Hybrid rice technology is based on the exploitation of heterosis, where crossing two genetically diverse parental lines results in a hybrid with superior yield performance. For commercial hybrid rice seed production, male sterility systems are essential, and these include cytoplasmic genetic male sterility (CGMS), also known as the three-line system, and environment-sensitive genetic male sterility (EGMS), which consists of the two-line system comprising photoperiod-sensitive genetic male sterility (PGMS) and temperature-sensitive genetic male sterility (TGMS). A major limitation of the CMS system is that the hybrid itself remains male sterile unless fertility is restored. Principle of cytoplasmic genetic male sterility (CGMS) The first step in the CGMS system is the development of a male sterile line, referred to as the A line. This is achieved through repeated backcrossing of a selected female parent with a wild abortive cytoplasm source, which provides male sterility, while using the female parent as the recurrent parent. This process is termed line conversion. For example, if a hybrid is to be produced by crossing parent P1 with parent P2, the wild abortive cytoplasm is first crossed with P1, followed by repeated backcrossing of the F1 progeny with P1, resulting in a male sterile version of P1, designated as P1MS. The second step involves the maintenance of the male sterile line. The male sterile A line is crossed with its corresponding maintainer line, known as the B line, which is genetically identical to the A line except for possessing normal fertile cytoplasm. This cross ensures the continued multiplication and maintenance of the male sterile line. The third step is the development of the hybrid. In this step, the pollen parent or male parent is converted into a restorer line by incorporating fertility restorer genes through backcross breeding, using the pollen parent as the recurrent parent and a known restorer source as the donor. This results in the development of a restorer line, designated as P2. Hybrid seed is then produced by crossing the male sterile A line (P1MS) with the restorer line (P2R), and the seed harvested from the A line constitutes the hybrid seed. Environment-sensitive genetic male sterility (EGMS) Environment-sensitive genetic male sterility refers to male sterility that is expressed under specific environmental conditions and is reversible. This system is further divided into two categories. In temperature-sensitive genetic male sterility (TGMS), male sterility is expressed at higher temperatures, generally around 23.3°C or above, as observed in rice TGMS lines such as Pei-Ai 645, while normal fertility is restored when temperatures fall below the critical threshold. In photoperiod-sensitive genetic male sterility (PGMS), the expression of male sterility is governed by day length, provided the temperature remains within a critical range of 23 to 29°C. Under long-day conditions exceeding 13 hours and 45 minutes, complete male sterility is expressed, whereas under short-day conditions, plants regain near-normal fertility. The two-line system is preferred for several reasons. It is simpler and more efficient than the three-line system, does not require a restorer line, eliminates the adverse effects associated with sterile cytoplasm, and facilitates easier exploitation of inter-subspecific heterosis, particularly in indica × japonica crosses. Hybrid Seed Production Selection of seed production fields Selection of seed production fields is a critical step and requires fertile soil, reliable irrigation, proper drainage, adequate sunlight, and effective control of insect pests and diseases to achieve high seed yields. Fields where rice was not previously grown are preferred to minimise volunteer plants. Seed production plots must be sufficiently isolated from other rice fields to ensure genetic purity so that pollination of the female parent occurs only from the designated male parent. Ideal season and weather conditions The rabi season is generally more suitable for hybrid rice seed production than the kharif season. Ideal climatic conditions include a daily mean temperature of 25–30°C, relative humidity of 70–80%, a day-night temperature difference of 8–10°C, bright sunshine during flowering, absence of rainfall, and a wind speed of 2–3 m/s to aid pollen dispersal. Isolation Isolation of hybrid seed production fields is essential. Foundation seed production of A, B, and R lines requires an isolation distance of at least 200 meters, while hybrid seed production fields require a minimum of 100 meters. Isolation can be achieved through distance, time, or barrier methods. Rice pollen grains are light and remain viable for only 3–5 minutes, but can travel up to 100 meters. The isolation distance may be reduced to 50 meters if at least ten border rows of the pollen parent are planted around the seed production plot. Time isolation involves staggering planting dates so that flowering does not overlap with nearby rice crops within 100 meters by at least three weeks. Barrier isolation utilises natural or artificial structures or tall crops exceeding 2.5 meters in height, with barrier widths ranging from 3 to 4 meters, depending on the crop used. Nursery raising For nursery raising, raised nursery beds enriched with basal manure are prepared. Seed rates of 20–25 kg/ha for the female parent and 10 kg/ha for the male parent are recommended. Raising seedlings under plastic film or in greenhouse conditions is considered advantageous. Seedling age at transplanting Seedlings aged 21 days are ideal for transplanting, as they promote timely heading and flowering. Older seedlings delay flowering, while younger seedlings advance flowering. If transplanting of the A line is delayed, transplanting of the B or R line should be delayed by the same duration to maintain synchronisation. Planting ratio The planting ratio between female and male lines is commonly maintained at 6:2 or 8:2. Spacing is generally kept at 15 cm between plants. Row spacing between two male rows is 30 cm, between two female rows 15 cm, and between male and female rows 20 cm. Row orientation should be nearly perpendicular to the prevailing wind direction to enhance cross-pollination, and rows are usually laid out in an east-west direction. Fertilizer management Fertiliser management in hybrid seed production fields typically involves application of 120:60:60 kg/ha of nitrogen, phosphorus (P₂O₅), and potassium (K₂O). Nitrogen is applied separately in three splits. The first split is applied 5–7 days after transplanting, the second 20–25 days later, and the third at maximum tillering. Zinc deficiency is corrected by applying 20–25 kg/ha of zinc sulphate or by foliar spraying of 0.2 percent zinc sulphate. Synchronization of flowering Synchronising the flowering of both parents is the key to increased yields. Technical measures such as staggering seeding dates of the male and female parents, sowing the male parents three times to extend the time of pollen availability, and predicting and adjusting flowering dates may be adopted. Adjusting sowing time: Seeding dates of the parents are adjusted in the seedbed so that they flower at the same time in the field. This is called differential seeding. The seeds of the Female parent are sown once, while the seeds of the male parent are sown three times at 3-day intervals in such a way that the 2nd sowing of male parent(R line) seeds coincides with the sowing of the female parent (A line) seeds. Case 1: Female and male parents are of the same duration - Sow one-third seeds of the male parent on the 1st day of the sowing schedule. Sow one-third seeds of the male parent and all the seeds of the female parent on the 4th day of the sowing schedule. Sow the rest one-third seeds of the male parent on the 7th day of the sowing schedule. Case 2: The Female is shorter in duration by 10 days than the male parent. Sow one-third seeds of the male parent on the 1st day of the sowing schedule. Sow one-third seeds of the male parent on the 4th day of the sowing schedule. Sow the rest of one-third seeds of the male parent on the 7th day of the sowing schedule. Sow all the seeds of the female parent on the 14th day of the sowing schedule. Case - 3: The Female is longer in duration by 10 days than the male parent. Sow all the seeds of the female parent on the 1st day of the sowing schedule. Sow one-third of/ ofseeds of the male parent on the 8th day of the sowing schedule. Sow one-third seeds of the male parent on the 11th day of the sowing schedule. Sow the rest of one-third seeds of the male parent on the 14th day of the sowing schedule. Adjusting flowering dates by fertiliser application: Beginning about 30 days before heading, 3 or 4 random samples of the main culm of both parents are taken at 3-day intervals. Young panicle development is compared under magnification. Ideally, the female parent should flower 1-2 days before the male line. During the first three stages of panicle differentiation, apply quick-releasing N fertiliser (2% urea) on the early parent to delay flowering or 1% phosphate/potassium fertiliser (e.g. potassium dihydrogen phosphate) on the late parent to enhance its flowering. This adjusts flowering differences of 4-5 days. Adjusting flowering dates by water management: Water management, e.g. drainage or flooding to the male line plot, is more responsive to flowering. During later stages of panicle differentiation, draining water from the field will delay male parent panicle development, while higher standing water will hasten panicle development. Methods of improving seed setting Supplementary pollination to increase outcrossing rate: Supplementary pollination is done on calm days during peak anthesis, by artificially shaking the canopy of the pollen parent to increase cross-pollination. It causes anthers to shed all of their pollen, disperses pollen grains uniformly over the seed parent plants and increases seed set in the seed parent. It is done by (i) Rope pulling: two persons pulling a one cm diameter rope across the two R lines, and panicles are shaken back and forth every 30 minutes until no pollen remains on the restorer line. (ii) Rod driving: one person shaking the canopy layer of the R lines with a bamboo stick, taking care not to break off the panicles at the neck. Flag Leaf clipping: Leaves taller than the panicles are the main obstacles to cross-pollination. Clipping leaves 1-2 days before initial heading increases the probability of pollination and outcrossing rate. The blade of the flag leaf is cut back 1/2 to 1/3 from the top. GA3 spray: Spraying seed parent with 50-100 g/ha GA3 @ 60 ppm concentration or more, 2 to 3 times (First spray when 15% of the tillers have started heading 40% of total (20 gm) in 500 litre water and second spray on the next day, or after 2 days-60% or 30 gm in 500 litres water) increases panicle exsertion and helps in increased seed setting. GA3 powder will not dissolve in water. It must be dissolved in a small amount of 70% ethanol before it is mixed with water. A liquid dish detergent or laundry detergent should be added to the solution. The detergent makes GA3 stick to the leaf surface for more efficient coverage of the plants. Gibberellic acid is sprayed for the following reasons- To adjust the plant height of both parents. To increase the growth rate of secondary and tertiary tiller panicles so that they can bear To enhance panicle exsertion from the flag leaf in the seed parent, as it has poor panicle exsertion. To increase the duration of the floret opening in the seed parent. To increase the rate of stigma exsertion and to lengthen the duration of stigma receptivity in the seed parent. Roguing Roguing is carried out at vegetative, flowering, and maturity stages to remove off-type plants, pollen shedders in the A line, diseased plants, and plants with abnormal panicle or grain characteristics. Field inspection A minimum of four field inspections is conducted. The first inspection occurs before flowering to verify isolation and planting accuracy. The second and third inspections take place during flowering to detect pollen shedders and off-types. The fourth inspection is conducted at maturity to confirm varietal purity. Harvesting During harvesting, male rows are harvested first to prevent mechanical admixture. Harvesting is done manually, and combines are avoided. Final roguing is performed before harvest, and seeds collected from the female parent constitute the hybrid seed. Hybrid seed yields generally range from 1.5 to 2.5 tonnes per hectare. Field standards FACTORS MAXIMUM PERMITTED (%) FOUNDATION CERTIFIED Off types in the seed parent 0.05 0.20 Off types in pollinator parent 0.05 0.20 Pollen shedding earheads in the seed parent 0.05 0.10 Objectionable weed plants(Wild rice (Oryza sativa var. fatua) 0.01 0.02