INTRODUCTION
Rice is the principal food for half of mankind. The people of Asia produce and eat 90% of all rice grown. To countless millions, rice is the center of their economic existence – it is life itself
The gap between research-predicted yield and actual field harvest is so big that, even with the advent of the Green Revolution, many Asian countries are not even self-sufficient in this basic commodity (Chandler 1979). About 3.3 billion people in the whole world eat rice (The Philippine Star 2007) and rice technology has to reach farmers in a timely fashion to improve productivity. Another big problem is the lack of logistics to transfer technology from research institutions to farmers’ fields.
Results of research carried out in developing countries are not always efficiently communicated to the scientific community. Much of the vital information does not reach the extension workers, the farmers, and others because there are not enough people trained to interpret, simplify, and popularize these findings. If this situation of low productivity is to improve, the flow of knowledge from research institutions to farmers should be hastened. This process can be facilitated with the development of a simple-to-understand manual on growing rice (Gorrez and Golden 1966).
THE RICE PLANT
The rice plant is an annual grass with round, hollow, jointed culms, rather flat leaves directly attached to the nodes of the stem, and terminal panicles (Chang and Bardenas 1965). It can grow in both irrigated and non-irrigated conditions.
Parts of the rice plant
The parts of the rice plant may be grouped into two: vegetative and reproductive (Vergara 1970, 1979). The vegetative parts of the rice plant include the root, stems, and leaves. The reproductive parts are the panicles.
Roots
The roots are responsible for the uptake of nutrients and water from the soil. If the downward movement of water happens freely and fast or if the soil is dry, the roots deve-lop downward easily, de-pending on the depth and type of topsoil. The deep-er the roots, the better the water- and nutrient- ab-sorbing capacity of the plant. This very important plant characteristic is useful in areas where there is unreliable water supply. The roots of rain-fed rice tend to extend downward, seeking water in the deeper part of the soil where there may be more moisture.
Having water at or even less than saturation point when direct seeding rice results in the downward development of strong root systems into the soil. With seeding on water (2-5 cm), ample germinating seedlings will be floating on the water with weaker roots anchoring into the soil.
Stem
The lower internode at the base of the stem is short, thick, and solid. The primary tillers grow from the lowermost nodes and give rise to secondary tillers, which, in turn, give rise to a third group of tertiary tillers. All tillers at later stages of growth are independent plants, since each produces its own root system. They can be separated when there is a need to replant missing hills, even 1-2 weeks after transplanting.
Leaves
The leaves of the rice plant can be distinguished from other grasses by the presence of both ligules and auricles, while the common weed species Echinochloa normally has neither. Generally, a grass leaf may have either a ligule or an auricle or none of both.
Varieties differ in the number of leaves produced. In early-maturing varieties with 14 leaves, the longest leaf is always the fifth from the flagleaf. This is fully developed before panicle formation starts. The succeeding leaves are similarly developed since there is competition for food between the leaves and the developing panicles. At the time of flowering, the first 10 older leaves are, by then, usually dead and only four to five leaves remain to support the plant to maturity. Green leaves at harvest time results in fully filled-up and healthy grains.
Spikelet
The spikelet (grain) develops after pollination and fertilization is completed, thus initiating the formation of the embryo and the endosperm. The embryo contains the embryonic leaves (plumule) and the embryonic roots (radicles). The endosperm consists mostly of starch granule, sugar, fats, crude fiber, and inorganic matter. It supplies the nutrients, which the seedling uses during the first few weeks of life. The dehulled grain (caryopsis), known commercially as broken rice, has a brownish pericarp. The pericarp is the outermost layer, which envelops the caryopsis. It is removed completely when rice is milled and polished.
Panicle
The reproductive part or the panicle is actually a group of spikelets (grains) borne on the uppermost node of the stem. Panicle initiation occurs and is usually fixed at 60-65 days from maturity. The young pani-cle, visible to the naked eye, is 1 mm long, with fine, white, hairy structures at the tip. Booting follows immediately after the start of panicle initiation stage, extending up to 30-35 days. The emergence of the panicle at the highest node and flagleaf known as flowering stage would mature in 25-30 days.
Growth stages of the rice plant
The entire life cycle of the rice plant may be divided into three phases (Chang and Bardenas 1965). But there may be up to nine phases, depending on the plant variety. The high-yield-ing non-seasonal varieties that mature between 100 and 120 days usually do not have the lag-vegetative phase. The illustration showing the nine growth stages can provide guidance in implementing specific field activities at the most appropriate time.
- The active-vegetative phase covers stages from seed germination to maximum tillering. During this phase, tiller number, height, and straw weight increase continually. The seedling stage includes the period of seedling emergence from the soil until just before the appearance of the first tiller. After the emergence of primary tillers, secondary tillers begin to form, and, as the plant becomes taller and larger, new tertiary tillers start to appear until the maximum tillering stage is reached.
- The lag-vegetative phase, also known as the photoperiod-sensitive phase, is from maximum tillering to panicle initiation stage. The plant is markedly affected by day-length and temperature, both of which can either shorten or lengthen crop growth duration. Traditional and late-maturing varieties, like those grown before the Green Revolution, have a photoperiod-sensitive phase. Early-maturing and non-seasonal varieties usually have a very short or no photoperiod-sensitive phase.
- The reproductive phase is from panicle initiation stage to flowering stage. It begins just before or immediately after the maximum tillering stage and is marked by the panicle primordial development of microscopic dimension inside the growing shoot. The enveloping sheath of the flagleaf swells about 15 days after panicle initiation stage and this is followed by the emergence of the panicle (flowering or heading stage) from the flagleaf sheath. Flowering occurs 25 days after panicle initiation, regardless of variety.
- The ripening phase starts at the flowering stage and ends at full maturity. The ripening period is characterized by grain development, with the grain increasing in size and weight, along with changes in grain color and aging of leaves. In the tropics, ripening takes place in 25-30 days, regardless of variety. The grain undergoes three distinct changes before it fully matures.
The panicles are in the milk stage when the starchy portions of the grain are watery and later turn milky in consistency; they are in the dough stage when the milky caryopsis or starchy portion of the grain turns into soft dough and later into hard dough stage. In the mature grain stage, the caryopsis is fully developed in size, hard, has changed from green to brown and at harvest, has a moisture content of 26%.
Duration of each growth phase
The duration of the repro-ductive and ripening phases is almost the same for all varieties (Vergara 1970). Differences in growth dura-tion from seeding to maturity, however, are determined by the number of days in the vegetative phase. These differences can be observed between non-seasonal varie-ties with fixed vegetative phase and seasonal, long maturing varieties with an increased or decreased vege-tative phase.
With short maturing 100-120-day varieties, there is almost no lag-vegetative phase. The active vegetative and reproductive phases of-ten overlap. It takes 35 days from panicle initiation to flowering and 30 days from flowering to maturity. The changes in length of the vegetative phase determine the differences in growth duration among varieties.
Day-length and plant types
Daylength is the duration of the light period during a given day. It is also defined as the interval between sunrise and sunset, known as photoperiod. Thus, varieties with growth duration increasing or decreas-ing in relation to photoperiod are called photoperiod-sensi-tive varieties. Rice is generally considered a short-day plant in the sense that short daylength decreases its growth period. Sunlight influences plant growth in two ways: light duration directly affects the entire development of the plant (photoperiodism), and light intensity affects the manufacture of carbohydrates (photosynthesis). There are varieties with fixed growth duration, regardless of daylength and season in which they are planted. These are the newer non-seasonal high-yielding varieties (HYVs) developed at IRRI. The rice plants have short stature, erect leaves, lodging resistance, good tillering capacity, pest and disease resistance, and high yield. The new HYVs mature in 100-140 days, have early vegetative vigor, have high tillering ability, are short (100 cm), possess erect leaves and sturdy stems that resist lodging, are resistant to pests and diseases, non-seasonal, and are high yielders.
On the other hand, rice plants whose growth duration is affected greatly by daylength (seasonal varieties), are traditionally tall, leafy, and susceptible to lodging. They have profuse tillering, late maturity, and low yield. The indica varieties grown for centuries in tropical Asia belong to this group. Scientists at the International Rice Research Institute, the Philippine Rice Research Institute, and the University of the Philippines Los Baños have been successful in developing nitrogen-responsive rice varieties from traditional indica varieties by crossing them with japonica and other plant types. There is no shortage of HYVs; the problem lies in testing and evaluating these materials in farmers’ fields.