Laboratory of Ann Hirsch


Sweetclover Methods

Crossing Techniques A

(Contributed by H.J. Gorz, University of Nebraska; also see paper by Miller et al., 1991)

Before you begin the cross:

  1. Convince yourself that making these crosses is possible.
  2. Lower your expectations. Don't try to do too many florets per raceme.
  3. Don't work too long without resting; one tends to get impatient and frustrated.
  4. Grow plants individually in small pots so they are portable and can be laid on their sides for emasculating.
  5. Be sure that the dissecting scope you use is properly adjusted and that you have adequate light.
  6. Use of the smallest, finest tweezers is important because the buds are so small. Proper fit of the tips of the tweezers is also important.

Technique:

  1. Pick a raceme with one or two flowers just opening at the base, or a raceme that still has buds at the tip.
  2. Remove all open florets and very large buds as well as florets that are two small at the tip. If very much of the keel is showing, the bud is probably too old. Anther dehiscence takes place during the bud stage.
  3. Choose several buds at the proper stage on opposite sides of the raceme for crossing. Remove the rest.
  4. Place the fingers gently between the two opposite rows of florets, holding the stem with very slight pressure on the calyx of one or two florets. Hold the floret near the bottom; do not pinch the embryo sac or the style.
  5. The florets tend to hang downwards on the raceme, so you must straighten them up between your thumb and forefinger while applying very gentle pressure.
  6. Start at the bottom of the oldest floret and work up. Do one side of the raceme, then turn it over and do the other side.
  7. It is important not to crush the lower florets while working on the upper ones
  8. Use only the very finest tweezers with tips that fit together perfectly. It may be necessary to grind the tips down somewhat. You should look at the tips under the dissecting scope and make certain that the tips come together perfectly.
  9. Remove the standard first, then the wings and the keel.
  10. Tease the petals out (moving the tweezers back and forth slightly) rather than pulling straight out.
  11. Of about 6 or 8 buds at about the right stage, expect that some will have shed pollen and some will pull off as you attempt to remove the petals. Carefully remove all ten anthers. Expect to wind up with 3 or 4 emasculated buds per raceme.
  12. Collect pollen from other anthers (flowers where the petals have just recently opened) with a flat toothpick that has some emery paper glued to it. Make sure the pollen is bright yellow; if it is white, it is too old.
  13. Place the pollen on the stigma of the emasculated floret. Repollinate the next day to make certain. Use a jeweler's tag to mark the cross.
  14. Rinse the forceps and the microscope stage in 95% alcohol before going on to the next pollination.
morphological stages of sweetclover

 

Morphological stages of sweetclover flower development suitable for use in cross-pollinations: (A) flower suitable for use as a seed parent. The two other unopened flowers below could also be used. (B) flower suitable for pollen parent. When collecting pollen, insert toothpick tip into the throat of the flower, between open petals, in the area indicated by Arrow B. (Photo, courtesy of D.R. Viands.)