Presented at the
AFA 9th International Annual Conference
Cairo, Egypt, 28-30 January 2003
Crop insurance against stress
with adequate potash
- Potassium has an unique role in plant development and yield formation
- Relating the function of potassium in plants with stress management
- What are the conclusions for nutrient management?
It is difficult to see any step in yield creating processes that are not influenced, directly or indirectly, by K because of its role in enzyme function and the creation of cell turgor responsible for the movement of water and solutes in the plants as the following examples show.
Fig. 2. K nutrition and nutrient cycling in plants
First: K plays a role in the transfer of nitrate from the roots to the shoots and leaves. Without adequate K, nitrate accumulates in the roots and a feedback mechanism to the root cells stops further nitrate uptake (Figure 2). Consequently, nitrate remains in soil at risk to loss to the environment, either when leached into surface and groundwater or denitrified and lost to the atmosphere as nitrogen gas or nitrous oxide, a greenhouse gas. In other words, too little K decreases the efficiency with which N fertilizers are used, a financial cost to the farmer leading to possible adverse environmental effects and a misuse of a natural resource in producing fertilizers.
Fig. 3. N metabolism of tobacco plants as affected by the K supply
Second: Nitrate in the plant is reduced first to amines and then incorporated into amino acids to ultimately form protein. With an inadequate K supply, the plant not only absorbs less nitrate from the soil solution, but less is converted into protein. Consequently, nitrate accumulates in the shoot and the protein content remains low (Figure 3). Both factors, nitrate and protein content, are important contributors to crop quality.
Third: Potassium plays a major role in the transport of assimilates from the leaves to the roots for them to function properly and also to storage organs like tubers and grains. Thus, plants with too little K because of an inadequate supply retain most of the low molecular assimilates, predominately sugars, in the leaves and shoots. For example K deficient bean plants contained 76 mg glucose/g DM in the leaves and exported only 1.6 mg in 8 hrs, whereas plants adequately supplied with K contained only 12 mg glucose/g DM because glucose was exported with a rate of 3.4 mg in 8 hrs (CAKMAK, 2002). As a consequence of the accumulation of low molecular weight carbohydrates in the leaves of plants with an inadequate K supply, it substantially decreases the efficiency of the conversion of sun energy into assimilates and excessive electrons released from the chloroplasts lead to the formation of highly toxic oxygen radicals (CAKMAK, 2002). Furthermore, lack of an adequate supply of carbohydrates to the roots restricts their growth and thus, the spatial exploitation of the soil for plant nutrients, especially phosphorus (P) and K. A shortage of carbohydrate for the same reason also leads to less nodulation and a lower efficiency of biological N fixation in leguminous plants like soybeans, faba beans or clover.
Forth: Energy conversion, carbohydrate formation and translocation, N metabolism and other metabolic processes in plants are controlled by enzyme systems, some 50 of which are activated by K. This again points to the versatile and multiple role, K plays in plants.
Fifth: In order to function properly, the metabolic processes require an appropriate hydration and acidity of plant cells. Potassium again is involved in both processes. It helps to adjust the osmotic potential in plant cells in the roots, to enable them to absorb water from the soil. In the whole plant, K activates the movement of water and solutes, it regulates the stomatae opening and thus, intake of carbon dioxide, the plants’ carbon source, and the loss of water to the atmosphere; it affects the pH of plant cells.
Biotic stress. Biotic stress is concerned with pests and diseases: As indicated above, plants with less than optimum K accumulate low molecular weight assimilates, i.e. sugars and amino acids in the leaves. Both groups of compounds are ideal food for invading parasites as summarized in IPI-Research Topic No. 3, “Potassium and Plant Health” (PERRENOUD, 1990). Numerous experiments have proved that the susceptibility of plants to pests and parasites increases sharply when their K supply is limited because of excessive amounts of low molecular weight assimilates in the leaves and shoots.
Furthermore, fissures, lesions, necrotic tissue, which are typical symptoms of K deficiency, give easy access to invading parasites.
Fig. 4. Pest incidences of soybean as affected by the K supply
An inadequate supply of K is commonly accompanied by an excessive supply of N. The very wide NK ratio of 1:0.1 in the West Asia/North Africa region (WANA) is a good example of the unfortunate practice of unbalanced fertilization. With excessive N and inadequate K, plant tissue is soft and offers little resistance to penetrating and/or chewing insects and parasites (Figure 4). Lodging of cereals and sugar cane can occur with unbalanced fertilization and a lodged crop provides an ideal micro climate for the build up of parasite populations to levels high enough for a successful attack on the host plants.
The yellowish discoloration of plants suffering from K deficiency is a kind of visual signal to attract aphids. A soft surface of the tissue with excessive N, assists aphids to penetrate the leaf surface and not only compete for assimilates but transmit viruses at the same time. Wilting plants, which is another indicator of K deficiency also attracts pests.
An excessive N supply with unbalanced fertilization requires carbon to metabolise it and this leaves little C from the Krebs cycle for synthesis of secondary compounds such as phenols and quinones. Those phenolic compounds play an important role in the host/pathogen relationship being the basis for many defence mechanisms. They act as phytoalexins or as precursors of lignin and suberin which act as mechanical barriers in leaves and stems against pest and insect attack.
Abiotic stress – drought: As indicated earlier, plants growing without an adequate K supply wilt quickly when there is insufficient water because of inadequate control of the stomatal opening and closing. Associated with drought stress the rate of photosynthesis decreases and, the lower the K supply the stronger is the reduction in carbon fixation, because the supply of CO2 through the stomata is decreased. A consequence of a shortage of CO2 is that surplus electrons, from light induced reactions in the chloroplast, reduce oxygen to toxic reactive oxygen species, e.g. superoxide radicals or hydrogen peroxide (CAKMAK, 2002). These reactive oxygen derivates are known to be responsible for the impairment of cellular functions and result in growth depression under stress situations. CAKMAK (2002) also showed that the activity of NADPH oxidase, which also generates the production of oxygen radicals, increases with decreasing K supply.
High light intensity in conjunction with drought stress, a common feature in countries of the WANA region, aggravates the oxidative damage in the chloroplasts and thus impairs biomass production. Therefore, K deficient plants grown under high light intensity very rapidly become chlorotic and necrotic. In other words, plants subject to drought and high light intensity have a particular high requirement for K from the soil solution.
WYRWA et al. (1998) showed that drought reduced the grain yield of triticale by 54% on soils low in K whereas, with an adequate K supply the grain yield was reduced by only 16%. EL HADI et al. (1997) also found under Egyptian conditions remarkable yield increases for a wide range of annual crops from applying K when the water supply was restricted.
Fig. 5. Impact of soil moisture and soil texture on the concentration of K in the solution (µM) needed to sustain a K uptake rate of 5 kg/ha/day by diffusive transport
In addition, with declining soil moisture, as frequently happens in arid countries, the diffusive K flux in the soil solution is drastically reduced. This could induce spatial K deficiency at the root surface at times when the plants‘ demand for K is greatest, i.e. just before flowering and grain set in annual crops, in spite of there being sufficient K in the bulk soil. GÄTH (1992) showed that with improving soil K status the diffusive K flux was increased. JOHNSTON et al. (1998) also found that the soil K concentration has to be increased with decreasing soil moisture to maintain a particular rate of K uptake, their results agreeing with those of GÄTH (Figure 5). All these results confirm the need for an ample K supply in drought affected regions.
Abiotic stress – salinity: A high salt concentration in the soil solution has two effects on plant growth, (i) the high osmotic pressure induced by salt impairs water uptake by plants, and (ii), for most cultivated plants, the glycophytes in particular, sodium (Na), which is the common cation under saline conditions, is an unwanted cation, especially when present in large concentrations. To overcome the high osmotic pressure of saline soils extra energy is used by the plant to buildup an osmotic potential in the roots that is sufficiently large to compete successfully for soil water. Also, Na accumulating at the root surface, especially at declining soil moisture levels, is an obstacle for K uptake unless the plants have a high selectivity for K.
Wilting, due to lack of water induced by osmotic stress from saline soil, triggers the same mechanism as described above, namely reduced carbon fixation excessive electrons from the photo reaction formation of reactive oxygen species damage to the chlorophyll and plant tissue growth retardation decrease in yield.
KAYA et al. (2001) recently published results which show that applying additional K under conditions of salt stress substantially alleviated the stress symptoms. The Na treatment substantially reduced dry matter production and the K concentration close to the deficiency level, together with a decrease in the chlorophyll content and the water uptake of cucumber and pepper while the membrane permeability was increased. Applying supplementary K, together with P, resulted in dry matter production, the chlorophyll and K content, the water uptake and the membrane permeability being restored to levels similar to those of the control treatment without Na.
The alleviating effect of K for plants suffering salinity stress implies that farmers have to take particular care to ensure that plants have an adequate K supply when plants are grown on saline soils.
Abiotic stress – low temperatures: At low root temperatures K uptake by roots is considerably decreased (KAFKAFI, 1997). The same applies to nitrate and phosphate while the uptake of Na and chloride (Cl) was less affected by root temperature (Table 1). For example, when carnations were cultivated in open fields in Israel there was an increased percentage of stem brittle after cold nights due to impaired K uptake when the K supply with the irrigation water was low. Increasing the K concentration in the irrigation water, and thus improving the K uptake, reduced the incidence of frost damage. Similarly, GREWAL & SINGH (1980) found a fairly strong negative correlation between the leaf K content and the degree of frost damage in potatoes. The higher the K content of the plant, the lower the damage, and thus the larger the final tuber yield. BOGDEVITCH (2000) observed in field trials testing K and growing oat in Belarus that late frost had a devastating effect on plants grown on soil with 132 mg/kg exchangeable K, while plants on the adjacent plot with 234 mg/kg exchangeable K showed hardly any frost damage. The final grain yield was 2.27 t/ha from the frost damaged plants on the control plot, and 3.65 t/ha from the plants receiving adequate K.
CAKMAK (2002) makes a connection between impaired K uptake at low temperatures and the photo-oxidative damage associated with K deficiency. This view is supported by the fact that NADPH oxidation and thus, the production of oxygen radicals is much larger at chilling than at normal temperatures (SHEN et al., 2000).
Food demand in the WANA region will increase further. The population in the WANA region has increased in the last 50 years from less than 120 million to currently 431 million and will increase further to 720 million within the next 30 years (FAO, 2002). Correspondingly, IFPRI expects that demand for cereals in the WANA region will almost double from currently 119 Mt to some 226 Mt in the year 2020. This would probably increase cereal imports from currently 40 Mt to almost 75 Mt. The implications for the financial situation in these countries is very obvious. To reduce the financial burden, the productivity per unit area of land has to be increased to close the gap between production and demand. To balance supply and demand the current cereal yield has to increase from almost 2 t/ha to around 2.8 t/ha, and ultimately to 5.2 t/ha to meet the demand 20 years from now.
Larger yields means more nutrients. Taking all harvested crops and appropriate parts of the plant residues into account, the total production of all crops in the WANA region represents an estimated removal of 3.64 Mt N, 1.67 Mt P2O5 and 3.99 Mt K2O. This contrasts a current nutrient supply with mineral fertilizers of 4.42 Mt N, 1.78 Mt P2O5 and 0.39 Mt K2O. In other words, N and P use approximately balances the removal by the harvested crops. The problem is the very large difference between the K removed by the crops and K applied in fertilizers. The difference has increased from 1.9 Mt K2O in the early 1970s, 2.4 Mt in the 1980s and 3.2 Mt K2O in the 1990s to currently 3.6 Mt K2O today (Figure 6). This indicates considerable mining of soil K reserves. Experience in other countries, like India and China, suggest that even taking the nutrient input with irrigation water and organic wastes like FYM into account, the K balance remains negative. Furthermore, of the nutrients removed by crops at harvest, more than half will be transferred with the crops into towns because more than half of the population in the WANA region lives in towns and hardly any nutrients from the urban areas are recycled to farm land. Thus a substantial amount of nutrients, K in particular, are required to replenish the nutrients taken from the fields.
Fig. 6. K consumption in the WANA region in relation to K removal by crops
It is high time to reverse the current imbalance in fertilizer use. Currently there exists a vicious circle. Farmers in the WANA region hardly apply any potash fertilizers for a number of reasons, lack of knowledge, seeing no visual response by crops or by getting inappropriate advice. With declining soil K reserves, the K release rate decreases and this restricts crop growth far below the genetic potential of the crop. In addition, with emerging K deficiency the plants loose their resistance to pests and diseases and become more vulnerable to climatic and soil borne stress, e.g. drought, frost, salinity. This again lowers the ultimate yield and eventually, the farmers’ income. With declining income the farmer may restrict fertilizer purchase to those that show an immediate and visible response, namely N and P. Continuing the imbalance in fertilizer use results in declining of soil fertility and thus, the farmers’ source of income which leads eventually into the poverty trap. At the same time, the society has to carry higher costs for food imports, for social security in rural areas and for poverty alleviation.
The reverse of this picture is that, if balanced fertilization is practiced, yields will increase due to a better balance in nutrient supply leading also to better resistance to biotic and abiotic stress. Balanced fertilization according to the crop and site specific characteristics will improve the quality of the harvested crops improving the competitiveness of the farmer in the market and providing additional income. With increasing income, the farmer will reinvest in soil fertility and purchase non-agricultural products attracting other businesses and creating jobs.
Farmers, the extension service and the fertilizer industry in the WANA region should combine forces to reverse the trend of unbalanced fertilization before severe problems arise from the loss of fertility in exploited soils.
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