The rice genetics program has two main objectives: to investigate new and more efficient breeding methods and to -develop improved germplasm lines useful to U.S. rice breeders. The project also cooperates with various international programs.

A collaborative program in 1970 with a senior scientist from Taiwan achieved the first success in inducing mutations in U.S. rice. Using cobalt-60 gamma radiation of Calrose rice seeds, mutations for short stature and early maturity were obtained. One of the short-stature mutants was later released as Calrose 76. This source of short stature produced the short-stature trait in most newer varieties.

Disease screening

A collection of 40 weedy, or wild type, rice species obtained by the USDA geneticist in 1974 from the International Rice Research Institute in the Philippines was screened for stem rot resistance in 1978. Species that showed disease resistance were crossed with cultivated rice. The progeny from these crosses had improved resistance to stem rot but were unacceptable for commercial use. The lines with improved resistance were turned over to the breeders at the Rice Experiment Station in 1980 and 1981 to continue the effort to produce stem rot resistant commercial varieties.

A similar program to find resistance to sheath blight began in 1981 in cooperation with the Department of Plant Pathology at UC Davis. The first hybrids with sheath blight resistant parents were grown in 1982. In 1983 the crossed populations were screened for sheath blight resistance, and resistant selections were saved for further improvement.

Hybrid rice

Figure 6. Male sterile rice flower (left) with shrunken anthers that are devoid of pollen, and a fertile flower. Genetic male sterile lines are being used for natural crossing in the field.

Studies were conducted from 1970 to 1973 by the USDA geneticist to determine if hybrid rice was superior to pure line selection. Results were somewhat encouraging, but the mechanisms for producing hybrid seed appeared too cumbersome and costly to be practical. Consequently, the program was discontinued. Later, it was learned that the Chinese had successfully produced several million acres of hybrid rice. The program was reinstated in 1979. Genetic lines are now available to produce hybrid seed, but to become successful in the U.S., the percent of natural crossing and grain quality must be greatly improved. Possibilities for accomplishing this are being investigated.

A program started in 1983 is using the male sterile technique to produce large numbers of crosses by natural field crossing. The procedure has proven feasible. Still to be proven is whether promising lines can be selected from this simplified crossing system.

Improved germplasm has been developed and released to U.S. rice breeders for cold tolerance, short stature, early maturity, and resistance to stem rot and sheath blight. The USDA rice geneticist, working in cooperation with the rice breeders at the Rice Experiment Station, released a mutant line that could increase natural crossing for hybrid rice production. Another project is investigating the use of the antibiotic drug, streptomycin, to induce cytoplasmic male sterility in major varieties. This type of male sterility is needed for hybrid rice production. Mutants also are being produced and screened for resistance to herbicides and other chemicals.