4.2: Resource Acquisition in Plants
Plants obtain the majority of the nutrients they need, including water, nitrogen, phosophorus, etc from the soil through their roots. The only exception is carbon, which is taken up in the form of \(\ce{CO2}\) from the atmosphere.
Cation Exchange
Many of the nutrients that plants need from the soil are positively-charged ions, called cations . These include potassium (\(\ce{K^{+}}\)), magnesium (\(\ce{Mg^{2+}}\)), and calcium (\(\ce{Ca^{2+}}\)). Soil particles often carry a negative charge, and so these compounds adhere to soil particles due to the attraction between opposite charges. In order for plant roots to take up these compounds, they must first be separated from the soil particles. Plants accomplish this separation through cation exchange (Figure \(\PageIndex{1}\)). In cation exchange, the proton (\(\ce{H^{+}}\)) concentration in the soil is increased in two ways. First, the root hairs exude \(\ce{H^{+}}\) ions directly into the soil. Second, respiration in root cells releases \(\ce{CO2}\) into the soil, which reacts with soil water to form carbonic acid, which itself ionizes into bicarbonate and free protons:
\[\ce{CO2 + H2O <=> H2CO3 <=> H^{+} + HCO3^{-}}\]
The \(\ce{H^{+}}\) ions adhere more strongly to the negatively-charged soil particles than do mineral cations such as \(\ce{Mg^{2+}}\) and \(\ce{Ca^{2+}}\). Consequently, the \(\ce{H^{+}}\) ions take the place of the mineral cations on the soil particles, leaving these compounds loose in the soil where they can be taken up by plant roots.
Root Mutualisms
In order to increase the quantity of nutrients obtained from the soil, plants have developed several mutualisms with other soil organisms. Mutualism is a type of species interaction in which both species benefit from the interaction. In this section, we will discuss two root mutualisms: mycorrhizae and root nodules.
Mycorrhizal relationships form between the roots of many plant species and a variety of soil fungi. Recall that the majority of the fungal body is in the form of branching filamentous structures called hyphae. In a mycorrhizal relationship, the hyphae of a fungus wrap around the root and/or penetrate the root. The plant root and the fungal hyphae can then exchange nutrients. Fungi are heterotrophs, so the fungus benefits by receiving glucose from the plant. Since many hyphae are microscopic and have a very high surface-area-to-volume ratio, they can access soil nutrients on a much finer scale than macroscopic plant roots. Consequently, the plant benefits by greatly increasing its ability to absorb nutrients of all kind throughout the soil, wherever the fungal body exists.
Mycorrhizal relationships fall into two broad categories: ectomycorrhizae and endomycorrhizae. Ectomycorrhizal fungi (Figure \(\PageIndex{2}\)) form a sheath around the outside of the plant root and hyphae penetrate the root and wrap around the cell walls of individual root cells. Ectomycorrhizal relationships are common on woody plants, including trees. Endomycorrhizal fungi penetrate the cell walls of roots and grow between the cell walls and cell membranes of the root cells sometimes forming tree-like structures called arbuscules (Fig. 4.2.2). In both ecto- and endomycorrhizal relationships, the plant is able to access nutrients in any portion of the soil profile that contains the extensive fungal hyphae networks, significantly increasing the quantity of nutrients available to the plant. Some mycorrhizae specialize on providing particular nutrients (such as phosphorus) while the majority are generalists, providing any soil nutrients the plant requires in exchange for glucose from photosynthesis.
Root nodules are another form of root symbiosis. Root nodules form only between legumes (members of the plant family Fabaceae, including peas and beans) and a group of soil bacteria called rhizobia . This relationship is also mutualistic, though also more specialized than the mycorrhizal exchange. In this case, the heterotrophic rhizobia also receive glucose from the plant, and the plant receives nitrogen from the bacteria (Figure \(\PageIndex{3}\)). Nitrogen is an important plant resource and is incredibly difficult for plants to obtain. Most nitrogen on Earth is in the form of N 2 gas in the atmosphere, which plants cannot use because of the powerful triple bond between the two nitrogen atoms, which renders it inert. Rhizobia bacteria are capable of converting N 2 into biologically useful forms (such as ammonium, NH 4 + ), a process called nitrogen fixation. Unlike mycorrhizae, root nodules are macroscopic, and can be easily seen with the naked eye. You will study an example of root nodules in Lab 2: Resource Acquisition in Eukaryotic Organisms.
Photosynthesis
At its simplest, photosynthesis is the process by which plants use light energy and \(\ce{CO2}\) from the atmosphere to build glucose (sugar) molecules. Glucose is an energy storage molecule, and later breakdown of glucose in respiration will release the stored energy, making it available for the plant to use for growth, reproduction, etc.
Visible light contains wavelengths from 300-750 nanometers (nm) and plant leaves contain photosynthetic pigments that absorb portions of the visible spectrum. The three most common photosynthetic pigments are Chlorophyll A, Chlorophyll B, and carotenoids such as beta-carotene (Figure \(\PageIndex{4}\)). These pigments absorb wavelengths in the dark green and blue ranges as well as orange and red. They reflect yellows and greens, which is why most plants appear greenish in color.
Recall that photosynthesis is made up of two stages: the reactions in which light energy is absorbed by pigments and stored in short-term energy molecules such as adenosine triphosphate (ATP) and NADPH; and the reactions which use those energy molecules to power \(\ce{CO2}\) uptake and its conversion to glucose (Figure \(\PageIndex{5}\)). In this text, we will refer to the first set of reactions as the light energy harvesting reactions, though they are also sometimes called the light-dependent reactions, or simply the light reactions. We will refer to the second set of reactions as the glucose-generating reactions, though they are also sometimes called the light-independent reactions or the Calvin(-Benson) Cycle. We consider the names "light energy harvesting reactions" and "glucose-generating reactions" to be more accurate and descriptive. For example, the glucose generating reactions are not truly independent of light, as they rely on energy stored in the form of ATP and NADPH by the light energy-harvesting reactions.
There are three different ‘types’ of photosynthesis, termed pathways. The most common photosynthetic pathway is the C3 pathway (Figure \(\PageIndex{6}\)), so named because the product of the first reaction is a 3-carbon molecule. Approximately 85% of all plants on Earth perform C3 photosynthesis and it was the first photosynthetic pathway to evolve, appearing on Earth about 2.5 billion years ago. In the C3 pathway, all mesophyll cells in the leaf perform photosynthesis. Mesophyll cells include all cells that are not vascular tissue (the leaf ‘veins’) or epidermal tissue (the outer layer of cells on the leaf surface).
The C3 pathway has two major costs. The first cost is water loss: in order to get \(\ce{CO2}\) from the atmosphere into the leaf, where it can be taken up by the mesophyll cells for photosynthesis, the leaf must open its stomata (pores in the epidermis). Anytime the stomata are open, water evaporates out of the leaf tissues to the atmosphere. Most of the water a plant obtains from the soil is lost this way. The second cost is a process called photorespiration , so named because it consumes O 2 and produces \(\ce{CO2}\) (like respiration) and because involves the photosynthetic protein Rubisco . The molecule that takes up \(\ce{CO2}\) from the air, named Rubisco, can also bind to O2. When Rubisco binds oxygen instead of carbon dioxide, the chloroplast is still able to produce glucose, but at a much slower rate and at a much higher energetic cost. Various environmental conditions, such as temperature and \(\ce{CO2}\) concentrations affect how much photorespiration occurs in a cell. Consequently, the C3 pathway is most beneficial under cool and wet environmental conditions and when \(\ce{CO2}\) concentrations are high.
Plants that perform C4 photosynthesis have developed a mechanism to avoid these drawbacks to C3 photosynthesis. The C4 pathway (Figure \(\PageIndex{7}\)) is named for the fact that the product of the first reaction is a 4-carbon molecule. C4 plants differ from C3 plants in that carbon fixation (when \(\ce{CO2}\) is picked up from the air spaces of the leaf) and the Calvin Cycle (when \(\ce{CO2}\) is converted into glucose) occur in different cells of the leaf. In C3 plants, carbon fixation and the Calvin Cycle occur in the same place and time – Rubisco obtains a \(\ce{CO2}\) molecule from the leaf air space and commences the Calvin Cycle immediately. In C4 plants, mesophyll cells use the molecule PEP-C for carbon fixation, which then moves to bundle sheath cells, where Rubisco is held to complete the Calvin Cycle. In this way, C4 plants avoid photorespiration by highly concentrating \(\ce{CO2}\) in the cells where Rubisco operates and by using PEP-C (which cannot bind to oxygen) for the first uptake of CO2. Concentrating \(\ce{CO2}\) in the bundle sheath cells also allows C4 plants to close their stomata when environmental conditions are hot or dry, allowing them to save water while still keeping their photosynthetic activity high. Moving \(\ce{CO2}\) into the bundle sheath cells, however, operates against the concentration gradient and so requires energy. Consequently, every glucose molecule that a C4 plant generates costs 2 more ATP energy molecules than under C3 photosynthesis. The C4 pathway is therefore most advantageous in environments that have higher temperatures and drier conditions (though not extremely arid) than those that favor C3 plants. The concentration of \(\ce{CO2}\) in bundle sheath cells also means that C4 plants perform better than C3 plants under low \(\ce{CO2}\) concentrations.
The third photosynthetic pathway is called CAM, an abbreviation of Crassulacean Acid Metabolism because this pathway was first discovered in the plant family Crassulaceae. Where C4 plants separate carbon fixation and the Calvin Cycle in space (where in the leaf they occur), CAM plants separate these processes in time (Fig 4.2.8). During the night, CAM plants use PEP-C to fix carbon, which is then converted to malic acid and stored overnight in the cell’s vacuole. During the day, malic acid is taken out of vacuole and converted back into \(\ce{CO2}\) , where Rubisco then completes the Calvin Cycle. This process allows CAM plants to keep their stomata closed during the day and only open at night, making it the most water efficient of the three pathways. Like C4 photosynthesis, however, the conversion of \(\ce{CO2}\) to and from malic acid, and its storage in the vacuole overnight costs CAM plants two extra ATP energy molecules (compared to C3 photosynthesis) for every glucose molecule generated. In addition, having stomata open only at night reduces the amount of \(\ce{CO2}\) CAM plants can fix, and so plants using this pathway tend to grow much more slowly than C3 or C4 plants. Consequently, CAM photosynthesis is generally only advantageous under conditions of extreme aridity, such as in deserts.
Limiting Nutrients and Growth Trade-Offs
Plants face particularly complex trade-offs when it comes to growth since investing in above-ground growth (leaves, stems, etc) reduces access to below-ground resources and vice versa. The availability of resources above ground (sunlight, CO2) and below ground (water, other nutrients) consequently influences patterns of plant resource investment in growth. An important consideration in these trade-offs is which nutrient is most limiting for plant growth in a given environment. Justus von Liebig, a 19th century German chemist, developed an analogy to describe how varying nutrient levels influence plant growth. Liebig likened plant resources to a barrel made of wooden slats (Fig 4.2.9). Each slat represented a particular nutrient and the slat’s height represented the relative availability of that nutrient in the ecosystem, compared to how much of that nutrient the plant needs (meaning that even if a nutrient is rare in the ecosystem, its slat may still be high if a plant requires very little of that nutrient). The quantity of water the barrel can hold represents plant growth in that ecosystem. The amount of water the barrel can hold is not driven by the height of the tallest slat, or even the average height of the slats; it is controlled entirely by the height of the shortest slat. Similarly, plant growth is not determined by the overall availability of resources, but by the availability of the limiting resource. The limiting resource is the resource that has the lowest availability compared to how much of that resource the plant requires (ie. the shortest slat). This concept is termed Liebig’s Law of the Minimum because the resource with the minimum availability will determine plant growth in the ecosystem.
Figure \(\PageIndex{9}\): Diagram of Liebig’s Law of the Minimum barrel analogy. Modified from Wikimedia Commons 2 .
The slats in Fig 4.2.9 include above-ground and below-ground resources. Changes in the availability of resources above- or below-ground will influence plant investment in growth in these areas. Scientists discuss the balance of above- and below-ground growth in terms of the root-to-shoot ratio, abbreviated root:shoot. In this case, ‘shoot’ includes all above-ground growth including stems, leaves, and reproductive structures like flowers or cones. An increase in plant investment in below-ground growth (roots) will increase the root:shoot ratio while an increase in plant investment in above-ground growth (shoots) will decrease the root:shoot ratio. An increase in the availability of below-ground resources will lead the plant to invest more in above-ground tissues, causing a reduction in the root:shoot ratio. At first, this may sound counter-intuitive; however, if below-ground resources become more abundant, they are then relatively less limiting than above-ground resources and so the plant will be benefitted by increasing its investment in the above-ground resources by increasing its investment in shoot growth, which will reduce the plant’s root:shoot ratio. The reverse is also true, if above-ground resources increase in availability.
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