Most essential elements have several functions in plants

The functions of the 16 elements essential for plant growth are summarized in table 7.1. A quick survey of this

Some Functions and Deficiency Symptoms of Essential Elements

ESSENTIAL ELEMENTS

NUMBER OF PERCENTAGE ATOMS OF DRY RELATIVE TO

TISSUE MOLYBDENUM FUNCTIONS

DEFICIENCY SYMPTOMS

Micronutrients molybdenum (Mo) 0.00001

copper (Cu)

0.0006

0.002

manganese (Mn)

0.005

0.002

0.01

chlorine (Cl)

Macronutrients sulfur (S)

0.01

1 part of nitrate reductase;

essential for N fixation

100 component of plastocyanin, a plastid pigment; present in lignin of xylem elements; activates enzymes

300 necessary for formation of pollen; involved in auxin synthesis; maintenance of ribosome structure; activates enzymes

1,000 photosynthetic O2 evolution;

enzyme activator; electron transfers

2,000 essential for growth of pollen tubes; regulation of enzyme function; possible role in sugar transport

2,000 required for synthesis of chlorophyll; component of cytochromes and ferredoxin; cofactor of peroxidase and some other enzymes

3,000 activates photosynthetic elements; functions in water balance

30,000 part of coenzyme A and the amino acids and cysteine and methionine; can be absorbed through stomata as gaseous SO2

chlorosis or twisting of young leaves young leaves dark green, twisted, and wilted; tips of roots and shoots remain alive; rarely deficient chlorosis, smaller leaves, reduced internodes; distorted leaf margins; older leaves most affected interveinal chlorosis; appears first on older leaves; necrosis common; disorganization of lamellar membranes death of apical meristems; leaves twisted and pale at base; swollen, discolored root tips; young tissue most affected interveinal chlorosis; short and slender stems; buds remain alive; affects young leaves first wilted leaves; chlorosis; necrosis; stunted, thickened roots interveinal chlorosis; usually no necrosis; affects young leaves; rarely deficient

NUMBER OF PERCENTAGE ATOMS ESSENTIAL OF DRY RELATIVE TO DEFICIENCY

ELEMENTS TISSUE MOLYBDENUM FUNCTIONS SYMPTOMS

phosphorus (P)

magnesium (Mg)

calcium (Ca)

potassium (K)

nitrogen (N)

oxygen (O)

carbon (C)

hydrogen (H)

60,000 part of nucleic acids, sugar phosphates, and ATP; component of phospholipids of membranes; coenzymes

80,000 part of chlorophyll; enzyme activator; protein synthesis

125,000 membrane integrity; in middle lamella; functions as "second messenger" to coordinate plant's responses to many environmental stimuli; reversibly binds with calmodulin, which activates many enzymes

250,000 regulates osmotic pressure of guard cells, thereby controlling opening and closing of stomata; activates more than 60 enzymes; necessary for starch formation

1,000,000 part of nucleic acids, chlorophyll, amino acids, protein, nucleotides, and coenzymes

30,000,000 major component of plant's organic compounds

35,000,000 major component of plant's organic compounds

60,000,000 major component of plant's organic compounds stunted growth, dark green pigmentation; accumulation of anthocyanin pigments, delayed maturity; affects entire plant; second to N, P is element most likely to be deficient chlorosis and reddening of leaves; leaf tips turn upward; older leaves most affected death of root and shoot tips; young leaves and shoots most affected chlorosis and necrosis, weak stems and roots; roots more susceptible to disease; older leaves most affected general chlorosis, stunted growth; purplish coloration due to accumulation of anthocyanin pigments; N is element most likely to be deficient rarely limiting enough as nutrient to cause specific symptoms rarely limiting enough as nutrient to cause specific symptoms rarely limiting enough as nutrient to cause specific symptoms

table leads to two important generalizations. First, plants require different amounts of different elements. For example, most plants require about 35 million times more carbon than molybdenum. These differing requirements reflect the various uses of these elements: carbon is in almost all compounds in plants, whereas molybdenum occurs in only a few. Secondly, most elements have several functions. Potassium is involved in starch synthesis, affects protein shape, and activates enzymes. Interestingly, the functions of most micronutrients in plants are similar to those in animals.

Although the functions of essential elements are diverse, they can be grouped into four general categories.

  1. Essential elements can be parts of cell structures. Carbon, hydrogen, and oxygen are part of all the biological molecules, such as carbohydrates. Similarly, nitrogen is an integral part of proteins and nucleic acids.
  2. Essential elements can be parts of compounds involved in energy-related chemical reactions in a plant. Magnesium is part of chlorophyll, and phosphorus is part of ATP and nucleic acids.
  3. Essential elements can activate or inhibit enzymes. Magnesium stimulates several enzymes involved in cellular respiration, whereas calcium inhibits several enzymes. In some cases, these enzymes may be those responsible for synthesizing plant hormones.
  4. Essential elements can alter the osmotic potential of a cell. For example, the movement of potassium into and out of guard cells causes them to open and close stomata, the tiny openings in the epidermis of leaves.

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