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» High seed germination percentages and uniform germination rates result in uniform plant supports that help maximize vegetable production.
» Germination and emergence are affected by seed quality and environmental conditions.
» Managing the temperature, humidity and texture of the bed helps to promote good germination and emergence.
THE GERMINATION PROCESS
Plant seeds are composed of an embryo, some form of food storage, and a seed coat. The embryo is the part of the seed that will become the root, stem, and sprout of the new plant.1 Seeds remain dormant until internal and external conditions allow the seed to break dormancy and begin the germination process. External conditions that influence dormancy and germination include temperature, humidity, air, and light (for some species).
The germination process is divided into three stages. The first phase is imbibition, when water is absorbed through the seed coat, causing the seed to expand and the seed coat to crack.1,2Phase two is the intermediate or delay phase. This phase implies the hydration of the cotyledons and the activation of the internal mechanisms of physiology and metabolism of the embryo. The seed begins to use its stored food in this phase.2,3 The third phase is the growth phase, when the radicle emerges from the seed and begins to develop into the root (Figure 1).1,2As the root develops, it anchors the plant and begins to absorb water and nutrients from the surrounding substrate (soil or potting mix). The hypocotyl begins to expand, taking the seed from germination to emergence.
Figure 1. Structures that develop during seed germination.
Once the cotyledons emerge from the soil, the plant enters the molting stage. Vegetables are classified as either monocotyledonous (monocotyledonous) or dicotyledonous (dicotyledonous) based on the number of cotyledons (seed leaves) they produce. Dicots produce two cotyledons and include crops such as tomatoes, peppers, brassicas, cucurbits, beans, beets, and lettuce. Monocots produce one cotyledon and include crops such as sweet corn, onions, and asparagus (Figure 2).1Some plants, such as beans, use their cotyledons to store food, which is used during germination and emergence until true leaves form. Other plants, such as broccoli or corn, use their cotyledons to produce food through photosynthesis.1 Once the seedlings emerge, true leaves begin to form and take over the food-producing function.
Figure 2. Seedlings (A) dicotyledonous and (B) monocotyledonous.
QUALITY OF SEEDS
Seed quality ratings are based on the percentage of seeds that germinate and the germination rate, usually defined as the number of days from sowing to 50% of seeds germinating. Both percentage and germination rate are affected by temperature and humidity conditions, so seed quality tests are usually carried out under ideal conditions for germination.2,4The evaluation of germination under field conditions is also useful, as some seeds that perform well in seed quality tests may not germinate and emerge well under more adverse field conditions. The term seed vigor is used to describe seed response to non-ideal conditions. Rapid, uniform germination and stand establishment under field conditions are indications of good seed vigor.2
Temperature is one of the most important environmental factors that affect the percentage and rate of germination. Seeds germinate faster and more evenly in their optimal temperature range.1For example, at 86°F, pepper seeds germinate in 8 days, while at 58°F, it takes 13 days for seeds to germinate.3The amount of heat required for 50% germination is expressed as the sum of heat (similar to degree days). Crops such as onions, leeks, and celery have relatively high sum heat requirements and germinate slower than other crops.4
Adequate moisture is necessary for seed germination, as the absorption of water by the seed initiates the germination process. Good seed-soil contact is necessary to allow the seeds to absorb water and remain hydrated.1 Soil moisture contents of 50 to 75 percent of field capacity are generally best for promoting seed germination. At moisture levels above 75%, air spaces in the soil fill with water and reduced oxygen availability can slow root growth and lead to root rot.2 Good gas exchange in the soil to remove carbon dioxide carbon and replenish the supply. of oxygen.3 Humidity levels below 50% can decrease the germination rate. Root seed coat emergence is the phase of the germination process most sensitive to moisture levels.4
While most seeds can germinate in the dark, some need light to encourage germination. Lettuce and celery are examples of crops that need light to germinate. On the other hand, tomato seeds germinate best in the dark, and light can inhibit germination in some tomato varieties.2,5Once seedlings have emerged, they require adequate light levels to grow properly.
GERMINATION AND EMERGENCY MANAGEMENT
Germination test: If the seeds are stored for more than one year, they may show less vigor and a lower germination rate. To assess germination percentage, wrap at least 25 seeds in a damp paper towel and keep damp but not soaking wet at room temperature. Check the germination percentage after five to ten days. Consider buying new seeds if the germination percentage is below 85-90%.1
It is desirable for all seeds in a crop to germinate at approximately the same time for uniform support. Uniform growth after germination is also desirable. Whenever possible, plant in a well-prepared bed with even soil moisture and when soil temperatures favor rapid germination to maximize support uniformity. In greenhouse plantings, use germination heating mats to provide even temperatures. For direct seeding in the field, a fine-textured seedbed, allowing good contact between seed and soil, and an even planting depth will help maximize stand uniformity.1,2Seed conditioning can help increase the germination rate and improve stand uniformity.
Adjust the sowing rate according to the seed germination rate percentage and the soil temperature at the time of sowing. Too dense stands can result in spindly seedlings, while sparse stands are a waste of space and can lead to uneven crop development.5The ideal planting depth varies according to the crop. In general, small seed crops (lettuce, brassicas) germinate and emerge better at a shallower planting depth, while larger seed crops (beans, cucurbits) germinate and emerge better when a shallower planting depth is used (table 1 ).5
The ideal temperatures for seedling growth can be different (often lower) than the ideal temperatures for seed germination. Seedling growth is generally slower at lower temperatures and faster at warmer temperatures within the temperature range for acceptable growth. In greenhouse systems, uniform 24-hour temperatures generally favor germination and emergence, but vegetative growth of seedlings may be better when nighttime temperatures are lower than daytime temperatures.1
Deep watering helps promote good root development and root growth deeper into the soil profile. Shallow watering can result in the development of a shallower root system and lead to increased plant stress later in the season. Excessive watering can reduce oxygen levels in the root zone and promote root rot. In greenhouse and seedling production systems, different amounts and schedules of watering may be required for seeds and seedlings. In no-till, sprinkler irrigation may be required initially to promote uniform emergence and prevent soil crusting. Growers can then switch to drip irrigation after emergence.3,5
1 Stivers, L. 2017. Understanding seed and seedling biology. Extension of the state of Pennsylvania. https://extension.psu.edu/understanding-seeds-and-seedling-biology.
2 Heuvelink, E. 2018. Tomatoes, 2nd edition. CABI
3 Stivers, L. and DuPont, T. 2012. Biology of seeds and seedlings. Extension of the state of Pennsylvania. https://extension.psu.edu/seed-and-seedling-biology.
4 Brewster, J. 2008. Onions and Other Garlic Vegetables. CABI.
5 Maynard, D. e Hochmuth, G. 1997. Knott's Handbook for Vegetable Growers, Fourth Edition. John Wiley & Filhos. Nova York.
Sites verified on 6/1/2022.
For additional agronomic information, contact your local seed representative.
Yield may vary from location to location and from year to year as local cultivation, soil and weather conditions can vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower's fields. The recommendations in this article are based on information obtained from the cited sources and should be used as a quick reference for vegetable production information. The content of this article should not replace the professional opinion of a farmer, farmer, agronomist, pathologist and similar professionals who deal with growing vegetables.
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4050_SE_S1 Published 01/20/2022
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