Agronomically, it is convenient to divide the life cycle of rice in terms of three phases – vegetative, reproductive and ripening phase. Development in each phase is further divided according to 0-9 numerical scale to identify the growth stages of a rice plant. Each number in the scale corresponds to a specific growth stage.
Life cycle of rice plant is, generally, completed within the range of 100-210 days, with the mode falling between 90 and 150 days. In temperate countries, the average duration from sowing to harvest is about 130-150 days. Varieties with growth duration of 150-210 days are usually photoperiod sensitive and planted in deep water areas.
From germination until harvest, crop plant develops through different growth stages, which are determined by environmental factors, cultivars and cultural practices. Knowledge of these growth stages is useful in deciding the right time for cultural operations such as topdressing of nitrogen and for plant protection measures.
Identification of growth stages is also useful in many physiological studies to identify the critical stages in the life cycle, which are sensitive to environmental factors (such as moisture sensitive stages) to take necessary measures for improving crop yields.
The most common method for identifying growth stages in cereals is known as Feekes scale, which is based on external appearance of the plant or plant organs. The Feekes scale was improved by Zadoks (1974) and later reproduced by Tottman (1987).
Therefore, three growth phases consist of a series of 10 distinct stages, as indicated in Table 1.2. These growth stages are based on data and characteristics of IR 64, a modem, a high yielding, semi-dwarf variety but apply, generally, to other rice varieties.
i. Vegetative:
It refers to a period from germination to the initiation of panicle primordia. It is characterised by active tillering, gradual increase in plant height and leaf emergence at regular intervals. Tillering may start when the main culm develops fifth or sixth leaf.
Active tillering refers to a stage when tillering rate (increase in tiller number per unit of time) is high. When tiller number per plant or unit area is maximum, the state is called maximum tillering stage, before or after the initiation of panicle primordia depending on the growth duration of the variety. Tillers developed after this stage may or may not produce panicles.
ii. Reproductive:
It is characterised by culm elongation, decline in tiller number, emergence of flag leaf, booting, heading and flowering. Initiation of panicle primordia usually dates back to about 30 days before heading. Internode elongation usually begins around the initiation of panicle primordia and continues until heading. Panicle exsertation is called heading. Spikelet anthesis (flowering) begins panicle exertion or on the following day.
Consequently, heading is considered synonym for anthesis. It takes 10-14 days for a crop to complete heading due to variation in panicle exertion within tillers of the same plant and between plants in the same field. Agronomically, 50 per cent of the panicle exsertation is called heading.
Anthesis, normally, occurs between 0800 and 1300 hrs in tropical environment and fertilisation completed within 5-6 hrs later. Within the same panicle, it takes 7-10 days for all the spikelets to complete anthesis.
iii. Ripening:
Ripening follows fertilisation, which may be subdivided into milky, dough, yellow-ripe and maturity stages. It is characterised by leaf senescence and grain growth: increase in grain size and weight and changes in grain colour. The length of ripening, largely affected by temperature, ranges from about 30 days in tropics to 60 days in cool temperate regions such as Japan, New South Wales and Australia.
Differences in growth duration are primarily due to differences in length of the vegetative growth stage. Length of the reproductive stage plus the ripening period may be considered about the same for any variety under a particular environment. Early maturing varieties have short vegetative stages. Hence, they may initiate panicle primordia before the maximum tiller stage and heading may be staggered as later tillers may also produce panicles.
Late maturing varieties have long periods of vegetative stage and may reach maximum tillering stage before initiation of panicle primordia. The period from maximum tillering stage to initiation of panicle primordia is also referred to as vegetative- lag phase.
Direct seeded rice normally starts tillering earlier than transplanted rice. However, it produces 2-5 tillers as against 10-30 in transplanted crop. Growth duration of the same variety may be shorter by about a week under direct seeding than transplanted crop.