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WATER (HUMIDITY)

As mentioned earlier, water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stomata, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of a solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. By its gradual evaporation of water from the surface of the leaf, near the stomate, helps stabilize plant temperature.

Relative Humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure expressed as a percent. For example, if a kilogram of air at 75°F could hold 4 grams of water vapor and there are only 3 grams of water in the air, then the relative humidity (RH) is:

RH =	water in the air —————————— water the air could hold (at constant temperature and pressure)

so, RH = 3/4 = .75 expressed as a % = 75%

Warm air can hold more water vapor than cold air. If the amount of water in the air stays the same and the temperature increases the relative humidity decreases. Water vapor will move from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity the faster water will move. The relative humidity in the air space between the cells within the leaf approaches 100%. When the stomata are open water vapor rushes out. As the vapor moves out, a cloud of high humidity is formed around the stomate. This cloud of humidity helps slow down transpiration and cool the leaf. If air movement blows the humid cloud away transpiration will increase as the stomata keep opening to balance the humidity.

NUTRITION

Many people confuse plant nutrition with plant fertilization. Plant nutrition refers to the needs and uses of the basic chemical elements in the plant. Fertilization is the term used when these materials are supplied to the environment around the plant. A lot must happen before a chemical element supplied in a fertilizer can be taken up and used by the plant.

Plants need 18 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, phosphorus, potassium, magnesium, calcium, and sulfur are found in the soil. The latter six elements are used in relatively large amounts by the plant and are called macronutrients. There are nine other elements that are used in much smaller amounts; these are called micronutrients or trace elements. The micronutrients, which are found in the soil are iron, zinc, molybdenum, nickel, manganese, boron, copper, cobalt, and chlorine. All 18 elements, both macronutrients and micronutrients are essential for plant growth.

Most of the nutrients that a plant needs are dissolved in water and then absorbed by the roots. Ninety-eight percent of these plant nutrients are absorbed from the soil solution and only about 2% are actually extracted from the soil particles by the root. Most of the nutrient elements are absorbed as charged ions or pieces of molecules. Ions may be positively charged cations or negatively charged anions. Positive and negative are equally paired so that there is no overall charge. For example, nitrogen may be absorbed as nitrate (NO₃^-) which is an anion with one negative charge. A potassium ion (K⁺) is a cation with one positive charge. Potassium nitrate (KNO₃^-) has one potassium ion and one nitrate ion. Calcium nitrate (Ca(NO₃)₂) has one calcium cation that has two positive charges and two negative, single charge, nitrate ions to match the two positive charges of the calcium.

The balance of ions in the soil is very important. Just as ions having opposite charges attract each other, ions having similar charges compete for chemical interactions and reactions in the environment. Some ions are more active than others or can compete better. For example, both calcium (Ca) and magnesium (Mg) are cations with two charges but magnesium is more active. If both are in competition to be absorbed the magnesium will be absorbed. This explains why the results of a soil test may indicate there is sufficient calcium in the soil, but the plant may still exhibit a calcium deficiency because of an excess of the more active magnesium. What may be expressed as a deficiency in one micronutrient may really be caused by an excess of another.

In order for the ions to be easily absorbed, they must first be dissolved in the soil solution. Some combinations of ions are easily dissolved such as potassium nitrate. When other ions combine they may precipitate or fall out of solution and thus become unavailable to the plant. Many of the micronutrients form complex combinations with phosphorous and calcium and precipitate out of the soil solution so the nutrients cannot be easily taken up by the plant. The pH, which means potential hydrogen, is a measurement of acidity or alkalinity. The pH greatly affects chemical reactions. If the soil pH is extremely high (alkaline), many of the micronutrients precipitate out of the solution and are unavailable to the plant. When the soil pH is extremely low (acid), some of the micronutrients become extremely soluble and ion levels may become high enough to injure the plant. The effect of pH varies with the ion, the types of ions in the soil, and the type of soil. Therefore, not only is the amount of the nutrient important but also the soil pH. For further information on what pH's effect on nutrients in the soil see Chapter 2 – Soils

Adequate water and oxygen must be available in the soil. Water is required for nutrient movement into and throughout the roots. Oxygen is required in the soil for respiration to occur to produce energy for growth and the movement of mineral ions into the root cells across their membranes. This is an active absorption process utilizing energy from respiration. Oxygen is not transported to roots from the shoot. Without adequate oxygen from the soil environment there is no energy produced for nutrient absorption. This also stops active absorption in which water flows into the cell due to the higher concentration of nutrients that were actively absorbed.

Anything that lowers or prevents the production of sugars in the leaves can lower nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficiency problems may develop. The stage of growth or how actively the plant is growing may also affect the amount of nutrients absorbed. Many plants go into a rest period or dormancy during part of the year. During dormancy few nutrients are absorbed. Plants may also absorb different nutrients just as flower buds begin to develop.

Nutrients transported from the root to the cell by the vascular system move into the cell through a cell membrane. There are three different ways this happens. First, an entire molecule or ion pair may move through the membrane. If the cell is using energy, called active transport, to absorb ions then only one of the ions in the pair is pulled into the cell. The other will follow to keep the charges even. Most anions (negative ions) are actively absorbed.

The second way of keeping the charges inside the cell balanced and absorbing a new ion is to exchange one charged ion for another ion with the same charge. A hydrogen ion (H⁺) is often released so that the cell can absorb another positive ion such as potassium (K⁺). Since this is a simple passive exchange, absorption energy may not be required. Cations may be absorbed by this passive method.

Both of the methods mentioned above may be passive or active. The third method, the carrier system, is always active absorption, requiring energy. Scientists have discovered that within the cell membrane there are specialized chemicals that act as carriers. The carrier, through chemical changes, attracts an ion from outside the cell membrane and releases it inside the cell. Once the ion is inside the cell it is attached to other ions so that it does not move out of the cell. Complex chemical reactions are involved in the entire process.

Although nutrients can be absorbed passively, research has shown that active absorption must take place if the plant is to grow and be healthy. The factors discussed earlier about absorption by the roots are also true for absorption by the cell. Some of the factors that affect nutrient absorption are the type of ion, soil pH, solubility of ion pairs, water, soil oxygen, sugar supply, plant stress, and temperature.

Foliar Absorption, a Special Case
Under normal growing conditions plants absorb most nutrients, except carbon, hydrogen, and oxygen, from the soil. However, some nutrients can also be absorbed by the leaves if they are sprayed on with a dilute solution. The factors that affect absorption by the cell are still important because the nutrient must enter the cell to be used by the plant. Care must be taken that the concentration of the nutrient is not too high or the leaf will be injured. Also, high temperatures can cause nutrients to injure leaves. An example is sulfur that is applied to leaves when the temperature is or will be above 90° F. Remember the leaf is covered by a thin layer of wax called the cuticle that the nutrient must get around or through before it can enter the cell.