Breeding A New Lettuce

Lettuce:  Red Grenoble x Salinas F2

The OSP made a small number of seeds of this interesting second generation lettuce cross from USDA-Salinas to a number of organic farmers several years ago.  Last year we multiplied the lettuce seed on our farm (a simple job) and this year we're selecting for vigor, taste, texture and bolt resistance.  You can see some of the diversity in the plants .  Apparently, the seed was been sitting around in Salinas, California for a while without generating much interest since this is clearly not a lettuce for distance shipping. It is; however, resistant to leaf miners and combines the flavor of the old French heirloom with the texture and heat tolerance of Salinas.  And for that reason, the folks at Salinas thought that it might be of interest to the organic farming community and passed it on to OSP and through the partnership, to us.

 These are the four primary phenotypes that appeared in the F3 of the Salinas cross.  Fine flavored and with a definite bolt resistant population, we are enjoying this loose-head lettuce which has enough of a heavy texture to please my finnicky niece.

We always grow a few types of lettuce. I've grown different varieties side by side in hopes that some would just cross but they never do.  So how is it that the professional breeders develop new lettuce varieties.

Lettuce is in a group of plants called composites, like daisies, sunflowers, zinnias, dandelions and scores of other plants. They produce a flower which technically is actually better called a "head" since it consists of many flowers or florets. Some of those florets can be ray flowers, they are arranged in a concentric way around the outside of the head as a single circle of petals each attached to the ovary or fruit which ripens to contain a single seed. Ray flowers may extend inward from their outer ring to a series of rings which gives the flower head a distinctive doubling effect (double marigolds, for example). Inside there may be disc flowers, often dozens crammed together making up the central part of the head. These little florets don't have the conventional showy petal attached like the ray flowers do but they are attached, each to an ovary which can also mature into a single seeded fruit called an achene. Unless you eat sunflower seeds you probably won't notice that seed of a composite like lettuce is actually contained in a shell-like fruit.

Lettuce flower heads are more like a dandelion than a sunflower and consist of a multitude of densely packed ray florets which open at the same time. As the florets open, the central stigma pushes through a crown of stamens and automatically is pollinated. Therefore, lettuce tends to be self pollinating. In fact, hand crossing lettuces is as tedious as hand crossing peas. There are some very effective insect pollinators that can cross lettuces, I am told. Therefore, you will want to separate varieties by a few meters if you are growing seed crops and want to maintain the purity of a variety.


How does one hybridize two lettuces to combine the characteristics and have a new pool of traits to select from?

That was a question that I asked George Moriarty of Cornell who has worked with lettuces.  He explained that what he does is find an unopened flower bud on the lettuce that will be the female parent.  This bud has to be within a day of opening.  Remember that the bud opens into a head of many small flowers and that the stigma of each tiny flower pushes through the stamens above which provide the pollen.  Cut off the top half of the flower bud (which removes the male stamens). Then find a fresh, open flower from your selected male parent.  Place a drop of water on the remaining exposed base of the female flower head and then swab the male into the water to release the pollen into the film of water.  Tag the blossom.  This special blossom with all your hand labor is the one that you will watch ripen and produce seed.  Each of the dozen or so florets on the head will produce a seed. 

The seed is now F1 hybrid.  Actually, there is a good possibility that some stray pollen contamination was introduced and so not every seed in that head will be a hybrid.  George says though, that the hybrid vigor of the F1's will be apparent when the seed germinates and that you should select those to grow out.