Germination initiates when the seed has absorbed adequate water and the temperature ranges between 50° to 107° F, with an optimum of 87° F. Germination begins when a young shoot and root are seen sprouting through the seed coat at one end of the seed. Emergence occurs when the shoot reaches the soil surface. When rice is planted on the soil surface, germination and emergence happen almost simultaneously. However, when rice is drilled, germination and emergence are separated by the time it takes for the shoot to grow through the surface. The coleoptile emerges through the soil surface and the prophyll, or the first sheathing leaf, emerges through the coleoptile. It is not a true leaf because it lacks a leaf blade.
Seedling growth is usually denoted by the number of leaves formed. During the seedling stage, the first to fourth leaves (V1-V4) form 14-25 days after emergence. The seminal root continues to grow deeper into the soil, and the secondary roots develop. When a leaf has fully developed another is emerging.
During this stage, tillers appear as secondary shoots and the fifth leaf is formed. 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 from plant population and variety. This stage takes two to three weeks to develop. Tillering stages are numbered according to the number of tillers present (V5-Vnth).
At this stage, active tillering is declining and marks the beginning of the reproductive stage. The plant height and diameter continue to increase but at a slower rate. The length of this phase depends on the maturation period of the variety. Phenoxy herbicides should be applied at this stage.
This is the first stage, R0, in the reproductive phase and reproductive growth stages can be defined as Rnth (ex. R0, R1…). The growing panicle is microscopic inside the stem, and panicle initiation can usually be associated with the beginning of stem internode formation. This phase is also known as the green ring stage in rice. A thin green band is visible just above the top node. This represents the beginning of internode elongation. The duration from R0 to R1 is typically 20 to 30 days.
During this stage, chlorophyll begins to build up between the nodes that are to separate in the process of forming the first stem internode between them. A total of five internodes can be produced in the formation of a stem of rice. This stage is also referred to as the jointing stage. Stem internodes can be distinguished from root internodes by the green color in the stem wall.
During this stage, internode and panicle formation continue. The forming panicle becomes visible and has grown about one-eighth of an inch. This growth stage will continue for 20-30 days until the collar has formed on the flag leaf. This is a critical stage in the plant’s development, as the number of potential grains per panicle is determined by the end of R1.
As the panicle continues to grow and develop inside the stem, the growth stage is referred to as booting and is identified by the length of the panicle. Early boot is when the panicle is two inches in length. Middle and late boot is when the length of the panicle is two to five inches and five inches or longer, respectively. During late boot, the panicle is developed completely. Environmental stress during this stage may cause a reduction in yield. The growth stages that follow booting occur after the panicle is visible outside the stem.
This is the stage the panicle begins to grow out of the rice stem. Heading Date is defined as when 50% of the panicles have partially emerged, and a field has “headed” when 100% of the panicles have totally emerged from the boot. From this stage forward, the growth stages are based on the state of the panicle outside the stem.
This stage is also known as anthesis and refers to the events between the opening and closing of the spikelet and lasts for an hour to two and a half hours. Pollination can occur during those hours. Rice is primarily a self-pollinating plant. Flowering begins at the tip of the panicle branches and moves down the branch to the panicle base. Flowering for the entire panicle is typically four to seven days. The potential number of filled grains per panicle has been established by the number of fertilized flowers. The number of filled grains per panicle is primarily determined by the conditions and events during this stage since only flowers fertilized can be filled grains.
The developing starch grains in the kernel are soft and the interior of the kernel is filled with a white liquid resembling milk. The interval development from R6 to R7 is about three to six days.
This stage is a continuation of the milk stage. The starch in the grain is beginning to become firm but is still soft.
This stage includes the end of grain filling, R7, and the grain drying stage, R8. The grain is now firm during this stage and is almost ready for harvest. The moisture content for the entire crop is above 22 percent. The interval for development from R7 to R8 stages is about two to five days.
The grain is hard and ready for harvest. During this stage, the moisture content is about 20 to 22 percent. Depending on the region and weather, an additional two weeks may be required to get the proper moisture content.