The growth cycle of the rice plant begins with fertilization and subsequent development of the embryo nucleus. With the introduction of heat and moisture' the embryo germinates and develops into a seedling. Feeding initially on the food supply contained within the grain (endosperm) and later drawing nutrients from the air and soil, the seedling grows into an adult plant which eventually produces flowers and then seeds. The extension agent must be able to recognize and understand the growth stages of rice in order to time management practices properly (e.g. transplanting, irrigation, fertilization, weeding, harvesting).
I. The Vegetative Phase
The vegetative phase begins with germination and ends with panicle initiation. Unlike the reproductive and maturity phases which are of equal duration for all rice varieties, the vegetative phase varies considerably in length and can last from 21 days to over 10 months. The vegetative phase can be divided into four stages:
a) Seedling Stage
The seedling stage begins with the emergence of the radicle and lasts until the onset of tillering, usually a period of 15-30 days depending on seed preparation practices, nursing techniques, nutrient inputs, and climatic conditions. During the early part of the seedling stage the plant's root system undergoes rapid and extensive growth; once the root system is able to draw nutrients from the soil, a substantial growth of the leaf surface follows. In rice nursed for c, transplanting, the appearance of the fourth leaf is generally considered to signal the end of the seedling stage.
b) The Transplanting Stage
The so-called transplanting stage is not really a natural stage at all but represents the 5-10 day period of growth impairment caused by the shock to the seedling of uprooting and transplanting. The transplantig stage can be shortend significantly by gentle handling of the seedlings and by early transplanting (younger plants recover most rapidly from the shock of uprooting because their relatively undeveloped root systems suffer little damage).
c) The Tillering Stage
The tillering stage begins with the appearance of the first tiller or shoot from the auxiliary bud on the lowermost internode. The tillering stage continues on through the formation of secondary and tertiary tillers. The number of tillers produced by a plant varies and is affected by genetic determinants, the availability of nutrients (including water and sunlight), and the general health of the plant. The tittering stage continues up to the point of maximum tillering, at which all effective tillers have been produced (an effective tiller is one which bears a panicle on which the grains will ripen fully).
d) The Photoperiod Sensitive Stage
The photoperiod sensitive stage lasts from the point of maximum tillering until panicle initiation and may vary extremely from one variety to the next (as much as 0-200 days). Photoperiod sensitivity is a natural mechanism based on the plant's ability to distinguish precise differences in daylength/nightlength. Photoperiod sensitivity ensures that the plant will enter its reproductive phase at the optimal time of year, i.e. when temperature and moisture will permit successful reproduction (in temperate zones this is usually in spring or early summer, in the tropics usually during the onset of the seasonal rains). The biological mechanism causing photoperiod sensitivity is quite complex and need not be explained in detail. It is important simply that the extension agent understand the effect of photoperiod sensitivity: namely, -that some varieties should be planted only during: certain time- of year to ensure that prevailing daylength/nightlength conditions will trigger panicle initiation when desired. Consult local authorities for information about which locally grown varieties are photoperiod sensitive.
II. The Reproductive Phase
The reproductive phase includes the period during which the panicle forms and emerges from the base of the tiller. The reproductive phase begins at panicle initiation, when the panicle begins to develop at the end of the last internode deep inside a protective covering of leaf sheaths. The reproductive phase lasts approximately 45 days among all varieties and can be divided into three stages:
a) The Booting Stage (Internode Elongation Stage)
By the time the panicle becomes visible to the naked eye (as a tiny, transparent growth less than 2 mm in length buried within the leaf sheaths near the base of the plant), the booting stage is already underway. During the booting stage, which lasts approximately 15-20 days among all rice varieties, the internodes undergo a rapid growth sport and quickly lengthen (like a radio aerial extending), causing the culm to shoot up from the base of the plant bearing the developing panicle. During this period of rapid growth the plant's demand for nutrients is high, making the early booting stage a crucial time for fertilization. However, it is sometimes difficult to detect panicle initiation and the earliest onset of booting. When in doubt consult the farmer who will often be able to tell when the rice has "become pregnant."
b) The Heading Stake
The booting stage is followed by the emergence of the panicle from the protective flag leaf sheath. The heading stage lasts until 90% of the panicles have emerged from their sheaths - Generally about 10 days in most varieties.
c) The Flowering Stake
The flowering stage begins with the emergence of the first anthers from the 'uppermost spikelets on each panicle.
Each individual spikelet flowers for only several hours daring the middle of the day on two or three successive days.
Flowering begins among the uppermost spikelets and continues for
approximately 15 days regardless of variety as the remaining spikelets
successively open (the lowermost spikelets flowering last). During flowering,
pollen from the anthers is transported by wind and insects to the stigma, which
carry it down into the ovaries where fertilization of the ovules
(Note: Never apply fertilizer or pesticide during periods of active flowering, as the pollenization process is extremely sensitive and can easily be disrupted by the presence of agro-chemicals )
III. The Ripening Phase
The ripening phase begins at fertilization and continue, through grain filling and ripening, approximately 25-35 days regardless of variety. Grain filling occurs as nutrients and water are transported from one part of the plant to another; the process is affected by the availability of water and nutrients, and by temperature. Grain filling and ripening can be broken down into four stages:
a) The Milk Stage
The endosperm first begins to form as a milky liquid. Rice at the milk stage is very susceptible to attack by sucking insect pests.
b) The Dough Stage
The milky liquid begins to solidify into a sticky white pas tot Bird pests generally begin to be a serious problem.
c) The Maturity stage
The grain is mature, or ripe, when the endosperm becomes hard and opaque. While the grains ripen, the leaves of the plant begin to turn yellow as nitrogen is transferred from the leaves to the seed. The full maturity stage is reached when more than 90% of the grains in the panicles have ripened. Mature grains usually undergo a change in color and turn a golden brown, but under wet climatic conditions ripe grains may remain somewhat greenish
d) The Over-ripe Stage
If the grains are not harvested on time, the vegetative parts of the plant - stems, leaves, and roots - begin to die off. Then the over-ripe grains fall off the panicles onto the ground in a process known as shattering. Some rice varieties are particularly susceptible to shattering, and serious crop losses may occur if harvesting is not completed on time. In rare instances, over-ripe grains left too long on the panicle may undergo germination 'varieties exhibiting -this characteristic are said to lack dormancy.
IV. Chart: the growth stages of rice