Effects of Light on Seed Germination and Plant Growth
Germination occurs when a seed comes out of dormancy and grows its primary root and shoot. Light isn’t strictly necessary for germination in most plant species. However, some seeds germinate best in absolute darkness, and others perform well with continuous sunlight. Light does become vital for every species after germination, because the initial sprout will not survive if it cannot reach a light source.
Basic Requirements for Seed Germination
All plants need sunlight to grow and survive, but seeds do not necessarily need light to come out of dormancy. This makes sense when you think about how many seeds you’ve buried under an inch of topsoil that have successfully germinated. According to the Iowa Agriculture Literacy Foundation, the three necessities for seed germination in all plant species are water, oxygen and a warm temperature.
Each of these must be present in the correct measurements, or germination may not occur. For example, many seeds will not germinate in flood waters because there isn’t enough oxygen present. Moist, well-oxygenated soil that’s either too cold or too hot also won’t lead to germination.
However, there’s plenty of variation between plant species. Each has its own critical soil moisture level and optimal soil temperature for germination. What one species considers “too cold” or “not moist enough” could trigger germination in another species. It’s important to understand the unique requirements of the seeds you want to grow in order to ensure successful germination.
What Happens During Germination
Germination starts when water softens the protective seed coat in a process called imbibition. Once water has penetrated the seed coat, it reaches the endosperm, a starch reserve that makes up the bulk of the seed. This starch converts to sugar as it interacts with water, and sugar is the primary energy source needed to launch the development of the embryo. A root will begin to grow down and anchor the seed into the soil, while a shoot will begin to grow up.
The shoot will continue to grow upward until it is exposed to light. At that point, the first set of leaves will unfurl, and photosynthesis – sugar production triggered by sunlight – can begin. Until it is exposed to light, the shoot relies on the energy reserves in the endosperm. If the shoot uses all of this energy before it reaches an adequate light source, it dies.
Effect of Light on Seed Germination
According to the Pennsylvania State University Extension, most seeds germinate best in darkness. However, begonia (Begonia spp.), primrose (Primula vulgaris) and coleus (Plectranthus scutellarioides) definitely need light to germinate. Thompson & Morgan also names petunias (Petunia spp.), blanketflower (Gaillardia aristata), gerbera daisy (Gerbera jamesonii), ficus tree (Ficus benjamina) and others as requiring or preferring light for germination. In contrast, seed germination experiments in light and dark conditions show that members of the Allium family, blue tansy (Phacelia tanacetifolia), pot marigold (Calendula officinalis) and delphinium (Delphinium elatum) are among those species that require total darkness.
Thompson & Morgan also point out that many seeds will germinate in both dark and light conditions. The number of fussy seeds are few and far between. In fact, most commercial seeds have been selected over time for easy germination, among other factors.
The size of a seed and its dispersal method both offer some clues as to its light preferences. Large seeds have a much larger endosperm, which means a young shoot has plenty of energy reserves to push through inches of soil and mulch before it can start photosynthesis. In contrast, small seeds have a small endosperm and need to immediately reach a light source for survival. In addition, seeds typically buried by animals may have adapted to prefer darkness when germinating.
Effect of Low Temperature on Seed Germination
If you’re struggling to germinate seeds from a plant species that doesn’t seem to have a light requirement, low soil temperature could be the culprit instead. Soil temperatures outside a species’ optimal range can severely slow germination or prevent it altogether. For example, Pennsylvania State University Extension notes that peppers (Capsicum annuum) germinate in just eight days when soil temperatures remain at 86 degrees Fahrenheit, but germinate in 13 days when soil temperature drops to 58 degrees.
Some species can tolerate a relatively wide range of temperatures for seed germination, but you’ll get faster results by planting seeds during optimal temperatures. For example, onion (Allium cepa) has been recorded as having germinated in soil temperatures as low as 35 degrees Fahrenheit and as high as 95 degrees, but germination occurs the fastest when onions are placed in soil that’s 75 degrees. In contrast, cucumber (Cucumis sativus) seeds prefer being planted in soil around 95 degrees Fahrenheit and can even germinate in conditions up to 105 degrees.
Therefore, it’s important to know whether your seeds are best sown outdoors in the spring or summer. Keep in mind that air temperature and soil temperature are different. In fact, according to Corn and Soybean Digest, soil tends to be cooler than the air in the spring and warmer in summer due to more sun exposure and heat absorption. When starting seeds indoors, consider using a heating mat to better control soil temperature.
Other Seed Germination Tips
Some seeds require unusual conditions or extra-special attention in order to germinate. For example, some species like the lodgepole pine (Pinus contorta) have seeds that must be exposed to fire to germinate. This incredible adaptation gives them a competitive edge, as they are among the first species to repopulate land cleared by wildfires. According to Encyclopedia Britannica, fire melts a resin on the seeds so that water can penetrate the seed coat, or chemicals present in the smoke trigger germination.
According to Diane’s Flower Seeds, some species have seed coats that are very tough and require a lot of moisture to soften up. When you’re in a hurry to germinate seeds, you can either soak the seeds overnight, scuff the seed coats on sandpaper, or snip the sides with clean nail clippers to help water reach the endosperm.
Other seeds won’t germinate until they’ve experienced a particular sequence of temperatures. For example, columbine (Aquilegia spp.) germinates best after it experiences the cold, moist conditions of winter. The ‘Purple Sensation’ onion needs to alternate between cool and warm temperatures before it will germinate. Because it can be tricky to simulate all the nuances of nature, sometimes it’s best to sow these seeds outdoors and see what happens.
Effect of Light on Continued Plant Growth
Once a seed has germinated, all plants require light to grow. The exact amount of light depends on the plant. Some species have adapted to live in shady environments, whereas others need maximum sun exposure to thrive. Knowing what your plants need can help you keep them vigorous and beautiful.
Furthermore, plants need adequate exposure to the correct type of light. Sunlight emits all the UV wavelengths that plants need throughout their life cycle, including red and blue light. Where gardeners may run into trouble is with grow lights that don’t give off sufficient quantities of red or blue light.
According to Greenhouse Product News, red light stimulates plant growth. In fact, when plants are grown only under red light, they tend to become more “leggy” than plants grown with both red and blue light. Michigan State University Extension points out that blue light actively suppresses plant overgrowth and also regulates the opening and closing of stomata, which is crucial in transporting and conserving water within the plant. Blue light may also regulate flowering.
- Michigan State University Extension: Effects of Blue Light on Plants
- Greenhouse Product News: Red Light and Plant Growth
- Diane’s Flower Seeds: Germinating Difficult Seeds
- Encyclopedia Britannica: Playing with Wildfire: 5 Amazing Adaptations of Pyrophytic Plants
- Corn and Soybean Digest: 7 facts you didn’t know about soil temperature
- Thompson & Morgan: The Effect of Light on Germination and Seedlings
- Pennsylvania State University Extension: Seed and Seedling Biology
- Iowa Agriculture Literacy Foundation: Science 101: Germination
Cathy Habas has been a full-time freelance writer since 2014. She graduated from Indiana University Southeast with a Bachelor’s degree in English. She enjoys writing content that helps homeowners feel motivated and confident in handling projects around the home. Her work has been published around the web, including on home improvement sites like Ron and Lisa.
Effects of Light on Seed Germination and Plant Growth. Plant hormones that respond to light govern the growth of plants from germination through the growing season. Some plants require full sun; others grow in the shade. Their light needs determine where you should plant them outside and whether or not you can grow …
The Effect of Light on Germination and Seedlings
Do seeds need light to germinate? And how does light affect the germination of seeds? T&M’s former Technical Manager, David Batty, investigates these questions and discusses the question of whether seedlings need light. Some plants germinate well in darkness, some prefer continuous light, and others have no preference either way.
Apparently it was custom in Ancient Egypt, before finally sealing the tomb, to leave a little pile of moistened corn near the sarcophagus. One can imagine the seed germinating in the pitch darkness, stretching itself upward feeling for light which was not there and finally toppling over having exhausted its food reserves.
It is a fact of life that most plants need light to grow and keep them healthy, but not all plants need light to germinate, and, as we shall see, some seeds find light a hindrance. If we look at the matter from the gardener’s point of view, however, we can use the rule of thumb that most cultivated plants on sale in seed form prefer to germinate in the dark. There are some notable exceptions however, some greenhouse perennials, epiphytes, many grasses, and even tobacco all prefer light and a large number of seeds are not fussy either way.
The reason is that commercially produced seed is bred and selected for its ease of germination as well as other more obvious characteristics and so peculiarities such as light or dark requirements do not often occur. On the other hand seed which is obtained non-commercially, in small quantities from the home gardener, seed lists, or the more unusual items from seed merchants may prove to be much more fussy in its requirements. In fact, research has shown that with seeds other than cultivated forms there is a great deal of variation. We can divide seeds of this type into those which germinate only in the dark, those which germinate only in continuous light, those which germinate after being given only a brief amount of light and those which germinate just as happily in light or darkness.
As long ago as 1926 experiments were carried out by Kinzel to find out the light requirements of hundreds of plant species. He found about 270 species which germinated at or above 20°C (60°F) in light, and 114 species germinated at the same temperature in the dark. He also found 190 species which germinate in light after experiencing hard frosts and 81 species likewise germinated in the dark. Fifty-two species germinated in the light and 32 species in the dark after light frosting and finally there were 33 species which were unaffected by light or dark.
Unfortunately, as with all gardening matters, things are not quite this simple. Other factors, it seems, can also affect the seed’s light requirements, for example, with some species (e.g. Salvia pratensisand Saxifraga caespitosa) light requirement only exists immediately after harvesting whereas with Salvia verticillata and Apium graveolens (Celery) this lasts for a year and to confuse matters further other species develop a light requirement while in storage. Chemicals also, such as nitrates in the soil, can substitute for light in stimulating seeds to germinate so that some light requiring seeds will still germinate if covered with fertile soil. Still it all makes for interesting gardening doesn’t it?
For a fairly comprehensive list of the Light/Dark requirements of seeds we refer you to Thompson & Morgan’s booklet ‘The Seed Sowing Guide’, which they will be pleased to send you for only 99p if you drop them a line. This is a helpful general guide but it is worth remembering that not all seeds in the same genus behave in the same way. For example Primula ohconia needs light and Primula spectablis needs darkness for germination, so there is still a lot to learn, much of which can only be gained by personal experience and sharing that information gained with others.
The explanation of how light affects some seeds and causes them to be in a state of readiness for germination and yet prevents other seeds if necessary from germinating is highly complex. Suffice it to say that it is mainly the light’s effect upon a plant pigment called phytochrome within the seed. This relates to the type of light which the seed receives. As a generalisation, light in the red wave length usually promotes germination whereas blue light inhibits it.
In a practical vein the light requirements of a seed may relate to the habitat in which the seed parent usually grows, so as to ensure that those which fall in an area conducive to growth will germinate and those which fall in less salubrious circumstances bide their time. For example a seed requiring light to germinate might fall into the deep shade of another plant where growing conditions would be very poor, whereas a seed falling into an open, well lit space would germinate quickly and flourish. On the other hand, it may be essential for the establishment of the young seedling that part or all the seed needs to be covered with soil or in the shade, perhaps, to protect the young root.
In such a case with a seed which required darkness, uncovered seed, which is exposed to light will not germinate. Sometimes only part of the seed is light sensitive. Phacelia is light sensitive at only two points on its surface and in a lettuce at only one. The micropyle where the water is absorbed, is light sensitive perhaps to ensure that only correctly oriented seed with the best chance of survival germinates.
Of course, the effect of light on seeds should not be over emphasised, no real hard and fast rules can be laid down, as other factors interact with light. To the gardener, the two questions he needs to have answered are ‘How deep should I sow my seed?’ and ‘Should I cover the seed tray to exclude light or not?’
In answer to the first question, depth of sowing depends a lot upon the size of the seed. Very tiny seed should normally be sown and left uncovered. Small seed which needs light will usually receive it even if you cover it with a light sprinkling of compost or vermiculite because light does travel a short distance through the soil and with some seeds exposure does not need to be long or continuous. For example tobacco seed receives all the light it needs to germinate, after it has taken up water, in 0.01 seconds of sunlight and even moonlight will do!
It is not just the very tiny seeds which sometimes need light to germinate, an average seed like Impatiens is light sensitive too and should be covered with a fine sprinkling of vermiculite after sowing and left in diffused light, placed in polythene to provide a high humidity until germination which usually takes 10-14 days at 21-42°C (70-75°F).
Medium sized seeds and upward, unless they have a light requirements (and we do not know of any really large seeds which do) should generally be sown just below the surface, enclosed in a polythene bag or cling film and placed in diffused light.
Some, but not all, popular seeds which prefer light for germination are:
- most Primula
Seeds which will only germinate in darkness should be sown at the correct depth and then covered in black plastic or similar to exclude all light until germination takes place. Cyclamen is a subject which should be treated in this way. Normally a difficult subject to germinate it proves far less so if sandwiched between moist filter paper and placed in a plastic container in total darkness. Usually germination occurs in about a month at 15-20degC (60-68degF) when the tiny corms can be transplanted into compost and grown on. The temperature, however, should be no higher than 20degC (68T) as high temperatures will induce a different form of dormancy!
Some other popular types which prefer darkness for germination are:
- Primula sinensis
Providing artificial light should not normally be necessary for seeds sown in greenhouses, well lit propagators etc. but if light is a problem or, more importantly, if you want to ensure rapid, healthy growth of your seedlings after germination then some form of additional light may be necessary. This would particularly be the case in raising seeds early in the season and quite a number of flower and vegetable seedlings respond to supplementary light. For example, tomatoes and cucumbers where vigour and earliness have been improved, also Antirrhinum, Stocks, Gerbera, Gloxinia and Gesnaria have all responded with a higher growth rate when given extra light in the winter months.
Tuberous begonias when sown in late winter must have supplementary lighting if they are to develop properly. They are sensitive to day length and when this is less than 12 hours they form tubers instead of making vegetative growth. In order, therefore to produce healthy young plants lighting must be given to extended the day lengths to more than 12 hours.
To provide this light, fluorescent tubes of the Gro-Lux type, to give light something akin to sunlight should be used, suspended around 2 feet (60cm) above the seedlings. As there will be so much moisture about use only approved horticultural fittings when installing the lights and fit a time clock if possible so that the lights can be on for 12 hours each day.
David Batty is a former Technical Manager at Thompson and Morgan Seeds, where he looked after the seed-testing laboratories.
Source of article
Growing From Seed – Spring 1989 Vol. 3 Number 2
Copyright: The Seed Raising Journal from Thompson & Morgan
Most plants need light to grow and keep them healthy, but not all plants need light to germinate, and, as we shall see, some seeds find light a hindrance.