Laboratory of Ann Hirsch

Sweetclover Methods

Crossing Techniques B

(Contributed by Wei-gang Yang, UCLA)

Drawn by Margaret Kowalczyk based on a diagram by Weigang Yang

Female flowers; selection and treatment (done under a dissection microscope; see diagrams):

1. Choose a raceme that has several florets with the petals showing about 0.5 mm beyond the calyx, just before the petals begin curving at the tips; the bud should not be longer than 1 mm. This stage of floral development is just before anthesis, but the stigma is receptive.
2. Choose three or four buds per raceme for crossing. Remove the rest.
3. Carefully remove the tip of the back sepal. Then use a very fine forceps (#5) and insert it in a vertical direction into the folded petal to open up the standard. Use extreme caution and patience . This produces an opening into the bud by exposing the wings and the keel which enclose the carpels and the stamens.
   
   • For larger Melilotus plants (e.g., U389 and derivatives). Using the forceps, put a gentle pressure on the bud to open it slightly. You do this by placing the forceps (in a horizontal orientation) around the keel and wings). This exposes the stigma. Do not use too much pressure or you will crush the bud. Releasing the forceps will cause the bud to close. Leave it under the microscope until you have the pollen ready.
     
   • For smaller Melilotus plants (e.g., U390). Open up the standard as in 4a but instead of applying pressure on the keel and wings to open up the bud, make a small hole in these petals to expose the stigma.

Male flowers; selection and treatment (see diagrams):

1. Choose florets that are close to blooming for pollen collection (no more than 3 mm long).
2. Remove floret and, using the forceps, take off or displace the petals, starting with the standard. Take care not to scatter the pollen. The anthers and carpel should be visible.
3. The pollen should be yellow in color and sticky.

Crossing (done under a dissection microscope; see diagram):

1. Using the male flower as a brush, put the pollen on the stigma of the female flower. It should be visible through the small hole (U390) or by applying pressure to the inner petals (U389). Use one hand to carry the pollen and the other to hold the forceps to apply slight pressure.
2. After pollination is accomplished, release the pressure applied by the forceps and close the flower carefully. Use a jeweler's tag to mark the cross.
   

Notes: This method does not require that you emasculate the flower of the female parent. The anthers should be at a stage prior to dehiscence of the pollen. However, you can remove the anthers if you want to be absolutely sure that the flower does not self.

It is necessary to use extreme caution and patience at all stages to avoid damaging the flowers.

We have had about 10-15% success rate using this method.