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Water is the single most important limiting factor for crop production in large areas of the world

By S.M. MUJTABA 
 and              
 S.M. ALAM, NIA
April 01 - 07, 2002

The total geographical area of Pakistan is 79.61 million hectares (mha), of which only 25% or 19.82 million hectares are currently by various crops under cultivation. Out of the total land area of Pakistan, the total land area of Punjab is 20.6 mha, out of which 54% or 11.04 mha are cultivated. The total land area of Sindh is 14.1 mha, out of which nearly 39% or 5.45 mha are cultivated. The total land area of Balochistan is 34.7 mha, out of which only 4% or 1.4 mha are cultivated, and the total land area of NWFP comprises of 10.2 mha, out of which nearly 10% or 1.93 mha are cultivated. Nearly 5 mha, or 24% of the total cultivated area of the country is rain-fed. About 15% of the total cropped area of the Punjab and 54% of the total cultivated area of Sindh, 56% of the NWFP and 55% of Balochistan, 50% of FATA, 94% of Azad Kashmir and a significant part of northern areas are encompassed by rain-fed agriculture. The balk of rain-fed areas constitutes arid and semi-arid lands, which do not receive development priority, as they are considered high risk for agriculture. About 10% of the national wheat production comes from rain-fed agriculture, which is higher than the total irrigated and rain-fed wheat production either in NWFP or Balochistan and is 67 and 14% of the total wheat production in Sindh and Punjab, respectively.

Pakistan falls into arid and semi-arid lands of climatic conditions. Areas receiving a rainfall less than 100 mm to 250 mm to less than 500 mm are called as semi-arid lands. These lands constitute about 88% of the country's total geographic area of 79.6 million hectare (mha). Out of this 40.9 mha are arid lands including 10.5 mha of hopper arid sandy deserts and 29.3 mha (36.8 %) are semi-arid. The agriculture in these arid and semi-arid areas are mainly dependent on the scanty and erratic rainfall. Pakistan is a country of great land with largest irrigation network in the world. The country faces sometimes acute shortage of water for irrigation purpose. This condition is known as drought.

Water is the single most important limiting factor for crop production in large areas of the world. If sufficient water is not available, application of fertilizers and high yielding crop varieties are useless. Water stress (drought) conditions of soils have wide-ranging effects on the morphological and physiological processes of plants. Numerous studies have indicated that water stress reductions in crop growth. It generally causes a decrease in cytokinin transport from roots to shoots and/or an increase in leaf abscisic acid. These changes in hormone balance cause changes in cell wall extensibility and therefore growth. The potential of roots to absorb nutrients generally declines in waterstress plants, presumably because of a decline in nutrient element demand. Concentrations of photosynthetic enzymes decline in response to drought by altering the hormonal balance. There are some evidences that roots are the primary sensors of water deficit, causing the observed physiological or biochemical perturbations in the shoots and decline in growth to be interconnected to changes in plant nutrition. CO2 balance, and water relations and growth pattern.

Drought is a worldwide problem and about 43% of world land are affected to various degree of drought. It is of special interest for Pakistan as almost 15 million hectares of cultivated land is affected by this syndrome. Agricultural production in these areas is primarily dependent on physical factors of climate and soil. These factors collectively determine the land potential, cropping pattern and crop productivity. Drought stress commences when the supply of water can no longer keep pace with the demand, created by the sunlight intercepted and transpiration by the plant leaves. This causes stomata to dose and thus photosynthesis is to be inhibited. When water becomes limiting, the availability of nutrients may also be restricted introducing another component to the phenomenon of drought. Drought is a complex scenario with three main components, (i) timing of occurrence during the season, (ii) duration, and (iii) intensity. These factors vary so widely in nature that it is very difficult to define specific plant attributes required for crop improvement under drought stress conditions.

Wheat, rice, cotton, are the cereal vegetable / fodder crops of Pakistan and grown in almost every part of Pakistan. The average yields of many crops are quite low in such areas, which is mainly due to shortage of water. Water stress not only affects the morphology, but also severely modifies the metabolism of the plant. The extent of the modification depends upon the cultivar, duration and intensity of stress. The physiological and biochemical responses of plant to water stress have long been of interest, mainly because of need to understand better effects on economically important crops like wheat. Crops production have been increased tremendously from 1947 to date, but with the passage of time many constraints were occurred, which affected the overall growth of all the crops of the country. Presently, due to absence of large-scale rain fall, there happened to be the shortage of irrigation water in the country. This situation has resulted in the shortage of both Rabi and Kharif crops. Still the situation is not favourable. However, this situation can be changed substantially with proper planning and by giving top priority to wheat production and mobilizing a vigorous national effort, the looming good situation resulted from water shortage could have been averted.

Under drought conditions the plants, which are able to adopt osmotically tend to survive. Osmotic adjustment results from the accumulation of solutes within cells, which lowers the osmotic potential and helps to maintain turgor or both shoot and roots. Turgor-driven processes such as stomatal opening and expansion of growth thus continue. During stress, the biological parameters of a crop is severely affected, thus alters the positive functions of OC-amylase, protease, reducing and non-reducing sugars, protein and total amino acids, peroxidase activity and many other enzymes such as: glutamine synthetase activity, glutamate synthase activity, glutamate dehydrogenase activity, nitrite reductase, activity, nitrate reductase activity, ribulose, l, 5, bisphosphate, carboxylaxe/oxygenase, DNA and RNA contents, total phenols, proline, betaine, ammonia, nitrate and chlorophyll and carotenoids content.

The productivity of crops plant could be accelerated through studying the nature of adaptation of cereal crops to drought and to find out possible physiological markers, which impart tolerance under these conditions. This is essential for evolving the best-adapted and high yielding wheat variety for these areas. In addition to wheat other crops, which are seriously affected by the shortage of water (drought) are: rice, cotton, sugarcane, maize, guar, sorghum, pearl millet, sugarbeet, numerous vegetable crops, fruit trees, orchard trees, forages, legumes, fodder, etc.

The sum of the total cations generally increased. Dry matter usually decreased faster than total nutrient uptake. It is suggested that growth is more susceptible to water stress than nutrient uptake. Ratios of divalent to monovalent cations were higher for dicotyledonous than for monocotyledonous plant. The uptake of ions from the soil or growth medium was closely related to plant, water content, transpiration or water flow. The effect of water stress was rapid, but damage was not permanent until after extended exposure to the stress. The length of time the plant roots could be exposed without permanent damage depended on the method used to impose the water stress, the land of stress, and the plant species. Although, nutrient eIements were absorbed by plant root systems actively at low water stresses, uptake at high water stresses way by passive mechanisms. This was supported by the leakage of ions and the large changes in the permeability of membranes. The active transport systems were generally damaged at high water stresses, and thus various ions present in the growth medium responded quite differently to crop in the growth systems. Under water stress conditions, the uptake of N decreased in soybean plants. This decline in shoot N uptake can be attributed to the decreased transpiration rate to transport N from roots to shoots. Nitrogen deficiency sensitized cotton plants to water stress, causing the effects of stress to occur at a higher water potential. The high N level under moisture stress has largely been attributed to the proline accumulation in grasses.

Drought stress condition has tremendous negative effects on the nutrients uptake by crop plant. The internal transport of nutrients in crop plants largely depends on the synthesis, utilization and translocation of photosynthesis because the switching over of these processes is generally regulated by nutrient metabolism water stress, in general reduces nutrient uptake by roots and transport from roots to shoots because of restricted transpiration rates and impaired active transport and membrane permeability, resulting in a reduced root-absorbing power of crop plants.

Nutrient uptake from soil solution is closely linked to plant root and soil water status. A decline in soil moisture is associated with a decrease in the diffusion rate of nutrients from the soil matrix to the absorbing root surface. The reduction in uptake and transpiration are usually associated with a reduction in the water content of the shoots and stomatal aperture, because that water stress has developed in the leaves. Maximum water uptake occurs in young roots, but continued aging of the roots after cessation of growth would result in a reduction in the root permeability to water and nutrients. Water and nutrient uptake due to reduced root permeability causes a disturbance in root metabolism. Changes in the soil moisture regimen can alter root morphology and anatomy, pore size distribution, and the angle of root penetration, which affect root proliferation. Mineral uptake decreased when water stress increased.

Plant species, and genotypes within species, vary in their responses to mineral uptake. These variations may explain some of the differences observed that are attributed to the reduced availability of nutrients, especially within the root zone under water stress conditions. It is suggested that nutrient uptake under water stress condition is influenced by the capacity of the root to absorb nutrients. However, the transport of nutrient to the shoots may occur even at a reduced transpiration rate. The slower growth rate under moisture stress prevents the dilution effect of nutrients. Although, water and nutrient uptake are independent phenomena, the transpirational equilibrium under irrigation favours the leaching of soluble forms of nitrogen and phosphorus. Therefore, water acts as a vector for nutrient transport within the system.

The uptake of mineral elements was maximum when the water potential was near that of field capacity, or about 0.3 bar. When water was increased to saturation or to very low stresses, the uptake of mineral elements decreased. This might be expected, since oxygen would be limited in water saturated soil and limited metabolic energy would be available for nutrient uptake. The reduction in nutrient uptake by plants under water stress, mineral elements in many range and forage species generally increased with water stress. Water stress favoured an increase in Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na) and Chloride (Cl) and decrease in Phosphorus (P) and Iron (Fe).

The common opinion that the high N level of plants subjected to drought is due to the fast accumulation of free amino acids that are not incorporated into proteins. The relative decrease in nitrogen accumulation as a function of increased water stress was less than the decrease in dry matter yield and the decrease in the accumulation of other nutrients. In water stress conditions decreased phosphorus concentration in crop plants and that water stress did not effect the phosphorus status of plants, but decrease and increase in mineral element concentrations with increased water stress. It is general that the uptake of phosphorus by plants is reduced in dry soil. However, that phosphorus absorption was not affected by increasing water stress in wheat, that the plants do not damage by witting, plants subjected to water stress, the rate of phosphorus uptake decreased in water stress conditions.

The reduction is the relative uptake of phosphorus by maize seedlings nearly linear with the soil moisture stress. The values for relative phosphorus uptake under stress are 100, 94, 80, 54 and 35 % of the control for the water stress levels of-0.3, -0.5, -1, -3 and -9 bar, respectively. The amount of phosphorus absorbed by plants was directly proportional to the amount of water applied to the soil surface. The rate of phosphorus absorption by plants not affected by increasing water stress until the plants were subjected to wilting, absorption then decreased markedly. Phosphorus uptake depends upon the soil phosphorus level as much as on the magnitude of water stress, and phosphorus deficiency appears to be one of the earliest effects of mild to moderate levels of water stress in soil grown plants, and the absorption of phosphate is severely inhibited when water stresses increased to -12 and -15 bar.

Under water stress then uptake of potassium and calcium by maize plants increased under such condition potassium is absorbed in preference to other nutrients. Drought - tolerant wheat varieties can accumulate more potassium than the other susceptible varieties, and plants well supplied with potassium had a higher stomatal resistance, which resulted in a low transpiration rate. It is observed that when increased water potential of wheat plants with increased potassium fertilization. Greater efficiency of potassium uptake associated with soil moisture, that soil moisture influences both the diffusion of potassium in the soil and plant root growth. The uptake of potassium totally depends on the soil moisture. The relative amount of potassium, calcium and magnesium increased considerably more in barley than in rye, when water stresses were imposed.

Moisture stress induces definite increase in the nitrogen level, a decrease in the phosphorus level, and variable effects on the potassium level. In all the plants, however, a definite reduction in the potassium level with moisture stress was also observed in addition to the increase in the nitrogen and the decrease in the phosphorus level. The nitrogen and phosphorus were decreased relatively early in the drying cycle and preceded water stress effects on biomass yield. It was concluded that the drought stress conditions substantially affect the growth and development of plants and ultimately the nutrient's uptake.