MORPHOLOGY AND RICE GROWTH
Modern approaches to crop protection rely on management rather than control or eradication
By Dr. S.M. ALAM, NIA
July 22 - 28, 2002
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).
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 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 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 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 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 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.
Growth: 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.
Vegetative 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.
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 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 environment.
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.
Insect pests: 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).
Rodents: 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);
Weeds: 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.