Cultivated rice is generally considered a
semi-aquatic 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 5 m in some floating rices. The
morphology of rice is divided into the vegetative phases (including
germination, seedling, and tillering stages) and the reproductive phases
(including panicle initiation and heading stages).
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.
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 40oC. 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 eoleoptile emerges it splits and the primary leaf
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 claw like
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
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 80 cm. Most rice varieties reach a maximum depth of 1 m or deeper
in soft upland soils. In flooded soils, however, rice roots seldom
exceed a depth of 40 cm. That is largely a consequence of limited 02
diffusion through the gas spaces of roots (aerenchyma) to supply the
growing root tips.
Panicle and Spikelet:
The major structures of the panicle 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 plant 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, i.e., panicle exertion. The time after
heading is better known as the ripening period (see chart below).
Potential grain yield is primarily determined before heading. Ultimate
yield, which is based on the amount of starch that fills the spikelet,
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 d in the vegetative
phase, 30 d in the reproductive phase, and 30 d in the ripening phase.
The vegetative phase is characterized by active tillering, gradual
increase in plant height, and leaf emergence at regular intervals.
Tillers that do not 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 unfavorable conditions.
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 d before heading when the
panicle has grown to about 1 mm long and can be recognized visually or
under magnification following stem dissection. Spikelet anthesis (or
flowering) begins with panicle exertion (heading), or on the following
day. Consequently, heading is considered a synonym for anthesis in rice.
It takes 10- 14 d for a rice crop to complete heading because there is
variation in panicle exertion 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 exerted.
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 d for all the
spikelets to complete anthesis; the spikelets themselves complete
anthesis within 5 d. 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 d. Ripening is also affected by temperature with ranges from about 30
d in the tropics to 65 d in cool, temperate regions such as Hokkaido,
Japan, and Yanco, NSW, Australia.
Pests, diseases, and weeds of rice:
Many species of organisms inhabit rice fields. Most of these organisms
are not harmful. For exarnple, some 500 species of arthropods (insects
and spiders) may appear in a rice field in a given season, but only a
very few are a potential treat. Most are beneficial or innocent, and
include a wide range of predatory and parasitic natural enemies that
contribute to keeping the insect pest organisms in check. Weeds are an
almost universal companion of rice in the tropics. In many situations,
weed growth ins prolific and weeds are a major constraint on crop yield.
The direct loss in rice production due to weeds in farmers fields in
Asia is reported to average about 20%, with loses reaching 40-100% where
weeds are not controlled. Weeding is a major production cost, with
estimates of 50-15o person days per hectare required for manual weeding,
depending on the number of weedings and type of rice culture. For many
farmers, weeding requires the greatest labor input during the
agricultural cycle, labor that is often not available when weeds are
most damaging to the crop. Upland rice more than any other crop shows
the ravages of lack of proper weeding. Sometimes, when the land is too
weedy the crop is abandoned.
The demands of transplanting and manual weeding, and
increasing shortages of labor, have encouraged the move to direct
seeding in irrigated and rainfed lowlands. Weeds become major issue in
these systems because rice and weeds emerge at the same time, and weed
control by flooding is difficult in seedling rice. Herbicides are being
used more to control weeds, and herbicide-resistant weeds and pollution
are emerging issues in direct-seeded systems. Other species of
potentially harmful organisms include insects, pathogens, molluscs, and
rodents. Some are herbivores that feed on the rice plant, others are
parasitic disease organisms. Although only a few pest species cause
sufficient yield reductions to require intensive control measures, they
are frequently cited as major constraints to rice production. Potential
losses of up to 55% before harvest have been estimated, but these
estimates often represent the worst case or highest levels of loss.
Actual losses are much lower. There have been serious
outbreaks of insects such as brown plant hopper (BPH) and diseases such
as blast, tungro virus, and sheath blight over large areas. But
considering the myriad opportunities for pest-crop interactions, such
outbreaks can be considered as rare events. When these outbreaks occur,
however, farmers suffer substantial economic losses. For instance, in
1975 the BPH infested nearly 80% of the rice fields in the Republic of
Korea. Crop loss in affected fields reached 20-30%. BPH destroyed
200,000 ha of rice between 1975 and 1980. In Indonesia, a virus disease
destroyed 100,000 ha between 1972 and 1975 in South Sulawesi alone. In
the early l900s, rice field insects and diseases caused widespread
famine in Japan and China. Calamities such as these encouraged crop
protection approaches that emphasized eradication and prevention. Those
approaches caused the use of agricultural pesticides to increase
dramatically. Although insect pest and disease resistance genes have
been introduced into rice cultivars, pesticide use has not declined.
Pesticides are often uneconomical and they may disrupt the ecological
balance of pests and their natural enemies. BPH outbreaks are often a
direct consequence of insecticide use, which wipes out predators that
regulate the BPH and sometimes stimulates increased fecundity in
surviving BPH females. Besides often causing the pest problems,
agricultural chemicals may pose a serious threat to humans and the
Modern approaches to crop protection rely on
management rather than control or eradication. In this approach, a pest
species is considered a pest only when it reaches numbers that can cause
yield reduction. Natural factors — such as natural enemies — that
prevent a pest species from increasing are emphasized. Pesticides are
used only as a last resort to bring abnormal pest densities down when
crop loss is expected to exceed the cost of treatment. In addition, the
use of rice cultivars that are resistant to major pest species is
encouraged. These cultivars do not need prophylactic treatment to
control the insects or diseases to which they are resistant. Using a
combination of control tactics instead of relying on just one tactic
such as host plant resistance or pesticides — and basing the decisions
for control on sound economic grounds is called integrated pest
management or IPM. More sustainable integrated weed management (IWM)
technology is also being developed through the investigation and
promotion of weed competitiveness in rice biological control, rice
allelopathy, and a thorough understanding of the biology, ecology and
socio-economics of the major weeds in various rice ecosystems. IWM,
promoting biological and cultural control and minimizing use of
herbicides is seen as a key to sustainable rice farming systems.
Stem borers; yellow stem borer Scirpophaga Incertulas (Walker);
white stem borer Scirpophaga innofata (Walker); striped stem
borer Chilo suppressalis (Walker); darkheaded rice borer Chilo
polychrysus (Meyrick); defoliators; rice leaffolders Cnaphalocrocis
medinalis (Guen,e) and others; rice caseworm Nymphula depunctalis
(Guen,e); leafhoppers; green leaf hopper Nephotettix virescens
(Distant); N. nigropictus (Sttl.); N. parvus Ishihara et
Kawase; N. cincticeps (Uhler); planthoppers; brown planthopper Nilaparvata
lugens (Sttl.); whitebacked planthopper Sogatella furcifera Horvath;
rice bugs; malayan black rice bug Scotinophara coarctata (Fabricius);
rice grain bug Leptocorisa oratorius (Fabricius).
Rice field rats Rattus rattus argentiventer (Rob & Kloss); R.
r. mindanensis Mearns.
Diseases of rice:
Viral diseases and their vectors; rice tungro Nephotettix virescens
(Distant); N. nigropictus (Sttl.); ragged stunt Nilaparvata lugens (Sttl.);
L. acuta (Thunberg); bacterial diseases and their causal agents;
bacterial blight Xanthomonas oryzae pv. oryzae, fungal diseases
and their causal agents; blast pyricularia oryzae Cav.; sheath blight Rhizoctonia
solani (Thanatephorus cacumeris) [Frank] Donk);
Ageratum conyzoides L.; Cyperus difformis L.; Cyperus iria L.;
E:chinochloa colona (L.) Link; Echinochloa crus-galli (L.)
P. Beauv.; Fimbristylis miliacea (L.) Vahl.; Ischaemum
rugosum Salisb; Monochoria vaginalis Burm.f Presl.