Light and Seed Germination

Light-germinationPost by Catriona McLeod, TAS.

As the days start to become lighter earlier and for longer, and warmer, the time to plant seeds is nigh. Light is critical to seed germination and plant growth. Plants and seeds can sense light quality (that is, wavelength), light direction, periodicity and intensity.

Radical tipsAs I noted in my last post (‘Cold soil and seed germination’) the first part of the seedling to emerge from the seed coat is the root (or ‘radical tip’). The shoot then expands and emerges from the seed. If the process of imbibition (water absorption by the seed) and germination occurs in darkness, root growth slows after the shoot emerges and shoot elongation accelerates. Once a seedling emerges into the sunlight, the plant starts to produce leaves and chlorophyll. This increases the chance of the seedling emerging from the soil into the light to undergo photosynthesis. This light-dependent developmental transformation is called photomorphogenesis (literally ‘light grown’), and it is induced by red, far-red and blue light.

Emerging cucumber seedlingIn addition to photomorphogenesis, light induces phototropism (plant growth), chloroplast differentiation (chloroplasts are specialised subunits in plant and algal cells whose behaviour and role is behaviour is strongly influenced by environmental factors like light color and intensity) and various other responses such as flowering and germination.

Plants exhibit different growth habits in the dark and light. In the dark they have elongated stems, undifferentiated chloroplasts and unexpanded leaves (skotomorphogenesis). In the absence of light, plants become etiolated, that is, they become elongated, which may assist them in pushing upwards through the soil. Characteristics of etiolation include no leaf growth, rapid stem elongation and limited radial expansion of stem, limited root elongation, limited production of lateral roots and no chlorophyll production.

In contrast, photomorphogenesis involves the inhibition of stem elongation: once the seedling reaches the soil surface, light induces a rapid switch to photomorphogenic development, in which etiolation is inhibited and the greening process enables plants to capture light through photosynthesis. This rapid switch from dark to light development is called de-etiolation and is essential for seedling survival after germination in the dark.

The effects of light vary between genera and even between species and varieties. Some seeds are stimulated by light, and others are inhibited by light during germination. Knowing the light requirements of a seed to germinate determines seed depth at planting (again, I noted in a previous post that as a rough guide a seed can be planted at a depth of about twice its diameter) and the types of seed treatments a seed may need before planting.

Most seeds need only to be planted in or placed on warm, moist soil to encourage growth; however, some seeds may need assistance to allow maximum imbibition to occur.

Some seeds are clad in a protective coating that is hard and water-resistant. This is an evolutionary strategy to protect the internal seed (but not the soft pith) from total digestion by animals. Grit and even animals’ acidic stomach contents may slightly abrade the surface. This can be replicated with one’s own seeds by gently using medium-grade sandpaper, or a nail file. Alternatively, with smaller seeds, even gentler scarification can be achieved by placing them in a jar with coarse sand. Shake the jar until the seed coats have lost their sheen. Some growers advocate lightly nicking the hard coat of larger seeds with a sharp knife.

A second method of germinating seeds is by stratification – the process of pre-treating seeds to simulate natural winter conditions that a seed must endure before germination. Soak the seeds in water for 24 hours, remove any floating seeds (as they may not be viable) and drain the remaining ones. Fold large seeds into a piece of damp, long-fibred sphagnum peat moss (or a similar medium which holds moisture without being dripping wet) and mix small ones with barely-moistened vermiculite; place them in a plastic bag. Seal and label the bag. Refrigerate the seeds at about 5° C (the optimum temperature for home refrigerators is between 0o C and 5o C). Sow the small seeds with the vermiculite, but remove the larger seeds from the peat moss (or similar) before sowing.

‘Chitting’ is another method of ‘pre-sprouting’ seeds. The term is often used to describe the process of exposing seed potatoes to light, which encourages them to shoot before being planted into the ground. To ‘chit’ seeds, place them in a paper towel, place the paper towel into a plastic bag and spray the paper towel until it is wet. Place the bag in a brightly lit, shady location in the house and check it daily for root growth progress. As soon as the tiny radical tips begin to emerge from the seeds, remove them gently from the paper towel (if possible, and if not, plant them with the towel as it will decompose in the soil) and sow as normal in the garden. Spinach and peas seeds benefit from chitting as they take much longer to germinate in cold weather, but will flourish in cool soil once they have sprouted.

Another successful method of stimulating germination is by soaking. Presoaking softens the coats of seeds that are slow to sprout and primes them for germination. Soak the seeds in a jar filled with tepid water for at least four hours and up to one day. Skim off any floating seeds. Pour the remaining seeds into a mesh strainer and rinse well with fresh, cool water, then sow immediately. This works well for edible peas, sweet peas, parsley, beets and chard.

Emerging fern frondThese are only a few of the many methods for inducing germination in seeds. Another important factor is seed viability. For example, corn, parsnip and onion seeds have an average viability of two years; cucumber and endive seeds are good for ten years at least. Getting seed germination to occur sounds complex, but in fact with experience, it becomes second nature.

Dr. Catriona McLeod is a passionate organic gardener, writer, editor, environmental consultant and designer based in Tasmania. A former academic, she has a background in architecture and landscape architecture. Her focus is now design sustainability, horticulture and the environment. She is a passionate advocate of wild places and the preservation of threatened environments and species, and is an avid and always-learning gardener, who tries to unearth the ‘hows’ and ‘whys’ of plants and planting.

Leave a Reply

Your email address will not be published. Required fields are marked *