Top priorities are given to increased yields through new varieties and to the disposal of rice straw.

Scientists at UCD identified three genetic and physiological factors that limit yields: 1) lack of certain and uniform rice stands; 2) sterility of panicles; and 3) lodging. Beyond these, they concluded that basic studies were needed to identify other barriers to higher yields that appear surmountable.

Partial solutions to the residue problem could be obtained by: 1) increasing the ratio of grain to straw; 2) reducing plant height, which would decrease the amount of loose straw; and 3) early maturity, which would improve the prospects for fall incorporation of the straw.

Toward more certain and uniform rice stands, a method has been devised for accurate measurement of seedling vigor under the temperatures that prevail at seeding time.

Hundreds of varieties from the world rice collection were tested. Many were found to have excellent seedling vigor but were otherwise poorly adapted to California conditions. Later, two mutants from Calrose were discovered to possess good seedling vigor. These genes for seedling vigor, which may be associated with early maturity, are being transferred to adapted short stature breeding lines.

 

Figure 6. Another approach plant breeders are using to increase rice yield is to cross large-seeded lines (right) with many-seeded lines left). Center panicle is what we have today in our old varieties. (UC Cooperative Extension photo)

Excellent resistance to low-temperature-induced sterility has been obtained from crosses between Caloro and the Japanese variety Kitaminori.

We will soon know the heritability of tolerance to sterility and its relation to early maturity. We anticipate being able to use this source of resistance to sterility to reduce blanking of California varieties.

A very useful spin-off of UCD research on blanking has been the finding that blanking in susceptible varieties can be reduced through water management just before heading. To minimize blanking: 1) raise the water 2 inches higher than above your normal depth when the plants are at the sensitive stage (about 3 weeks before heading); 2) minimize spilling so that cooling from added water will be minimal; 3) about a week before heading, reduce water intake so the water sinks to the original 6-inch depth. These procedures are recommended where blanking has been a problem, especially in cooler areas.

Great improvements have been made in resistance to lodging with the California shorter-stature rice varieties.

The induced mutant gene for shorter-stature occurs in the relatively cold-tolerant varieties Calrose 76 and M7, while M9 carries the tropical source of short-stature. All three short varieties will be available to growers in limited quantity in 1977. These short-stature varieties show less height increase with high nitrogen fertility than do tall varieties. With proper water depth and fertility management, yield increases of about 10% above tall varieties are anticipated.

The search continues to find the physiological barriers I to further yield increases.

These studies have not progressed far enough to provide sure guidance, but some of the evidence points toward increased number of kernels per head as the most promising direction for raising yield barriers.

The ratio of grain to straw is increased in the new short-stature varieties.

In three 1976 tests, Calrose 76 produced about 10% less straw (and 10% more grain) than CS-M3. Since standing short-stature rice can be cut at a higher level than lodged tall rice, leaving more stubble, loose residue should be 20-30% less from the short-stature crop than from the tall crop.

Early-maturity genes are now available for short- and medium-grain types.

Derived from the induced mutation program, these genes are available in the high-yielding medium-grain background. Earlier harvest increases opportunities to burn, incorporate, or remove rice straw.

Future basic rice genetic and physiology research will focus in three major areas.

Growth-chamber studies will be designed and conducted to shed light on the relative importance of greater leaf efficiency and greater panicle size as methods for increasing yields. Once the rate-limiting factor is established, genetic selection can be directed toward maximizing these factors. Additional field and laboratory studies will be directed toward mutation breeding to improve rice quality. Based on the use of a screening method to observe mutations in large numbers of plants, the results, if successful, could be highly beneficial from a rice-marketing standpoint.

USDA geneticists propose to introduce low-sterility characteristics into useful short-stature japonica breeding lines. A companion field study will seek to answer the question of whether short stature alone reduces sterility in rice.