[Market-farming] Intentionally crossing tomatoes
wshoemak at inil.com
Fri Feb 11 09:41:44 EST 2005
This is a very abbreviated version but it will get you started.
There's a reason why plant breeders call their techniques "controlled
crossing". You want to make sure that no foreign pollen interfere's with
your goal. I've done a little tomato breeding in our greenhouse. You
designate one variety your "mother" plant and the other variety is the
pollen donor, or father plant. The father plant will create lots of flowers
and any one of them that just opened can be used to contribute pollen. So as
long as the plants are of similar age, the father will be available to
contribute pollen through the life of the mother. Just like a man, right?
This is best done in the absence of breezes and with a bit of shade
(10-20%?), hence the greenhouse environment. The mother plant will similarly
produce many flowers but because you want to control the process, clip off
any pollinated flowers or fruit that are not the result of your crosses.
That ensures that the plant will support your crosses no matter how poorly
they were done, unless you just didn't get any viable pollen to the flower
at all. When making the cross, timing is everything. The flowers are viable
for one day generally. Make your crosses in the morning, generally after 10
am, as they will be physiologically very active and receptive to pollen if
they're at the right stage of development. You look for flowers that are
just opening. Carefully, using a pair of tweezers, grip the end of the
corolla (the yellow part of the tomato flower) on the edge and gently tug on
it. It should slip off. You may want to practice this a few times until
you're confident in it. Make sure you don't injure the pistil, which arises
from the embryonic tissue at the base and will remain attached to the
flower. Watch to see if pollen blows up, like dust being disturbed. This is
a sign that it may be too mature. You need to remove the corolla before the
pollen is ready or you will contaminate the pistil (the female flower
organ). Once you're confident you can do this without contaminating the
pistil with pollen from the same flower, you're ready.
The next opportunity, remove the corolla from a healthy looking flower in
the right stage. Slip a plastic sandwich bag over it. You don't need to
tightly seal it, just secure it as a barrier against insects and foreign
pollen. Examine the pistil (the long tube in the center of the flower with a
"platform" at the end). Is the end of the pistil moist, looking like it has
a wet, sticky substance on it? That means it's receptive, ready to receive
the pollen. Then you can take a fresh flower from the "father" plant. Choose
one that is almost fully open. Just like with the mother, you can slip off
the corolla. Open it up and look for pollen that is being released. Take the
inside of the corolla and shake it very near the end of the pistil of the
flower you just prepared on the mother plant. You may want to repeat this
3-4 times with other viable flowers from the father plant. The important
thing is to make sure the female flower receives enough viable pollen. Every
eventual seed needs a single, viable pollen grain. After pollinating, return
the bag to serve as a barrier to foreign pollen. Then use a small string
tag and a permanent ink pen to identify the cross ("Mother variety x Father
variety) and put a date on it. Place the tag over the cross on the peduncle
(fruit stem). Continue to remove other flowers and fruit from the mother
plant and make several more crosses in the same manner. Let the fruit fully
mature and the seed inside will be viable.
After harvesting the very ripe fruit, crush it into a container, remove the
solids as much as possible, and let it ferment for several days. A white
film will develop and it will begin to smell. Don't let it sit like this too
long. Using a fine wire strainer, wash the fermented juice and seeds with
clear running water until the seeds look clean. Set them out on a paper
towel to dry. By the way, though seeds may have come from different fruit,
if they were the same mother and father, they can be bulked together. Once
the seed are dry, put them in an envelope and place in the refrigerator
until it's time to plant them out.
It'll take about 10 generations to arrive at a stable new variety. During
the first few generations you won't be able to see many differences. the
differences will really start to emerge in the 4th and 5th generations.
Select those you like, discard the ones you don't. Expect 1 in 1000 or fewer
to be worth keeping. The real challenge isn't making the crosses, it's being
able to make good selections over time, and seriously, viciously discarding
It takes patience and isn't for everyone but it can result in something
wonderful. By the way, I've been focusing on crossing 'Black' tomato with
'Sweet 100' and with a compact, determinate large-fruited early line. Good
luck to both of us.
Bill Shoemaker, Sr Research Specialist - Food Crops
University of Illinois - St Charles Horticulture Research Center
>For the past number of years I have had unintended cross pollination of
>of my tomato varieties, most frequently occuring with pototo leafed
>varieties like "Brandywine" and some of the cherry tomatoes. I always
>thought that it only appeared that way because the effects of these
>crossings are more easily observed when one starts seeing potato leafed off
>types among his non potato leafed transplants or when something besides
>cherry tomatos start appearing on the cherry tomato plants. Then I learned
>last year that crosses do indeed occur more more frequently among potato
>leaf and cherry varieties because they are more primative and have
>differences in stamen or pistol lengths (I don't remember which) with those
>of more modern varieties which renders them more susceptable to crossing.
>Well anyway, now I am thinking of trying to intentionally cross two
>varieties that I like a lot, but both have highly contrasting positive and
>negative characteristics I would like to combine in hopes of ending up with
>a new variety that would minimize the weaknesses of both of them and
>the best of both of the parent varieties. These are the two varieties:
>* Blosser-- a large pink beefsteak slicing type. Strengths are it's
>relatively good yield of smooth well shaped fruits with a moderate sized
>stem core compared to other heirloom varieties and a superb low acid
>Weaknesses are it's susceptability to mosaic virus diseases and cracking,
>the bane of many hierlooms.
>* Eva's Purple Ball-- a smaller tennis ball sized pink salad tomato.
>Strengths are it's superior resistence to cracking, disease resistence, and
>a small stem core. The main weakness is the lack of size. My customers
>looking more for the larger slicing types.
>I would like to breed the cracking resistence and smaller stem core of the
>EPB into the larger beefsteak style slicing tomato represented by the
>What would be the better "no brainer" method for accomplishing this cross?
>I recall reading somewhere that the guy who bred the "Morgage Lifter"
>variety in the 1930's did so by surrounding a pollen receiver plant with up
>to a half dozen pollinator plants and saving seed from the best fruits off
>the pollen receiver. My question would be "which variety should be the
>pollinator and which should be the pollen receiver?" Should I consider
>doing some in both directions? Maybe I should consider using "Brandywine"
>instead of "Blosser" for my larger sized variety in this cross. Both
>Brandywine and Blosser yield a similar sized fruit but I like the shape of
>the Blosser a little better. However, Blosser has the standard tomato leaf
>shape as opposed to the potato leaf shape of Brandywine. Based on previous
>experience, I wonder if Brandywine would cross easier with EPB than would
>Blosser and if it would be easier to identify the crossed plants among my
>seed starts the following spring. The potato leaf makes a good marker.
>possible I would like to avoid having to hand pollinate. How many years
>would I need to repeat this process in order to get a new true breeding
>variety. How would this differ from a hybred? I hope some of you with
>plant breeding experience can weigh in on this one.
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