Several growth stages help identify critical periods during the life cycle of the rice crop. They are generally separated into stages primarily associated either with vegetative or reproductive development. The vegetative phase occurs first and is associated with the six- to eight-week period following planting. The reproductive phase follows and is generally associated with the formation, development and maturation of the panicle and grain. (This information was primarily developed by Dr. Richard Dunand, retired Rice Research Physiologist at the Rice Research Station).
Germination and emergence are the first stages in the vegetative phase of growth and the basis on which stand or plant population is determined. Germination begins with the appearance of the young shoot and root through the seed coat at one end of the seed. As the shoot elongates and reaches the soil surface, emergence occurs. When rice is planted on the soil surface, germination and emergence occur almost simultaneously. In drill planting, where seeds are planted below the soil surface, germination and emergence are separated by the time it takes for the shoot to grow the distance between the seed and the soil surface.
Seedling growth follows emergence and is usually denoted by number of leaves (1-leaf, 2-leaf, etc., stages). Seedling stages are determined by the number of leaf blades that are fully developed. When a leaf has fully exerted, another is usually emerging. However, the emerging leaf should not be counted. Seedling stages occur during the first two to five weeks after planting.
As rice enters the 3- to 4-leaf stage, tiller (or stool) formation typically begins, initiating the tillering stages. Tillers appear as secondary shoots to the main shoot. Tillers are initiated at the base of the plant, emerging from the inside of the seedling leaves on the main shoot. The number of tillers is primarily determined by plant population and variety, and tiller formation occurs over a two- to three-week period. Generally, a stand of 10 plants per square foot will average two to four tillers per plant. When one tiller appears, the growth stage is the first tiller stage. Tillering stages are numbered according to the number of tillers present, and, generally, with a stand of 10 plants per square foot, plants in the fourth tiller stage are common.
Panicle initiation is the first stage in the reproductive phase of growth. The developing panicle is microscopic in size inside the stem, and panicle initiation can usually be associated with the beginning of stem internode formation.
As the internode elongation stage begins, a build-up of chlorophyll occurs between the nodes that are to separate in the process of forming the first stem internode between them. This accumulation of chlorophyll imparts a green color that encircles the developing internode producing a green ring or band. Thus, the internode elongation stage is sometimes referred to as the green ring stage. This stage is also referred to as first green ring because additional internodes that form on top of the first internode can also have the green ring effect as they begin to form. A total of five internodes can be produced in the formation of a stem of rice.
As internode and panicle formation continue, the panicle differentiation stage occurs. Panicle differentiation is the first stage in the reproductive phase when the newly forming panicle becomes visible. At this stage, the panicle inside the stem has grown to approximately 1/8 inch. In the metric system, the length is approximately 2 millimeters, and the stage is sometimes referred to as the 2 mm panicle stage. The panicle continues to grow and develop inside the stem. As it does, the growth stage is referred to as booting and identified by the length of the panicle. When the panicle is up to 2 inches in length, the growth stage is early boot. Middle boot and late boot occur when the length of the panicle is 2 to 5 inches and 5 inches or greater, respectively. Up to this point, identification of the growth stages in the reproductive phase requires the stem to be dissected (split in half).
During late boot, the panicle develops completely, and the growth stages that follow occur after the panicle has exerted and is visible outside the stem. The heading stage is noted when a portion of a panicle is observed growing out of the end of a rice stem. From this time forward, growth stages are based on the state of the panicle outside of the rice stem. Heading stages are identified by percentages, and the 50 percent heading stage occurs when 50 percent of rice stems are heading or headed (panicle completely emerged from the stem).
The flowering and grain filling stages begin within one to five days after heading, and grain filling is complete within three weeks. The grain filling stages are separated into the milk, dough and physiological maturity stages. The milk stage is observed when a milky white substance begins to accumulate, usually seven to 10 days after heading, and the dough stage occurs about a week later as the milky substance begins to change and become the texture of bread dough.
When rice grains first become firm, they are at the physiological maturity stage. Grain moisture at physiological maturity is around 30 percent. Depending on the weather, an additional two weeks are required for moisture content to drop to around 20 percent and the grain to ripen to harvest maturity.
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