How Are Rice Crosses Made?

Steven Linscombe  |  1/22/2013 11:26:40 PM

The making of rice crosses.

An important function of the Rice Research Station is the continual development of new, superior rice varieties to maintain and enhance the viability of the Louisiana rice industry. Variety development is a long-term process, and the typical variety takes an average of eight years to develop. To create a new variety, one must first create new pools of genetic variation. This is done by crossing two different varieties or lines to generate a full spectrum of recombinants for selection for different breeding goals. A cross is made through a tedious and meticulous approach.

Rice is a self-pollinated crop with a perfect flower that contains the pistil (stigmas, styles and ovary) and stamens (two-celled anthers and filaments). Prior to anthesis, which is the period during which the flower is functional, the designated female parent must have its anthers removed before being pollinated with the selected male parent.

The process to remove anthers is called emasculation. Hot water treatment and vacuum suction are the two most common emasculation methods. Pollen grains are more sensitive to high temperatures than is the stigma. When treated with water at 113 degrees Fahrenheit for 5 minutes, the pollen grains lose viability while the stigma remains functional. This differential response to high temperature is used for emasculation prior to artificial hybridization. Pollen grains from the spikelets that will flower the same day a cross is made will have the maximum chance to be killed by the hot water treatment. Female panicles with the top 1/4 or 1/3 of the spikelets already flowered make the best materials for hot water treatment. On a sunny day, females can be treated beginning at 8 a.m., while the whole emasculation process must be finished before 10 a.m. when the florets will typically begin to open naturally. The starting time needs to be adjusted for the weather and materials.

Immediately after being removed from the hot water, some spikelets should open. One-third of the glume must be clipped off of each spikelet, and remaining spikelets that have not opened are cut off with a pair of sharp scissors. At least 20-30 emasculated spikelets are needed for a successful cross. Once the emasculation is finished, a glassine cross bag is immediately placed over the panicle with the female label on the bag along with the date. Plants can be placed back into the greenhouse awaiting pollination.

Emasculation is also carried out by using a small vacuum pump to extract the unripe anthers from spikelets of selected female parents by suction force. The device contains an oil-free pump, plastic tube and filters for filtering pollen grains and incoming air. A glass pipette is connected to the end of the plastic tube, and the narrow nozzle of the pipette is used for sucking anthers from the clipped spikelets. A panicle of the designated female parent with only a few top spikelets that flowered the day before is the perfect material for vacuum emasculation. The panicle is trimmed from the bottom upward to leave 25-35 well-spaced spikelets to be emasculated.

An opening is then cut on each of the selected spikelets separately so that the anthers come free to be vacuumed out of the spikelet. The tip of the pipette is inserted into the clipped spikelet, and the vacuum sucks out anthers without damaging the stigma. The emasculated panicle is then covered with a labeled glassine cross bag and the plant is moved back into the greenhouse and is now ready for pollination.

The process of transferring pollen from selected male parents to the emasculated spikelets of female parents is called pollination. It may be done immediately after emasculation or may be delayed for up to several days. It can be done either by bagging a flowering male panicle above the emasculated female panicle in a glassine cross bag or by cutting off the entire or part of a flowering male panicle and swirling around the female inside the glassine cross bag. After pollinating the female, the appropriate information on the male parent is written on the glassine cross bag and the bag is placed back over the female panicle. The cross seed is mature and ready to be harvested in about 28-34 days. In a typical year, approximately 1,000 crosses will be made at the Rice Research Station.

Permission granted Jan. 13, 2013, by B. Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com

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