DIAGNOSING MICRONUTRIENT DEFICIENCIES

Chelate is derived from the Greek Word "CHELE" meaning claw

The function of a chelate (organic complex) is to chemically combine the positive charged cation (zinc, manganese, iron, copper, magnesium, or calcium) with an organic, which is negative charged chelating agent.

The organic molecule surrounds the positive charged metal and protects the new chelated form of cation from chemical tie up in the soil or in the fertilizer spray tank

Plant Growth Regulators

Growth and development of plants, like all organisms, is regulated by a combination of genetic factors and environment influences.

A multitude of growth activities are under specific chemical controls. Such chemicals are known as plant growth regulators, or plant hormones. Other growth activities are subject to environmental cues, including photoperiod, temperature, and pressure and moisture changes. One of the differences between plant responses and animal responses is that plant responses to the environment often involve differential growth patterns rather than behavioral activities in response to positive or negative stimuli.

Signal Transduction Pathways in Plants

Hormones may function as signal molecules that trigger the signal transduction pathways in cells. Such pathways often result in the synthesis of transcription factors that in turn promote synthesis of enzymes that facilitate chemical reactions within the cell (the response). Signal transduction pathways are equally important for chemical messaging in plants as in animals and both environmental cues and hormones serve as signal stimuli.

Plant Growth Regulators

Plant hormones, or plant growth regulators, are chemicals produced by plants that alter growth patterns and/or maintenance of the plant. They can be found in many cells and tissues, although plant hormones seem to be concentrated in meristems and buds (which are dormant shoot meristems).One should not confuse hormones, or growth regulators with enzymes. Any chemical reaction that occurs in a cell requires a specific catalyst, a molecule that brings the reactants together in an arrangement in which the chemical reaction can take place. Enzymes, which are proteins, are the catalysts found in the cells of all living organisms. Growth regulators function more in controlling events by sending chemical signals or messages to cells to do something or to not do something. Plant hormones inhibit as well as promote cellular activities.

In contrast to animal hormones, which generally have very specific effects, the hormones identified in plants most often regulate division, elongation and differentiation of cells. Most hormones have multiple effects in plants. Plant hormones work in very small concentrations, affecting membrane properties, controlling gene expression and affecting enzyme activity. One example previously mentioned was the action of plant hormones on increasing the extensibility of cellulose for cell wall expansion. In most cases, the effect plant hormones have on the plant depends on the location of and concentration of the hormone relative to other hormones in the specific tissues. Plant hormones often work in conjunction with each other, and have overlapping effects. They also work with environmental stimuli, as we shall discuss.

There are several classes of plant hormones, including a number of recently "discovered" ones. Auxins

The concept of chemical messengers in plants was proposed by Charles Darwin and his son, Francis, in 1881, who spent time looking at the phenomenon of phototropism in wheat seedlings. Plant shoots are positively phototropic. When a seedling is illuminated from the side, the shoot will bend towards the light. This directional growth makes sense, since plants need light for photosynthesis. However, the Darwins found that if the coleoptile of the wheat seedling was removed, the plant no longer curved towards light. They did a number of experiments and determined that a chemical located in the coleoptile traveled to the region of elongation and effected a differential elongation of cells furthest from the light sources. The chemical was subsequently studied and named auxin by Frits Went in 1926.

Chemically auxin is indoleacetic acid (IAA) which is synthesized from tryptophan. There are a number of synthetic "auxins" too. Auxins promote growth in molar concentrations of 10-3 to 10-8. A primary site of auxin production is the apical shoot meristem. Auxin moves down the stem parenchyma cells by polar transport (auxin becomes negatively charged) using proton pumps, an ATP requiring process.

Auxin Functions

• The most studied function of auxin is promotion of elongation and cell enlargement For cell elongation, proton pumps increase H+ concentration in cell walls which stimulates expansions, proteins that disrupt hydrogen bonds and break cross linkages in cellulose. This facilitates wall expansion when cells take in more water.
• Auxins are involved in tropic responses. Auxins migrate away from a light source, which accounts for the uneven elongation of cells on the shaded side of a plant unevenly exposed to light.
• Auxin stimulates the production of secondary growth by simulating cambium cells to divide and secondary xylem to differentiate.
• Auxin produced in apical buds tends to inhibit the activation of buds lower on the stems. This is known as apical dominance. This effect lessens with distance from the shoot tip. Cytokinins (another group of plant hormones) counter the apical dominance effect of auxins.
• Auxins also:
• Promotes lateral and adventitious root development
• Promotes other hormone production, especially ethylene when auxin concentration increases
• Promotes flower initiation
• Loss of auxin initiates leaf abscission
• Fruit development requires auxin produced by the developing seed. Can get seedless fruits using synthetic auxins

Auxins are toxic in large concentrations. They affect mostly dicots but not monocots, suggesting the effect is on secondary growth activities. Synthetic auxin herbicides include 2-4-D and 2-4-5-T (Agent orange of the Vietnam era contains synthetic auxin)

• Used as defoliants
• Used as weed killers

Cytokinins

Cytokinins are a group of phenyl urea derivatives of adenine, one of the molecules in DNA. The first cytokinin was chemically isolated in 1913 and cytokinins were studied using coconut endosperm for a number of years starting in the 1940's. However cytokinins were not chemically identified until 1963 in corn. That cytokinin was called zeatin.

Biologists have yet to identify the genes that code for the variety of cytokinins found in plants. One hypothesis today is that cytokinins are actually synthesized by symbiotic methylobacteria that live within plant tissues.

Cytokinin Functions

• Cytokinins promote cytokinesis. They are found primarily in root meristems, embryos and fruits, and migrate from roots to the shoot systems of plants in xylem tissue.
• The effects of cytokinins are often studied in tissue culture. Parenchyma cells grown in tissue culture will not divide and differentiate unless cytokinins and auxin are present. The ratio of cytokinins to auxin controls what tissues differentiate in the tissue culture.
• Cytokinins can promote axillary bud growth by over-riding the apparent inhibiting effect of auxin. This is one of the ways in which plants balance root and shoot growth. Shoot tip auxins inhibit lateral bud activation. Cytokinins produced in root meristems travel upward in xylem and if in higher concentration counter the inhibition of auxins to activate lateral buds.
• Removing shoot tip activates lateral shoot development.
• Cytokinins also retard leaf senescence, probably by stimulating RNA and protein synthesis and delaying degradation of chlorophyll.
• Some pathogens that promote tumor formation, called galls in plants take advantage of cytokinin function. The bacteria that form these tumors contain genes for the synthesis of cytokinins so promote rapid undifferentiated cell growth in the infected part of the plant. Agrobacterium tumefaciens, a common vector in plant biotechnology contains such genes.