Introductory Crop Physiology (1+1)

Characteristics of Phytohormones

• The word hormone is derived from Greek, meaning 'set in motion.' Plant hormones affect gene expression and transcription levels, cellular division, and growth.
• They are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can also effect plant growth.
• Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth, development, and differentiation of cells and tissues.
• The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized.
• Plants utilize simple chemicals as hormones, which move more easily through the plant's tissues.
• They are often produced and used on a local basis within the plant body, plant cells even produce hormones that affect different regions of the cell producing the hormone.
• Hormones are transported within the plant by utilizing four types of movements.
• For localized movement, cytoplasmic streaming within cells and slow diffusion of ions and molecules between cells are utilized.
• Vascular tissues are used to move hormones from one part of the plant to another; these include sieve tubes that move sugars from the leaves to the roots and flowers, and xylem that moves water and mineral solutes from the roots to the foliage.
• Not all plant cells respond to hormones, but those cells that do are programmed to respond at specific points in their growth cycle.
• The greatest effects occur at specific stages during the cell's life, with diminished effects occurring before or after this period.
• Plants need hormones at very specific times during plant growth and at specific locations.
• They also need to disengage the effects that hormones have when they are no longer needed.
• The production of hormones occurs very often at sites of active growth within the meristems, before cells have fully differentiated.
• After production they are sometimes moved to other parts of the plant where they cause an immediate effect or they can be stored in cells to be released later.
• Plants use different pathways to regulate internal hormone quantities and moderate their effects; they can regulate the amount of chemicals used to biosynthesize hormones.
• They can store them in cells, inactivate them, or cannibalise already-formed hormones by conjugating them with carbohydrates, amino acids or peptides.
• Plants can also break down hormones chemically, effectively destroying them. Plant hormones frequently regulate the concentrations of other plant hormones.
• Plants also move hormones around the plant diluting their concentrations.
• The concentration of hormones required for plant responses are very low (10-6 to 10-5mol/L). Because of these low concentrations, it has been very difficult to study plant hormones, and only since the late 1970s have scientists been able to start piecing together their effects and relationships to plant physiology.
• Much of the early work on plant hormones involved studying plants that were genetically deficient in one or involved the use of tissue-cultured plants grown in vitro that were subjected to differing ratios of hormones, and the resultant growth compared.
• The earliest scientific observation and study dates to the 1880s; the determination and observation of plant hormones and their identification was spread-out over the next 70 years.
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