In this article we will discuss about the effects of climate on wheat cultivation.
i. Temperature:
The minimum, optimum and maximum temperatures for wheat growth are 3° to 4°C, 25°C and 30° to 32°C, respectively. Therefore, the main climatic feature that determines the type of wheat to be grown is the temperature during winters. In regions of mild winters and warm dry summers, spring wheat is raised.
When winters are moderately cool and dry or when winters are severe but where a snow cover during winter season is likely to be available, winter wheats are raised. When the winters are very severe and/or the ground is not covered with snow, only spring wheats are sown.
All the wheats grown in India, although planted in winter are essentially of spring type. Whenever real winter wheats are grown in India, they generally do not come to flower. If they do, the flowering period coincides with summer leading to severe grain shriveling.
Attempts to grow winter wheats at higher altitudes of northern hills, where the temperatures are low and the growth period longer were not successful. In such areas, the fanner prefers taking a summer crop of wheat instead from the end of May or early June to September or the early October.
Of all the environmental factors, temperature plays the most dominant role to wheat adaptation in India. Both the start and end of potential wheat crop season is limited by the onset and end of favourable temperature regimes. Within the season itself, warmer temperatures shorten the vegetative crop duration.
This accounts for decrease in the crop duration as one proceeds from north to south in the wheat belt. A long duration variety sown early can have its duration reduced by a warmer season, while short duration varieties may have their earning period extended into the warm period on account of colder winter. Only in hill regions of India, wheat has duration longer than 5-6 months.
High temperature in early phase results in poor tillering, sparce vegetative growth and very early heading. High temperature at ripening stage leads to premature ripening. Satisfactory germination of seed is in a mean daily temperature of 20°-25°C. For good tillering, lower temperatures are needed.
Production of fertile tillers will be optimum when mean maximum and minimum temperatures in the tillering phase are 30° and 10°C, respectively. New varieties are more thermosensitive during anthesis and the number of grains per ear decrease if the minimum temperature drops below 7°C. Number of grains per year decrease if the maximum temperatures during anthesis and grain development increase above 32° and 30°C, respectively.
Asana and Saini (1962) reported that high temperatures initially increased the rate of grain filling but reduced the final grain weight. At Delhi, 5 per cent rise in maximum and minimum temperature reduced the test weight by 15 per cent.
Day temperature more than 25°C affects grain formation due to increased respiratory depletion of photosynthates. A mean maximum temperature of 25 °C and a mean minimum of 12°C is considered optimal for grain development.
Influence of temperature on wheat can be summarised as below:
1. Heat sum above base temperature of 10°C during vegetative phase is around 1800 and longer photoperiods reduce this duration.
2. Growth is optimum at mean temperature of 15°-20°C while temperatures above 25°-30°C have an adverse effect.
3. Early vegetative growth can tolerate very low temperature.
4. Moderately warm weather with adequate soil moisture favours optimum root development.
5. High soil temperature during primordial formation reduces the number of spikelets.
6. Frost as well as mean temperature around 30°-35°C during flowering especially in association with low relative humidity (less than 25%) are harmful.
7. Hot, dry or high humidity during ripening is not desirable.
ii. Light Intensity:
Bright sunny weather is important for good wheat yields. At Baroda, bright sunshine hrs of 1 to 4 from November to February resulted in poor crop yield. Variation in crop surface in relation to solar radiation assumes importance after the canopy closes so that the lower leaves receive insufficient light for normal photosynthesis. The overall efficiency of light utilisation by wheat crop increased to 2.2 per cent by early February when the LAI was 5.
Asana and Wattal (1965) concluded that the mechanism of starch synthesis would be affected at low light intensity. Wattal and Asana (1974) noted varietal difference in the capacity to use high light intensities. These studies indicated that the rate of grain development might be determined by other factors besides photosynthetic activity.
At Kharagpur, wheat sown in late October had high seedling mortality while that sown in November suffered due to high tiller mortality and longer vegetative period leading to reduced generative periods on account of high temperature of 35°C at flowering.
It was concluded that the optimal photothermal units above a base temperature of 4.4°C was 22,000 from sowing to maturity made up of about 10,000 units from germination to flowering and about 12,000 from ear emergence to maturity.
iii. Rainfall:
Wheat is grown in the world in rainfall ranges of 300-1130 mm. More than 50 per cent of the area under wheat in the word is in the 360-630 mm range, 15 per cent in areas where rainfall exceeds 880 mm and 10 per cent in areas with less than 380 mm rainfall.
In India, wheat is grown in rainfall ranges of 100 mm as in arid western plains to 4000 mm in humid eastern Himalayan region. Rainfall distribution is not very favourable for the wheat crop. Major rainfall in wheat growing regions in India is received during the monsoon period, which is not directly useful to wheat crop. However, this residual moisture is used to raise un-irrigated wheat crop. It is estimated that around 30 per cent of the wheat in the country is under un-irrigated conditions.
High humidity and rains especially at harvest are hindrance to wheat cultivation due to problems in harvesting and threshing. Such conditions exist in for the eastern region where wheat cultivation is not making progress due to adverse humid conditions. Humid conditions also encourage the incidence of pests and diseases.
In areas where there is a clear separation of rainy season and the winter period is only marginally conducive for wheat, sowing of wheat is taken up as soon as the temperature in the higher ranges become tolerable enough for satisfactory germination even though the crop is exposed to warm temperature in its early spell which in turn shorten the crop duration.
A delay in sowing to take maximum advantage of cool weather leads to poor crop stand on account of decreasing soil moisture in the soil surface zone with increasing time from cessation of time. While wheat can adjust to limited moisture availability through reduced growth, moisture availability in the root zone right up to ear filling is a must. Delayed sowing leading to the reproductive phase occurring at a time of high crop water needs has to be avoided at all costs.
iv. Soils:
Wheat can be grown on a variety of soils, with reasonable drainage and good water retentive capacity. In term of soil pH, wheat is considered medium acid tolerant with permissible range of 5.5 to 7.0. Wheat is also considered to be medium tolerant to soil salinity. Initial yield decline (threshold) due to salinity is reported to be about 6 dS m-1. However, differences in tolerance to soil reaction exist among different cultivars.
Wheat growing areas of India can be divided into five soil divisions:
1. Gangetic alluvium of Uttar Pradesh and Bihar
2. Indus alluvium of Punjab and Haryana
3. Black soils of central and southern India
4. Hilly soils of Himalaya and elsewhere
5. Desert soils of Rajasthan.
Of these five divisions, the first two divisions are most suitable for wheat cultivation.