Potassium in plant production

Introduction

The plant needs large quantities of potassium (K). The uptake of K is frequently as high as or even higher than the uptake of nitrogen. Potassium is an essential element for all living organisms. Not only is plant tissue content of K higher than that of other cations but it is also the most important cation in many physiological and biochemical processes. Although the overall effects of K on photosynthesis, carbohydrate and protein synthesis and on the water economy of the plant have been confirmed in numerous experiments, the actual functions of this element in the physiology of the plant and in yield formation have for long been obscure. Only recently, with the more detailed investigation of the manifold processes of plant metabolism, have some of the questions as to how potassium functions in the plant been answered.

A salient feature of K is the high rate at which it is taken up by the plant. Though, in contrast to many other indispensable elements, K is not a constituent of organic compounds, it is omnipresent in the plant and very mobile. This mobility and the participation of K in the activation of important enzyme reactions are two fundamental characteristics of this element.

K in plant metabolism and yield formation

Life on earth depends on the photosynthetic activity of plants: on the conversion of solar energy into chemical energy (fig. 1). Everything which helps plants to absorb more solar energy makes this process more efficient. POTASSIUM PROMOTES PHOTOSYNTHESIS (fig. 2). It activates those enzymes which are involved in the energy transfer, in the build-up of ATP (adenosine-tri-phosphate) which stores the energy needed for CO₂ assimilation and the synthesis of sugar, starch, proteins etc. ATP is the major carrier of energy in plant metabolism. Obviously, high concentrations of potassium are necessary for optimum efficiency of energy transfer.

A high rate of CO₂ assimilation can be maintained only if the assimilates are removed from the leaves to other plant organs, particularly to roots and storage tissues. This transport is as important as the photosynthetic process itself. POTASSIUM SPEEDS UP THE FLOW OF ASSIMILATES (fig. 3). How is K involved in these processes? Translocation of assimilates and other solutes takes place in the sieve tubes of the phloem tissue. The phloem sap is especially rich in sucrose and potassium and K seems to be directly involved in the process of "phloem loading". A high rate of phloem loading in the leaves i. e. at the "source" and of phloem unloading in the storage tissues ("sink") brings about a speedy flow of assimilates in the sieve tubes. Consequently, more sugar is transported from the source to the sink if plants are well supplied with potassium. The first observations concerning the positive influence of K on sugar transport were made with sugar cane (fig. 4). They have been confirmed later in experiments with many other plants.

Better delivery of assimilates improves the filling of storage organs as shown by results on root and tuber crops (fig. 5), cereals (fig. 6) or vegetables (fig. 7). Generalizing, one can say that POTASSIUM INTENSIFIES THE STORAGE OF ASSIMILATES. Taking cereals (fig. 6) as an example, we find no great effect of K on the first yield component, which is tillering i.e. the number of ears per plant or per unit area. But K has a marked influence on the other two yield components, number of grain per ear and weight per grain. Generally the number of florets within the ear exceeds the number of grains set because some of the florets degenerate. To keep a fair percentage of them alive, a sufficient supply of assimilates is needed and this is supported by the stimulating effect of K on photosynthesis and assimilate transport. The influence of potassium in single grain weight can be explained in a similar way. It has been observed repeatedly that the leaves of wheat plants well supplied with K remain green for a longer time during grain filling, thus providing the ears with assimilates over an extended period. As a result more starch can be synthesized and the grains grow larger. In addition, K also enhances the synthesis of lipids in oleaginous crops, thus improving oil production.

Potassium (K) and nitrogen (N)

The inorganic nitrogen taken up by the plant as nitrate (NO₃^- or ammonium (NH₄⁺) must be converted into organic N compounds which contain the nitrogen primarilyasNH₂ groups. The first products in this conversion process are amino acids of quite simple structure. They are the substrates for the synthesis of the more complicated organic N compounds, such as nucleic acids or proteins. The conversion of inorganic nitrogen and the synthesis of organic N compounds are both energy-consuming processes. POTASSIUMFAVOURSTHEPRODUCTION OF PROTEINS by stimulating a) the generation of energy-rich ATP, b) the reduction of NO₃ to NH₂ and c) the supply of assimilates for amino acid synthesis. It is of little use for the plant to take up much inorganic N unless this can be converted into amino acids and proteins. A high concentration of ammonia or nitrates 'in the plant would actually be poisonous. Good K nutrition favours the rapid turnover of inorganic nitrogen into proteins (fig. 8) and consequently, POTASSIUM IMPROVES THE EFFECT OF NITROGEN fertilizer. In fact, high rates of N can be utilized by the plant and transformed into high yields only in the presence of high K levels (fig. 9,10).

This strong positive N/K interaction is also effective in leguminous plants. These plants are able to bind atmospheric nitrogen through the agency of the Rhizobium bacteria living in their root nodules. They convert gaseous nitrogen (N₂) via ammonia into the NH₂ group of the amino acids. As the N₂ gas is very inert, complex enzymatic processes are involved in nitrogen fixation, and a considerable amount of energy is needed.

Considering the important role of potassium in energy transfer, it is not surprising that K ENHANCES THE FIXATION OF ATMOSPHERIC NITROGEN (fig. 11).

Recent investigations have shown that by improving the K nutrition of the host plant bacterial N₂ fixation can be considerably increased. Experiments, in which nitrogen was labelled with ¹⁵N and CO₂ with ¹⁴C, showed that K not only favoured the translocation of ¹⁴C labelled sugars from the leaves to the roots and root nodules but also the assimilation and turnover of molecular nitrogen within the nodules.

The result was an increase of amino acid production in the nodules, leading to improved protein synthesis and growth. Such data from greenhouse experiments help to explain why leguminous forage crops, such as clover and alfalfa, show better growth and nitrogen uptake when properly supplied with potash fertilizers (fig. 12).

K and the water regime of the plant

POTASSIUM IMPROVES WATER-USE EFFICIENCY (fig. 13). As mentioned earlier, much K is taken up by the plant. Accumulation of potassium in the cells leads to an increase of their osmotic pressure so that water moves into the cell and this, in turn, increases the turgor pressure of the cell. As turgor is essential for cell expansion, supplying the necessary pressure from inside the cell for cell wall extension, it can be concluded that K is involved in the basic process of cell enlargement.

Through its contribution to the osmotic pressure and turgidity of cells K has a dominant role in the opening and closing of the stomata, which regulate the transpiration of water and the penetration of atmospheric carbon dioxide into the leaf. In water stress, plants well supplied with K very quickly close their stomata, thus preventing excessive water loss by the plant. If, on the other hand, the plant obtains sufficient water the stomata open wide and CO₂ assimilation is high. Thus K improves water use efficiency.

According to recent investigations, the involvement in "osmoregulation" i.e. in the adjustment of plant cells to environmental conditions, seems to be one of the most important biophysical role of potassium. Thus it is plausible that K, in addition to its many biochemical functions, improves the tolerance of the plant to various stress situations, such as drought, low temperature or salinity.

References

* ifc = international fertilizer correspondent, published bimonthly by IPI

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