Steven Linscombe | 12/17/2013 4:03:43 AM
Rice breeders are often asked what they look for when making selections. The answer is complex. 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. This trait is controlled by a single pair of genes, each inherited from a 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 even though a plant possesses a complex of genes with potential for high yield, under unfavorable environmental conditions, high yields will not be realized. For example, the rice variety Mermentau has a high yield potential, but high yields will not be realized without adequate nitrogen.
Plant breeders face a tremendous challenge in breeding successful varieties. For example, the rice grain of a long-grain variety must have certain dimensions and the grains must be uniform in size. The height of the rice plant must be tall enough to provide a structure to support high yield as well as facilitate harvest, but not too tall to be 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 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. To facilitate screening for resistance to this disease, the breeder will typically inoculate plots and rows with 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 assure 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 another critical aspect of a successful rice variety. There are numerous factors that will influence milling quality. 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 as can the field conditions under which the plants are grown and the conditions under which the grain is artificially dried after harvest. Drying at excessive temperatures can dramatically reduce milling yields. This illustrates why this trait is a difficult one to select for, and the true measure of this trait comes when samples are actually milled.
Another important aspect of grain quality is the amount of chalk present. Chalky rice occurs when part of the grain is whiter than the rest because the starch has not developed properly. This is a point of weakness that can increase breakage during the milling process. Even if the grain does not break, chalky rice is aesthetically unpleasing and can greatly reduce the quality of the rice sample. This is another trait controlled by genetics but also highly influenced by the environment under which the grain develops. Typically, levels of chalk are higher when the rice grain grows under higher temperatures. Some varieties are inherently more resistant to chalk formation even under high temperatures, and their identification is important during the breeding process.
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 cropping) characteristics.
Putting all of these traits together in a package is what keeps rice variety development intriguing and worthwhile.
This project is partially funded by the U.S. Department of Agriculture National Institute of Food and Agriculture (NIFA).
Permission granted December 16, 2013, by B. Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com