2 Crop Production Levels and processes 1.2.1 Levels of crop production

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2 Crop Production Levels and processes 1.2.1 Levels of crop production
  1.2 Crop Production Levels and processes 1.2.1 Levels of crop production De Wit proposed a classification of systems of crop production based on growth-limiting factors (de Wit & Penning de Vnes. 1982; Penning de Vries & van Laar 1982) and distinguishes four levels of plant production. The crop production systems at any of these levels can be considered as members of a broad class of systems. In order of decreasing yield, these levels are: Production Level 1 The crop has ample water and nutrients and produces a higher yield than at any other Production Level. Its growth rate depends only on the current state of the crop and on current weather, particularly radiation and temperature Table 2. The relative values of important aspects of models in different phases of development With a full canopy, the growth rate of field crops is typically between 150 and 350 kg ha -1  d -1  of dry matter. Thi s is the 'potential growth rate’ and the crop yield ‘ the potential yield ’ . These growth conditions are realized on very intensive arable and grassland farms in Western Europe and often in glasshouses. Production Level 2 The growth rate is limited only by the availability of water for at least part of the growing season. This situation seldom occurs spontaneously, but in semi-arid regions applying fertilizers can result in crop growth at this Production Level. This may also occur in other climates under intensive cropping on light soils Production Level 3 The growth rate of the crop is restricted by nitrogen shortage for at least part of the growing season and by water shortage or poor weather for the remainder. This situation occurs frequently in agricultural systems all over the world. Nitrogen shortage occurs particularly in crops when fertilizer is not intensively applied In the natural environment, even nitrogen-efficient plants cannot always absorb sufficient nitrogen. Production Level 4  Crop growth is restricted by low phosphorus and other mineral nutrients in the soil for at least part of the growing season The growth rates are 10-50 kg ha -1  d -1  and the growing season often lasts less than 100 days. This situation usually occurs in heavily exploited areas where no fertilizers are used Rarely do cases fit exactly into one of these Production Levels, but it is practical to reduce specific cases to one of these four categories. This focuses attention on the dynamics of the main environmental factor and on the response of the crop to it. Environmental factors that have no regulatory effect can then be disregarded, because they do not determine the growth rate. The growth rate then sets the absorption rate or efficiency of use of non-limiting factors. If, for example, plant growth is limited by nitrogen, there is little use in studying CO 2  assimilation or transpiration to understand the current growth rate. All emphasis should be placed on nitrogen availability, the nitrogen balance and the plants response to nitrogen This analysis of plant production systems allows for considerable narrowing of the subject of study and permits more rapid research progress Growth-reducing factors, such as diseases, insect pests and weeds, can occur at each of these Production Levels and give them, in a sense, an extra dimension. The fact that actual situations are often more complex does not contradict the general usefulness of this scheme of Production Levels as a basis for distinction between causes and consequences of plant growth.  Note that this use of Production Levels has a crop physiological basis and is  not related to descriptions of production systems based on agronomic practice or crop ecology, such as the irrigated, rainfed lowland, deep water and upland production systems in rice growing (IRR1. 1984). 1. 2.2 Principal processes of the Production Levels This systemalic analysis of crop production can be taken a step further to formulate simple systems and models at the four levels of plant production. At Production Level 1 the intensity of radiation, the interception of light and the efficiency of energy use in the plant are key factors for understanding the growth rate. Figure 5. a relational diagram, indicates the essence of models at Production Level 1. Light is a driving variable Assimilated carbohydrates are stored, usually briefly, in an easily accessible form, such as starch ("reserves'), and are later used for maintenance or growth. Temperature is an external variable that can modify growth rates and  photosynthesis. In growth processe s, reserves are converted into ‘structural biomass’  with a specific efficiency. Structural biomass consists of those components that are not mobilized again for maintenance or growth processes elsewhere in the plant. The parti-tioning of biomass between roots, leaves, stems and storage organs is strongly related to the physiological age of the crop, which itself is a function of temperature. Figure 5. A relational diagram of a system at I'roductum Level 1. Light and temper-ature are driving variables, the photosynhetic efficiency is a constant. Rectangles represent quantities (state variables);   valve symbols, flows (rate variables); circles, auxiliary variables; underlined variables, driving and other external variables; full lines, (flows of material; dashed lines, information flow (symbols according to Forrester, 1961)  At Production Level 2, key factors are the degree of exploitation of soil water and the efficiency of its use by the crop (Figure 6). Water shortage leads to stomatal closure and to a simultaneous reduction of C0 2   assimilation and transpiration. Water use efficiency is the ratio of photosynthesis and transpiration rates. The ratio of the actual transpiration rate and the potential rate provides the link between the carbon and water balance. The extent to which the potential transpiration, and consequently the  potential photosynthesis rate, is realized, depends on the availability of water. The amount of water stored in the soil is a buffer between rainfall and capillary rise and the  processes by which water is lost. This buffering capacity and the simultaneous water loss through transpiration and non-productive processes cause the growth rate to depend only indirectly on rainfall. The relation of plant growth to the principal driving variable of this system is indirect (rather than direct, as at Production Level 1). At Production Level 3,   nitrogen in plant tissues is distinguished by two fractions: mobilizable and immobilizable nitrogen (Figure 7). The amount of nitrogen that can be mobilized for growth of new organs is often considerable. Figure 6. A relational diagram of a system at Production level 2. Water shortage is the main limiting factor. Rectangles represent quantities (state variables); valve symbols, flows (rate variables); circles, auxiliary variables; underlined variables, driving and other external variables; full lines, flows of material, dashed lines, information flow (symbols according to Forrester. 1961).   Figure 7. A relational diagram of a system at Production Level 3. Nitrogen shortage is the main limiting factor. Rectangles represent quantities (state variables); valve symbols, flows (rate variables); underlined variables, driving and other external variables; full lines, flows of material; dashed lines, information flow (symbols according to Forrester, 1961). The concentration of nitrogen in mature tissue may reduce to half or a quarter of its maximum value before the tissue stops functioning. Growth is directly related to the rate of nitrogen absorption only after the internal nitrogen reserve is used. This internal reserve of nitrogen makes the increase in plant dry matter at any moment largely independent of the current absorption of nitrogen. The relation of nitrogen uptake and growth is, therefore, quite different from that of water uptake and growth. The mobilizable fraction consists of enzymes and membrane  proteins that are broken down and exported as amino acids; not all can be considered reserves, because cells cannot function without them. The immobilizable fraction of nitrogen in the tissues is tied up in stable proteins. The growth rate at this Production Level is primarily determined by the availability of nitrogen from the soil and the internal reserve. Hence the rate of CO 2  assimilation is a consequence of the growth rate. The availability of nitrogen from the soil resembles that of water, a variable amount of inorganic nitrogen is present in the soil and most of it is readily available to roots that are sufficiently close. Soil microflora may compete with plants for this nitrogen and other processes may also interfere. Nitrogen in organic matter in the soil is not available to crops. But mineralization, i.e., breakdown of organic matter by microbes, releases nitrogen to the inorganic pool. Crucial processes of crop growth at Production Level 4 are similar to those at Production Level 3 (Figure 8). The concentration of phosphorus in ageing tissue decreases in the same way as nitrogen; and, as with nitrogen, plants also have an internal reserve of phosphorus. But the processes that make phosphorus available to roots differ considerably from those for nitrogen. Plants require a much higher root density for adequately exploring the soil for phosphorus; and the quantity of dissolved  phosphorus in the soil is so small that the rate of its replenishment determines the  phosphorus supply to roots. Mycorrhiza may enhance phosphorus uptake by increasing the explored volume of soil. Both organic and inorganic compounds in the soil may provide and capture dissolved phosphorus. Chapters 2,3,4 and 5 consider models for situations with ample nutrients for crop growth. For models of crops in situations with severe shortages of nitrogen and  phosphorus, see van Keulen (1982), Hansen & Aslyng (1984) and van Keulen & Wolf (1986). 1.3 Uses of crop growth models
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