Rice is the staple food of more than 60 percent of the world population. About 90 percent of all the rice grain in the world is produced and consumed in Asia. In India alone, rice is cultivated on more than 44.6 million ha (Mha) with an average productivity of 3.0 t/ha. In countries such as China, where rice is cultivated on nearly 30.5 Mha, average productivity is nearly 6.2 t/ha. Such differences in productivity depend mainly on the genotypes used, the quality of the seeds, the management practices adopted and the climatic conditions during the crop growth period. Since high quality seed is free from various diseases and has better seed health, it tends to produce healthy seedlings with no initial disease inoculums. A superior quality seed not only increases productivity per unit area, but it also helps produce uniform crops without any admixtures - important for obtaining high prices on the market. High-quality produce results in high milling recovery and better quality rice, which translates into increased profits.
In order to increase crop productivity, significant efforts in breeding new conventional varieties and hybrids have been made. Given the very large area cultivated under conventional varieties and the vast potential of hybrid rice in Asia, it is increasingly important to have genetically pure and good quality seed. Quality seed production procedures for conventional varieties are well known for different classes of seed. Since rice is a self-pollinated crop, maintenance of seed purity is much easier with varieties than with rice hybrids. This paper deals primarily with hybrid rice seed production procedures, which are different from the procedures adopted for varieties. It should be noted, however, that for parental line seed production the procedures are similar to those for varieties.
SEED PRODUCTION OF HYBRIDS THROUGH THE THREE-LINE SYSTEM
The success of hybrid rice cultivation depends on the success of the hybrid rice seed production programme which enables seed producers to produce high quality seed at an economical price. Hybrid rice seed production requires specialized techniques which must be fully understood by the production staff.
Hybrid rice seed production using the CMS system, i.e. the three-line system: A line (female), B line (maintainer) and R line (restorer) involves three steps:
1. Multiplication of A line
2. Multiplication of B and R lines
3. Production of hybrid seed (A x R)
In order to obtain the best quality F1 seed in the hybrid seed production programme, high genetic and physical purity of the parental lines is a prerequisite. Even a very low percentage of impurity or contamination in a parental line can cause the production plots to reject the seed. A slight impurity in parental lines can also lead to high costs because of the enormous efforts in rouging which can be very costly for the hybrid seed producer. Impure parental lines lead to variation in plant type, duration, plant height and grain size, and ultimately the quality of the F1 hybrid is affected. It is therefore essential to adopt methods to ensure quality seed production. Quality seed should have the following characteristics:
True-to-type genetic purity
No contamination or admixture in the seed
High germination capacity
Free from disease
Free from weeds, soil particles, sand and stones
No broken seeds
In order to achieve higher seed yields, rice seed production should be undertaken in the most favourable areas where irrigation is guaranteed, and with adequate and balanced use of fertilizers together with integrated nutrient and pest management. Some standardized seed production practices will be discussed in the next article of this sequel.