Lettuce seed germination research!

Here is a piece of information concerning lettuce seed germination I came across some time ago.

In related research, plant physiologists wondered why people have trouble with seed germination of lettuce in their vegetable gardens. They knew that seed germination was very high in Petri dishes in the laboratory.

It turns out that in lettuce seeds, unlike barley, a critical step in triggering seed germination is photo-activation. The seeds need to be exposed to light in order to germinate. I hope you are wondering, immediately, how germination responds to photon flux and wavelength of light!

Lettuce responds well to very low photon fluxes...it is not a photosynthetic process!

The wavelength of light is critical. The seeds germinate well in white light, but also to single "colors"...particularly red light (660 nm). On the other hand, far-red light (730 nm) strongly reduces lettuce germination.

It took a long time to identify, isolate, and characterize the photoreceptor. It is called phytochrome. Phytochrome exists in two different chemical forms: Pr and Pfr. Phytochrome in its Pr form absorbs light maximally in red wavelengths...hence Pr. Phytochrome in its Pfr form absorbs light maximally in far-red wavelengths...hence Pfr. The name of the form of phytochrome is determined by the color of light it absorbs maximally. What made characterizing phytochrome difficult was the fact that the two forms interconvert. As Pfr absorbs far-red light, it changes chemically into Pr! Similarly, Pr absorbs red light and changes chemically into Pfr.

If you think about how you might analyze a pigment (as you have done several times in lab!), you generally put an extract into a spectrophotometer and measure absorbance of a wavelength at which the pigment maximally absorbs light. With phytochrome, this is almost impossible...the light you would use to measure it, causes it to change to the other form! It is elusive!

Back to the lettuce, obviously the garden problem is planting the seeds too deep (in the dark!). Without light to photo-activate seed germination, the seeds fail to germinate to their potential. In the dark, the ratio of Pfr to Pr determines whether each seed will germinate. If exposed to red light, the phytochrome is all converted into Pfr and the seeds germinate. When the seeds are exposed to far-red light, the phytochrome is all converted into Pr and the seeds fail to germinate. Obviously the active form of phytochrome is Pfr.

Seed Storage and Germination Testing

First and foremost, you will need to protect the seed from any extremes. No high heat or high moisture allowed please. I like to store my seed in airtight plastic containers in the bottom of the fridge. This places them out of the way and insures they will stay nice and cool for the months that have to go by from one spring to the next.

Each little seed is actually a very tiny, but complete plant that is enclosed in a seed coat. The coat is tough and hard and protects it until conditions become right for growth. This usually happens in the early spring when the soil gets above 50F and moisture is then allowed to soak into the seed itself. When this happens an enzyme is activated that helps bring the tiny plant out of hibernation and back into an active growing state. But, how can you tell if the seeds you have left over are still viable? No one wants to spend a lot of time and effort getting seed planted and watered only to find out weeks later that they were not viable!

The best technique I have found requires just a few common items from around the house; a plastic baggie, a paper towel and some water. Just use enough water to get the towel damp but not soggy. Place a few ‘test’ seeds onto the paper towel and then fold it over so the seeds are covered on both sides. I usually place ten test seeds on the towel. Next place the seed into a baggie and seal it. You can place your ‘germination chamber’ anywhere where the temps are warm but also where it will be out of direct sunlight. Then, just wait a few days, check every 24 hours to see if germination is occurring. Seeds like spinach, lettuce and radish will germinate very rapidly, say in a few days at most. Other seeds like tomatoes or pepper will take a week or longer. But, if they are viable you should see most if not all sprouting at some point. The number of seeds that germinate is important to consider. If you get less than 50% then you should think about buying some new seed packets. If it’s over 50% then you should be able to get decent crops from the balance of the packet. Now, following are some suggested storage times for common garden vegetables. It’s important to remember that some seed, for whatever reason, will not store well;

1-2 years – Corn, Onion, Parsley, Parsnips and Peppers

3-4 years – Asparagus, Beans, Peas, Beets, Cabbage, Carrots, Squash and Tomatoes

5-6 years – Cucumbers, Lettuce, Melons and Spinach

So, when you think you’re ready, get those packets out and start testing!