solution to increase adhesion. Two days later, they can be harvested and marketed, reaching the market 3-5 days earlier than those ripened naturally.