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Climatic Requirement

Climatic requirements:

 

Rice crop is best suited to tropical and sub-tropical humid climate but it is grown in variety of climate except extreme cold temperate. The climatic factors that affect rice production are temperature, day length and humidity. These are known to influence the total yield and area under Sikkim. Sikkim has a sub tropical, humid and wet Himalayan climate. The positive aspects of Sikkim climate in terms of rice production are:

The moderate temperature limits (12° C to 24° C).
Assured rainfall through the growth phase.
Soil with high organic matter (about 2%).
At the same time the negative aspects of Sikkim climate for rice production are:

Low sunshine hours and high humidity prolong crop duration.
Lack of clear dry spell during of ripening phase.
Excess rainfall during flowering period.
Effect of Himalayan cool climate at the later stage of plant growth.
Temperature
The atmospheric temperature has considerable effect on growth and development of rice plant. Rice needs relatively high temperature for their optimum growth and development. Temperature requirement of rice is different for different growth stages. Rice crop can be grown successfully where average air temperature is 210C or more than 210C for 5 to 6 months. The critical mean temperature for flowering and fertilization ranges from 16 to 20°C. For vegetative growth a temperature range of 25 to 30 0C and for grain filling and ripening 20 to 250C temperature was reported best. High temperature especially during night leads to loss of reserved food through greater respiration. For higher grain yield a day temperature of 25 to 32°C and night temperature of 15 to 20°C is preferable. Temperature beyond 35°C affects not only pollen shedding but also grain filling. A higher mean temperature ranging between 25 to 32°C per day would reduce the growth duration and accelerate flowering whereas a mean temperature of less than 15°C would slow during vegetative growth and plants fail to flowers. Therefore, for vigorous vegetative growth moderately high temperature is required. It is well known that mild temperature of night and clear sunny weather during day time is better for high yield of rice, but temperature less than 150C is not conducive for panicle initiation as well as for crop growth.

Under Sikkim condition rice can be successfully cultivated at or above 20°C temperature which is the average day temperature during kharif in Sikkim. Optimum temperatures for different crop stages are noted below:

Critical temperature of rice crop for different growth stages (Yoshida, 1978):

 

 

Growth Stages    Critical temperature (o C)
     Low    High    Optimum
Germination    16-19    45    18-40
Seedling emergence and establishment    12-35    35    25-30
Rooting    16    35    25-28
Leaf elongation    7-12    45    31
Tillering    9-16    33    25-31
Panicle initiation    15    -    -
Panicle differentiation    15-20    30    -
Anthesis    22    35-36    30-33
Ripening    12-18    >30    20-29
 
Solar radiation:
Clear sunny weather during ripening and moist-humid during vegetative phase is desirable for rice crop. Low solar radiation would hamper ripening of grains and would increase chaff production enormously.

Sunshine hours:
Rice crop prefers brighter and prolonged sunshine for enhanced photosynthetic activity and higher yield. So, it should receive more than 300 sunshine hours during the last 45 days before harvest.

Humidity:
Moist humid weather during vegetative growth and dry-sunny weather during ripening is most desirable. A relative humidity of 60-80 % is said to be optimum.

Rainfall:
Rice crop requires about 1400 to 1800 mm water. Therefore, if this much rain occurs during crop season with well distribution will be sufficient for crop.

Wind:
High wind velocity or storm is not desirable, as it cause lodging or shattering depending upon the crop growth stage. Gentle wind is the best for rice cultivati