• Cultivated rice is generally considered as semiaquatic annual grass, although in the tropics it can survive as a perennial, producing new tillers from nodes after harvest (ratooning).
• At maturity the rice plant has a main stem and a number of tillers. Each productive tiller bears a terminal flowering head or panicle.
• Plant height varies by variety and environmental conditions, ranging from approximately 0.4m to over m in some floating rices. The morphology of rice is divided into the vegetative phase (including germination, seedling, and tillering stages) and the reproductive phase (including panicle initiation and heading stages).

Seeds

The rice grain, commonly called a seed, consists of the true fruit or brown rice (caryopsis) and the hull, which encloses the brown rice. Brown rice consists mainly of the embryo and endosperm. The surface contains several thin layers of differentiated tissues that enclose the embryo and endosperm. The palea, lemmas, and rachilla constitute the hull of indica rices. In japonica rices, however, the hull usually includes rudimentary glumes and perhaps a portion of the pedicel. A single grain weighs about 10-45 mg at 0% moisture content. Grain length, width and thickness vary widely among varieties. Hull weight averages about 20% of total grain weight.

Seedlings

• Germination and seedling development start when seed dormancy has been broken and the seed absorbs adequate water and is exposed to a temperature ranging from about 10 to 40°C.
• The physiological definition of germination is usually the time when the radicle or coleoptile (embryonic shoot) emerge from the ruptured seed coat.
• Under aerated conditions the seminal root is the first to emerge through the coleorhiza from the embryo, and this is followed by the coleoptile.
• Under anaerobic conditions, however, the coleoptile is the first to emerge, with the roots developing when the coleoptile has reached the aerated regions of the environment.
• If the seed develops in the dark as when seeds are sown beneath the soil surface, a short stem (mesocotyl) develops, which lifts the crown of the plant to just below the soil surface. After the coleoptile emerges it splits and the primary leaf develops.

Tillering plants

• Each stem of rice is made up of a series of nodes and internodes. The internodes vary in length depending on variety and environmental conditions, but generally increase from the lower to upper part of the stem. Each upper node bears a leaf and a bud, which can grow into a tiller.
• The number of nodes varies from 13 to 16 with only the upper 4 or 5 separated by long internodes. Under rapid increases in water level some deepwater rice varieties can also increase the lower internode lengths by over 30 cm each.
• The leaf blade is attached at the node by the leaf sheath, which encircles the stem. Where the leaf blade and the leaf sheath meet is a pair of clawlike appendages, called the auricle, which encircle the stem.
• Coarse hairs cover the surface of the auricle. Immediately above the auricle is a thin, upright membrane called the ligule.
• The tillering stage starts as soon as the seedling is self-supporting and generally finishes at panicle initiation. Tillering usually begins with the emergence of the first tiller when seedlings have five leaves.
• This first tiller develops between the main stem and the second leaf from the base of the plant. Subsequently when the 6th leaf emerges the second tiller develops between the main stem and the 3rd leaf from the base.
• Tillers growing from the main stem are called primary tillers. These may generate secondary tillers, which may in turn generate tertiary tillers.
• These are produced in a synchronous manner. Although the tillers remain attached to the plant, at later stages they are independent because they produce their own roots. Varieties and races of rice differ in tillering ability.
• Numerous environmental factors also affect tillering including spacing, light, nutrient supply, and cultural practices.
• The rice root system consists of two major types: crown roots (including mat roots) and nodal roots. In fact both these roots develop from nodes, but crown roots develop from nodes below the soil surface.
• Roots that develop from nodes above the soil surface usually are referred to as nodal roots. Nodal roots are often found in rice cultivars growing at water depths above 80cm. Most rice varieties reach a maximum depth of 1m or deeper in soft upalnd soils.
• In flooded soils, however, rice roots seldom exceed a depth of 40 cm. That is largely a consequence of limited O2 diffusion through the gas spaces of roots (aerenchyma) to supply the growth of root tips.

Panicle and spikelets

• The major structures of the panicles are the base, axis, primary and secondary branches, pedicel, rudimentary glumes, and the spikelets.
• The panicle axis extends from the panicle base to the apex; it has 8-10 nodes at 2- to 4-cm intervals from which primary branches develop.
• Secondary branches develop from the primary branches. Pedicels develop from the nodes of the primary and secondary branches; the spikelets are positioned above them.
• Since rice has only one fully developed floret (flower) per spikelet, these terms are often used interchangeably. The flower is enclosed in the lemma and palea, which may be either awned or awnless.
• The flower consists of the pistil and stamens, and the components of the pistil are the stigmas, styles and ovary.

• The growth duration of the rice plants is 3-6 months, depending on the variety and the environment under which it is grown. During this time, rice completes two distinct growth phases: vegetative and reproductive.
• The vegetative phase is subdivided into germination, early seedling growth, and tillering; the reproductive phase is subdivided into the time before and after heading, ie., panicle exsertion. The time after heading is better known as the ripening period.
• Potential grain yield is primarily determined before heading. Ultimate yield, which is based on the amount of starch that fills the spikelets, is largely determined after heading. Hence, agronomically it is convenient to regard the life history of rice in terms of three growth phases: vegetative, reproductive, and ripening.
• A 120-day variety, when planted in a tropical environment, spends about 60 days in the vegetative phase, 30 days in the reproductive phase, and 30 days in the ripening phase.

Vegetative phase

• The vegetative phase is characterized by active tillering, gradual increase in plant height, and leaf emergence at regular intervals. Tillers that do no bear panicles are called ineffective tillers.
• The number of ineffective tillers is a closely examined trait in plant breeding since it is undesirable in irrigated varieties, but sometimes an advantage in rainfed lowland varieties where productive tillers or panicles may be lost due to unfavourable conditions.

Reproductive phase

• The reproductive growth phase is characterized by culm elongation (which increases plant height), decline in tiller number, emergence of the flag leaf (the last leaf), booting, heading, and flowering of the spikelets.
• Panicle initiation is the stage about 25 days before heading when the panicle has grown about 1mm long and can be recognized visually or under magnification following stem dissection.
• Spikelet anthesis (or flowering) begins with panicle exsertion (heading), or on the following day. Consequently, heading is considered a synonym for anthesis in rice.
• It takes 10-14 days for a rice crop to complete heading because there is a variation in panicle exsertion among tillers of the same plant and among plants in the same field.
• Agronomically, heading is usually defined as the time when 50% of the panicles have exserted.
• Anthesis normally occurs between 1000 and 1300 h in tropical environments and fertilization is completed within 6 h. Only very few spikelets have anthesis in the afternoon, usually when the temperature is low.
• Within the same panicle it takes 7-10 days for all the spikelets to complete anthesis; the spikelets themselves complete anthesis within 5 days.
• Ripening follows, fertilization, and may be subdivided into milky, dough, yellow-ripe and maturity stages.
• These terms are primarily based on the texture and color of the growing grains.
• The length of ripening varies among varieties from about 15 to 40 days.