Wheat is the leading food grain of Pakistan and the staple diet of people. It occupies a central position in agriculture. The contribution of wheat to agriculture is 10.3 per cent and GDP is 2.2 per cent. The area of wheat cultivation is increase upto 4.4 per cent from last year and its yield also increased. The wheat production is enhanced due to timely rainfall at regular interval and favorable weather condition which is suitable for grain health (Pakistan Economic Survey, 2013-14). Very often the shortage of wheat supply is experienced in the country and the government is forced to import wheat by spending a huge amount of foreign exchange. Thus, it is imperative to enhance domestic production of wheat in order to ensure supply of wheat, especially in the face of increasing population of the country.
Many factors are responsible for low wheat yield in Pakistan, but limited water supply due to shortage of irrigation water or frequent droughts in the country contributes impressively in this regard. The canal head withdrawals have decreased in Kharif 2008 to 66.93 MAF as compared to 70.78 MAF during the same period last year, a reduction of about 5 per cent. The decreasing trend in seasonal rains is another important factor affecting wheat production in rain-fed as well as irrigated areas. Moreover, inefficient use of water is another notorious phenomenon in irrigation system of Pakistan and a significant portion of water is lost before it reaches to the farmers fields.
Water stress experienced by a wheat crop during growth is known to have cumulative effects expressed as a reduction in total biomass compared to well water potential. Decreased growth rate was caused primarily by reduction in radiation use efficiency when drought isat various growth stages such as tillering, booting, earing, anthesis and grain development stages. Better performance of crop depends upon availability of water during these stages. The water needed for a crop is related to moisture sensitive periods defined as “certain development phases in which the plant is, or appears by its observed response, to be more sensitive to moisture conditions than at other stages of development.”
Water stress at anthesis stage reduces pollination and thus less number of grains per spike are formed which results in reduction of grain yield. The rate of photosynthesis and translocation of carbohydrates to grains is increased if adequate water is available at or after anthesis period which enhances grain size and thereby lead to increased grain yield. Highest reduction in the grain yield occurs when plants are water stressed for 70 days after anthesis in wheat plants subjected to water stress for 15 days after germination, 25 days after the onset of tillering, 50 days after boot and 70 days after anthesis. Dry matter accumulation decreases with an increase in the water deficit. The intermittent stress is more harmful than continuous water stress and stress at heading (45-60 DAS) affected dry matter production more than stress at 15-30 DAS. Irrigation enhances the total dry matter (TDM) and mean crop growth rate (CGR) over the non-irrigated crops. The fully irrigated treatment show more LAI which leads to more yield of the crop. The water stress conditions resulted in the considerable decrease of the crop yield in durum wheat at different growth stages. The water stress condition significantly decreases the shoot dry weight in four wheat cultivars evaluated at different water regimes at different growth stages.
The grain yield is reduced by 28 per cent by the stress from tillering to anthesis as compared to unstressed treatment in irrigated wheat crops. The grain yield is reduced 36 per cent by the stress from booting to grain filling. The grain yield is reduced 16 per cent by the stress during grain filling. The undesirable response may be remedied by selection for varieties which are less sensitive to soil moisture deficit at grain filling. The grain weight and number of grains per spike are reduced by the stress that reduced the grain yield by 44 per cent in wheat. There is significant reduction in the above ground biomass, grain and straw yield per plant. The yield components reduces when stress is applied at seedling, booting and earing stage in wheat cultivars at various critical stages of plant growth. The plants under stress mature a week earlier at earing stage whereas stress applied at seedling and booting stage delays the maturity by one week compared with control. Booting stage is found to be critical for grain yield.
The identification of moisture stress periods for wheat crop would be an important implication for irrigation practices. Optimization of crop yields especially of wheat through development of water conservation facilities, better soil and water management and selection and development of drought tolerant crop varieties, therefore, are the potential approaches for enhances grain production, especially of wheat under water-limited conditions.