Plant Physiology (5) - Stomatal Conductance and Plant Nutrition
Stomatal conductance describes the diffusion of gas through plant stomata. Plants regulate their stomatal aperture in response to environmental conditions. This is described either as a conductance or a resistance (conductance is reciprocal of resistance). Stomatal conductance can be a good indicator of plant water status, as many plants regulate their water loss through stomatal conductance.
Do the stomata control water loss, or is another factor at play? In still air, the boundary layer resistance of the leaves controls leaf water loss. In moving air, stomatal resistance is what controls the water loss in the leaves. Boundary layer conductance depends on wind speed, leaf size, and the gas being diffused across the stomata. Blue lights stimulate stomatal opening.
There are some anatomical contributions to a plantās boundary layer resistance to water loss, including the leafās shape and thickness, surface area, presence or absence of trichomes (hairs), presence or absence of pits around the stomata, leaf orientation, and whether or not the leaf has a waxy surface.
The transpiration is the ratio of water los to carbon gain. A plant loses 400 water molecules per every carbon molecule. The reciprocal of this is water use efficiency. This depends on three factors: the gradient driving water loss is 50 times higher, carbon dioxide diffuses more slowly, and carbon dioxide has farther to diffuse in the cell.
Mineral nutrition is very important to the overall health of the plant. Keep in mind that plants are capable of making all their necessary compounds from inorganic compounds and elements in the environment. They are supplied with all the carbon, hydrogen, and oxygen they could ever need. In order to get the other elements they need, they must be obtained from the soil.
A mineral is an inorganic element that is acquired mostly in the form of inorganic ions form the soil. A nutrient is a substance that is needed to survive or is necessary for the synthesis of organic compounds. Plants require 9 macronutrients and at least 8 micronutrients, many of which are essential. An essential element is required for a plant to grow from a seed and complete its life cycle. There are 17 essential elements: nitrogen, carbon, phosphorous, copper, nickel, sodium, potassium, oxygen, manganese, molybdenum, hydrogen, calcium, iron, boron, sulfur, magnesium, zinc, and chlorine.
An element that is essential is universal for all plants. Its absence would prevent the completion of the life cycle and would lead to deficiency. An essential element is required for some aspect of mineral nutrition. A beneficial element is often limited to only a few species. These elements stimulate growth and development, and may be required in some species of plant.
There are four basic groups of mineral elements. Group one is composed of nitrogen and sulfur, and they form organic components of plants. These are assimilated via biochemical reactions involving redox reactions. Group two elements are phosphorous, silicon, and boron. These are used in energy storage reactions or maintaining structural integrity. These elements are present in plant tissues as phosphate, borate, and sulfate esters. Group three elements - potassium, calcium, manganese, chlorine, magnesium, and sodium - are present in tissues as ions or ions bound to substrates. These are particularly important in certain roles, such as enzyme cofactors and regulation of osmotic potentials. Group four elements are the rest of the essential nutrients, and these play important roles in reactions involving electron transport. Some are also involved in the formation and regulation of growth hormones and the light reactions of photosynthesis.
Nutrient deficiencies occur when concentration of nutrient decreases below the typical range. Deficiencies of certain nutrients lead t specific visual, often characteristic symptoms reflective of the role of that nutrient in the plant. A nitrogen deficiency will result in the yellowing of mature, lower leaves. Potassium deficiency will lead to leaf margin necrosis. Calcium deficiency will lead to blossom-end rot, in which case the plant will flower and produce fruit, but severe necrosis occurs on the fruit. In cases of magnesium deficiency, interveinal chlorosis occurs on the plant, and in iron deficiency the plant experiences generalized chlorosis.
The location of deficiency symptoms reflects the mobility of the nutrient. Nutrients are redistributed in the phloem. If deficiency is appearing in old leaves, the nutrient is mobile and has been directed to the newer leaves. If deficiency is appearing in new leaves, the deficiency is in a nonmobile nutrient.
The soil also affects nutrient absorption. The pH of the soil affects the growth of plant roots and soil microbes. Root growth favors a pH of 5.5 to 6.5. Acidic soil conditions weathers rocks and releases potassium, magnesium, calcium, and manganese. Decomposition lowers soilās pH. Rainfall leaches ions through the soil, which forms alkaline conditions, and negatively charged soil āholds onā to nutrients that are then released through cation exchange.
Mycorrhizal associations can be beneficial for plant nutrition, as mycorrhizal fungi can increase the surface area of the roots and provide nutrients to the plants in exchange for carbon.
