Rice breeders are often asked, “What do you look for when you are making selections?” This seemingly simple question requires a complex answer. Rice varieties are composed of many genes that control every function of the individual plant. Some plant traits are controlled by only one gene (simply inherited) while others are controlled by many genes (polygenic). An example of a simply inherited trait is the presence (pubescence) or absence (glabrous) of leaf hairs. The pure line varieties from the Rice Research Station are glabrous while the hybrids developed by RiceTec are typically pubescent. This trait is controlled by a single pair of genes, one each having been inherited from each parent. An example of a polygenic trait is yield. The yield potential of a variety is controlled by many genes. In general, the physical expression of polygenic traits is subject to environmental influence. This means that while a plant possesses a complex of genes with potential for high yield, under less than favorable environmental conditions, that high yield potential will not be realized. An example would be if one were to grow a field of the rice variety CL151 in Louisiana with no applied nitrogen. In spite of the high yield potential inherent in that variety, that yield potential will never be realized without adequate nitrogen.

Plant breeders face a tremendous challenge in breeding varieties that will be successful for production. A long-grain variety must have certain grain dimensions to be successful. Uniformity of grain shape and size are also critical. Plant height is important because plants need to be tall enough to provide a plant structure to support high yield as well as facilitate harvest. The plants cannot be so tall, however, that they will be more susceptible to lodging. Plant height is another example of a simply inherited trait. But the expression of that potential is highly influenced by environmental conditions, such as adequate nitrogen fertilizer.

Disease resistance is another important breeding objective for Rice Research Station breeders and pathologists. To select for this trait requires the presence of the proper disease causal agent. Sheath blight is the most troublesome disease in Louisiana rice production year in and year out. To facilitate screening for resistance to this disease, the pathologist will typically inoculate plots and rows with inoculum containing Rhizoctonia solani, which is the fungal organism that causes this disease. With diseases such as rice blast, planting highly susceptible varieties around breeding nurseries will typically ensure high disease pressure and eliminate the need for inoculation. This illustrates the importance of knowledge of each disease and the use of that knowledge to create the most favorable environment for effective screening for that particular disease.

Milling quality is also critical for a successful rice variety. There are numerous factors that will influence the quality of rice. One of the most important is the percentage of whole (unbroken) grains remaining after the milling process. This is an example of a trait with a high level of genetic control but also a substantial environmental influence. Grain shape and uniformity are important. Some rice varieties will always have fairly low whole grain milling yields, regardless of the environment. Others can have high milling yields in favorable environments but much lower yields under unfavorable conditions. Also, the grain moisture at harvest can have a significant impact on this characteristic. To further complicate this, environment here refers not only to the field conditions under which the plants are grown but also the conditions under which the grain is artificially dried after harvest. Drying at excessive temperatures can dramatically reduce milling yields. This will illustrate why this trait is a difficult one to select for, and the true measure of this trait comes when samples are actually milled when breeding lines reach the yield testing stage.

The traits discussed are just a few of the multitude of traits that must be considered when a rice breeder is making selections. Others include seedling vigor, cold tolerance (both at the seedling and reproductive stage), response to plant growth regulators, cycle (number of days from emergence to maturity), grain shattering, herbicide tolerance or resistance, insect resistance, panicle exertion, seed dormancy and ratooning (second crop) characteristics. Putting all of these traits together in a package is what keeps rice variety development worthwhile.

The 2011 Rice Research Station Field Day is scheduled to be held on Thursday, June 30. Please mark your calendar.

Permission granted March 18, 2011 by B. Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com.