Water and Mineral Nutrition in Plants

Water and Mineral Nutrition in Plants

We have just concluded a series of lectures looking at the structure of the "higher" plant body and some of its features. At this time we can look at how the plant functions (physiology) and how plant growth and development is regulated. In other words, how does a plant do the things it needs to do, such as:

Obtain nutrients for growth and survival, both from the "soil" and from the atmosphere
Maintain water balance and transport water throughout the plant
Transport nutrients and solutes to its cells and tissues
Regulate growth and developmental activities

You'll note the plants have many of the same problems that any animal does - its just that plants tend to solve their problems in different ways... Let's look now at some of these problems and how plants grow and survive in an often "hostile" environment.

Obtaining Nutrients 
Plants are autotrophs. They obtain raw materials from their environment. Plants need about 18 elements, mostly mineral ions, along with oxygen, water, and carbon dioxide. We will discuss the specific mineral needs in the laboratory exercise on mineral nutrition.
Plants then process these substances into their needed organic molecules for plant structure and function.
Plants produce few waste products because they have no need to extract nutrients from pre-formed organic materials (like we do), and their fuel needed to do cell work is provided by photosynthesis

How does the average plant obtain these raw materials?
Needed gases are obtained by diffusion from the atmosphere, and will be discussed a bit more later.

Water and most minerals must be absorbed as water-soluble ions from soil(never dirt) via the roots.

Soil serves as the:

• Reservoir for many mineral ions
• Storage for some mineral ions
  

The origins of any soil is the parent rock, of whatever type, which is "weathered" to tiny particles, often called clay, by mechanical and chemical processes (mostly involving water). The mineral component of soil is determined by the parent rock of that area.

Any soil "community" includes:

• Mineral particulates
• Living organisms
• Air and water spaces (30 - 60% of soil volume)
• Humus (decayed and decaying organic material) component
  

The availability and concentration of minerals is critical for growth. If a needed mineral is absent from the soil, the plant can not grow properly, if at all. Mineral cycling, which involves an entire food chain, including the vital decomposers, is an important process for ecosystems. Disturbed ecosystems may lose minerals in many ways, diminishing plant growth.

Soil type is very important in agriculture. The major agricultural areas of the world are characterized by having soils called Chernozem soils, which are calcium-rich with lots of humus. Acid soils, such as the podzols of many conifer forests and the iron or aluminum-rich laterite soils of the tropics are not well suited to cultivation and form "Hard-Pan" with disturbance.

Assuming minerals are available, how does the plant absorb and move minerals to needed locations in the plant body?


Obtaining minerals from soil
Water from rainfall percolates through soil spaces and forms a film around soil particles.
Soluble ions (minerals) dissolve into H2O from the surrounding soil
There is a competition for minerals by root absorption versus leaching minerals through soil as water percolates past root region.
Water and dissolved minerals move into root in the root hair region by diffusion and active transport 
From the root hairs, nutrients move through the Cortex, mostly between cells, called the apoplast pathway, and into the stele via the endodermis. Water and minerals can also move from cell to cell within the cortex, called the symplast pathway. From the stele, most minerals are moved along with water in xylem which is continuous throughout the plant.

The rate of mineral absorption is affected by:

• Available concentration
• "Ease" of movement or how soluble the mineral is in water
  

The solubility of many minerals can be altered by changes in soil conditions and other soil substances. Soil pH is one critical factor.

Mycorrhizae are also very important in mineral absorption for many plants. Mycorrhizae increase the area for absorption and actively absorb minerals which are in low concentration.


Water and Nutrient Movements in Plants
Plants are 80-90% water (wet weight) and soil and atmosphere usually contain a much lower proportion of water. Most plants present large surface areas to their surroundings; both the root and leaf surface areas are large (roots to absorb water and nutrients, leaves for exposure to sun for photosynthesis. Because there is so much surface exposed, plants need to be efficient in obtaining and conserving water for their cells and tissues.

Some ways plant have of conserving water
The stomata necessary for gas exchange open and close so that water loss by transpiration is minimized when plants have no need for CO2. This helps to maintain appropriate water balance.
Epidermal cells on above-ground structures are coated with a waxy cuticle layer (cutin) to prevent water loss, or, if the surface is cork, the walls contain impermeable suberin.
Plant cells have vacuoles to accumulate a volume of water, and cell walls to help maintain turgor. (This works better at preventing excess water than it does at preventing dehydration. Plants achieve "permanent wilt" when plasmolysis (loss of turgor) can not be reversed.)
Many cells and tissues need not be maintained because they're dead (saves energy)


Water Movement in Plants
How does water enter and move through the plant, especially when plants have no pumps, and how does a plant move water upward against the forces of gravity as much as 300 feet.

We know that water moves from the soil's environment by diffusion into the root through root hairs. It travels through the cortex of the root, mostly between cells, and from root stele upward through the xylem tissue, and out the leaves' stomata, with a few stops along the way for photosynthesis, turgor maintenance, and other water requirements. How does this happen? Let's look at the end point first, since this also functions in the overall movement of water throughout the plant.

Water diffuses out of the plant via transpiration through the stomata. Transpiration is the term used to describe the evaporation of water from the plant. Transpiration also plays a role in the movement of water throughout the plant as we shall discuss. Transpiration loss is significant. In corn fields, as much as 90% of the water absorbed by the roots is lost by transpiration.


How Water moves - The Tension-Cohesion Theory
Much water is lost via transpiration. This creates a negative water potential in cells which exerts a "pull" on H2O in cell walls which connects to H2O in xylem creating a tension in the xylem.
Since water molecules tend to cohere (stick to each other), this tension is transmitted to the xylem in roots making the root water potential negative, too. Water from soil is now attracted by a diffusion gradient.
Further, the xylem conducting cells have adhesive properties (like a capillary tube) so water is attracted (and sticks together along walls).
Water is also pure in the vascular stele. The casparian strips of the endodermis layer mean that endodermal cells screen everything as substances enter the stele.
This combination of forces is sufficient to move water upward against all forces of gravity.
It also means that a plant disadvantage, water lost by transpiration, can be turned around to do something beneficial for the plant.


Regulating Transpiration - The Stomatal Mechanism
Since it is important to conserve as much water as possible, plants have mechanisms to open and close their stomata, minimizing water loss during non-photosynthetic times, a mechanism which we will look at in more detail soon.

Stomata are open in daytime which permits diffusion of CO2 into the leaf for photosynthesis. At the same time water is lost through the stomata, via transpiration. The intercellular spaces of plant tissues are near 100% humidity, and the stomata are openings into the environment, which is usually not at 100% humidity. The diffusion gradient for water is from the leaf to the environment. This creates serious problems for water maintenance. The resolution of this problem is the closure of stomata at night so that water loss is restricted to the daytime hours when the plant is actively using CO2. It is important to remember that the primary function of stomata is gas exchange, a subject that will be discussed later.

How stomata work
The mechanical operation of stomata is a phenomenon of turgor, osmotic balance and active transport, helped by the structure of the guard cells. Most guard cells are bean shaped. The inner walls of each guard cell pair are thickened, while the outer walls are thin. When water is absorbed by a guard cell, it swells, stretching the cell. The thicker inner wall does not stretch so the rest of the cell gets distorted. This distortion of the pair of guard cells makes a shape which causes a gap between the guard cell pair's inner walls. This gap is the stoma. When guard cells lose turgor, they "shrink", and the collapsed cells force the inner walls of the guard cell pair together, closing the stoma.

To produce these changes in turgor, a ratio of potassium (K+) and H2O is maintained within guard cells that varies from daytime to nighttime. In contrast to other epidermal cells, guard cells contain chloroplasts, and the process of photosynthesis is used to maintain turgor when stomata are open.

Daytime

• K+ pumped in by active transport through channels in guard cell membranes in response to presence of light
• Draws H2O into cell in response to K+ solute concentration, increasing the cell volume (a turgor phenomenon) and swelling guard cells which "open" forming stomata.
• Photosynthesis maintains high solute concentration for turgor.
  

Nighttime
Stomates close at night

• K+ leaches out of guard cells. This may be activated by Calcium (Ca++)
• H2O follows.
• Guard cells lose turgor and "close"
  
  

Other Water Movements
Imbibition
Water can also be absorbed rapidly into cells by means other than osmosis. Certain molecules, especially starch and cellulose "attract" water molecules when they are wet because on surface charges (+ and -). Since water is polar, it is attracted to the surfaces of these molecules, and large amounts of water can be taken into cells in this manner. As discussed, imbibition is very important for the process of germination, causing the seeds to swell rapidly with the uptake of water.

Positive Root Pressure
Simple diffusion pressure in roots moves H2O upward - often forcing the H2O to be exuded from vein tips in leaves, a phenomenon called guttation. The special leaf tip cells are called hydathodes.

Guttation occurs when there is high soil moisture and low evaporation stress. You can typically see guttation in Seattle during the spring and early summer, in the early mornings before the sunlight evaporates the water. Grasses and many herbaceous plants, such as strawberries. guttate nicely. Guttation can not be observed during rain, since the rain drops coat the plant surfaces, and should not be confused with dew, which can condense from the atmosphere onto plant surfaces.


Other Means of Preserving Water
Plants undergo excessive evaporation stress when the soil is dry, the air is hot and dry, or when it is windy. In response to these conditions:

Stomata may close to prevent H2O loss. This closure is usually under hormone control.
Primary growth plants may wilt from the loss of turgor which results when transpiration exceeds root absorption of water
Some plants which are subject to frequent evaporation stress (especially desert succulents) may have reverse stomatal operation , in which CO2 can be absorbed at night and trapped in the form of 4-carbon acids. This CO2 trap is also used by C4 photosynthesis plants, although such plants need not have reverse stomatal operation.

As previously discussed, xeromorphic plants have a number of modifications to minimize water loss:

Temporary leaves (retain leaves only during moist times)
Leaves with stomatal crypts and thick epidermis and cuticle
Dense epidermal hairs, often reflective (silver)
Tiny leaves or No leaves, they may have photosynthetic stems instead
Short life span plants
Water storage tissues (succulence)

Movement of Solutes in plant
While some solutes are moved through xylem, most solutes are transported in phloem.

Transport in Phloem 
The discovery of phloem movement is credited to Malpighi who recorded that when one rings a tree, the tree dies by lack of "nourishment" below the ring. Although this was noted a long time ago, learning how phloem transports solutes was inhibited because access to the phloem tissue was difficult. (The cells collapsed and quit functioning when manipulated.)

Ultimately, the common aphid was used as a research tool. Aphids normally penetrate into phloem to feed, and their actions do not stop the phloem activity in the plant. Aphids merely divert the flow into the aphid body (and sometimes through the aphid body, since phloem movement is osmotic.) In addition, radioactive tracers were used to identify phloem solutes, mostly sucrose.

Sucrose has a high osmotic potential (hydrostatic pressure) . While sucrose, like any substance, can move by diffusion from where it is concentrated, the normal rate of phloem movement is much faster than simple diffusion.

Movement in phloem is always in a direction of more concentrated to less concentrated, or from where you have solutes (mostly sugars from photosynthesis) to where you need solutes (to use) Phloem can also be used to move solutes from storage areas to where solutes are needed. This pressure-flow gradient between the "source" of solutes and the "sink", the location to which the solutes are being moved explains how solutes are moved through phloem.


Movement in Phloem
Solutes are actively secreted from the source into a sieve tube. Special parenchyma cells help this.
Once the sugars are in the sieve tubes, movement is facilitated by osmotic potential (turgor increases the hydrostatic pressure in the cells) The presence of sugar in a sieve tube attracts water.
Water moves into the sieve tube increasing the pressure in the sieve tube
Increased pressure forces the solutes into the next cell of the sieve tube. This mechanism is called pressure flow .
Active transport is again used at the sink, to move the solutes into the cells where they will be needed or stored.

Obtaining Gases from the Atmosphere
Some nutrients are not in soil minerals. CO2 (needed for photosynthesis) and O2, (needed by all cells) are atmospheric gases. As we have discussed, CO2 diffuses into the plant through open stomata. Excess oxygen will diffuse out of the plant through stomata as well.

Other gas exchange mechanisms
Since all cells also require oxygen for cell respiration, there are additional mechanisms in plants for gas exchange, since oxygen produced during photosynthesis can not freely move to all parts of the plant. Substances will diffuse along the easiest route and gradient, and much oxygen is lost through stomata.

Primary growth stems have stomata in the stem epidermis.
Secondary growth stems have lenticels in the cork (bark)
Roots obtain oxygen strictly by diffusion through root epidermis which is not cutinized. Oxygen must compete for water for essential soil spaces, and water saturated soils may cause the roots to suffocate from lack of oxygen. Roots don't need CO2

Photosynthesis

Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae (Kingdom Protista). Plants need only light energy, CO2, and H2O to make sugar. The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.

 Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomates. The upper and lower epidermal cells do not have chloroplasts, thus photosynthesis does not occur there. They serve primarily as protection for the rest of the leaf. The stomates are holes which occur primarily in the lower epidermis and are for air exchange: they let CO2 in and O2 out. The vascular bundles or veins in a leaf are part of the plant's transportation system, moving water and nutrients around the plant as needed. The mesophyll cells have chloroplasts and this is where photosynthesis occurs.

 As you hopefully recall, the parts of a chloroplast include the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana. The chlorophyll is built into the membranes of the thylakoids.

Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the red and blue light that are absorbed that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis.

The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O (+ light energy)  C6H12O6 + 6O2. This is the source of the O2 we breathe, and thus, a significant factor in the concerns about deforestation.

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There are two parts to photosynthesis:

The light reaction happens in the thylakoid membrane and converts light energy to chemical energy. This chemical reaction must, therefore, take place in the light. Chlorophyll and several other pigments such as beta-carotene are organized in clusters in the thylakoid membrane and are involved in the light reaction. Each of these differently-colored pigments can absorb a slightly different color of light and pass its energy to the central chlorphyll molecule to do photosynthesis. The central part of the chemical structure of a chlorophyll molecule is a porphyrin ring, which consists of several fused rings of carbon and nitrogen with a magnesium ion in the center.

 

The energy harvested via the light reaction is stored by forming a chemical called ATP (adenosine triphosphate), a compound used by cells for energy storage. This chemical is made of the nucleotide adenine bonded to a ribose sugar, and that is bonded to three phosphate groups. This molecule is very similar to the building blocks for our DNA.

 

The dark reaction takes place in the stroma within the chloroplast, and converts CO2 to sugar. This reaction doesn't directly need light in order to occur, but it does need the products of the light reaction (ATP and another chemical called NADPH). The dark reaction involves a cycle called the Calvin cycle in which CO2 and energy from ATP are used to form sugar. Actually, notice that the first product of photosynthesis is a three-carbon compound called glyceraldehyde 3-phosphate. Almost immediately, two of these join to form a glucose molecule.

Most plants put CO2 directly into the Calvin cycle. Thus the first stable organic compound formed is the glyceraldehyde 3-phosphate. Since that molecule contains three carbon atoms, these plants are called C3 plants. For all plants, hot summer weather increases the amount of water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomates closed during hot, dry weather. Unfortunately, this means that once the CO2 in their leaves reaches a low level, they must stop doing photosynthesis. Even if there is a tiny bit of CO2 left, the enzymes used to grab it and put it into the Calvin cycle just don't have enough CO2 to use. Typically the grass in our yards just turns brown and goes dormant. Some plants like crabgrass, corn, and sugar cane have a special modification to conserve water. These plants capture CO2 in a different way: they do an extra step first, before doing the Calvin cycle. These plants have a special enzyme that can work better, even at very low CO2 levels, to grab CO2 and turn it first into oxaloacetate, which contains four carbons. Thus, these plants are called C4 plants. The CO2 is then released from the oxaloacetate and put into the Calvin cycle. This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown.

There is yet another strategy to cope with very hot, dry, desert weather and conserve water. Some plants (for example, cacti and pineapple) that live in extremely hot, dry areas like deserts, can only safely open their stomates at night when the weather is cool. Thus, there is no chance for them to get the CO2 needed for the dark reaction during the daytime. At night when they can open their stomates and take in CO2, these plants incorporate the CO2 into various organic compounds to store it. In the daytime, when the light reaction is occurring and ATP is available (but the stomates must remain closed), they take the CO2 from these organic compounds and put it into the Calvin cycle. These plants are called CAM plants, which stands for crassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plant Sedum) where this process was first discovered.