A (1) Adenine, a purine base in DNA and RNA; pairs with thymine (in DNA) or uracil (in RNA). (2) Adenosine, the nucleoside containing adenine as its base.
aberrant Different from the normal type of species, genus, or higher group in one or more characters, but not readily assignable to another group.
absolute growth rateSee growth rate.
acceptability, animal Readiness with which animals select and ingest a forage; sometimes used interchangeably to mean either palatability or voluntary intake.
achlamydeous Of flowers: Lacking petals and sepals; e.g., willow (Salix spp.).
acid-detergent fiber Insoluble residue following extraction with acid detergent (van Soest method); i.e., cell wall constituents minus hemicellulose. Abbr.: ADF. See also neutral-detergent fiber.
acid-detergent lignin Lignin determined on the residue following extraction with acid detergent. Abbr.: ADL.
ad libitum feeding Daily feed offerings in excess of consumption; generally, 115% of consumption.
adenineSee A.
ADP Adenosine 5'-diphosphate.
adventitious Part II.
adventiveSee adventitious.
aeration, mechanicalSee cultivation, turf.
aflatoxin C17H10O6. A polynuclear substance and a known carcinogen derived from molds; specifically, that produced by a fungus occurring on peanut.
agamospermy All types of apomixes in which seeds are formed by asexual means. Does not include vegetative reproduction.
agroforestry Land-use system in which woody perennials are grown for wood production with agricultural crops, with or without animal production.
aleurone A layer of protein granules found in the endosperm of ripe seeds or fruits.
alkaline hydrolysis Cleavage of covalent chemical bonds by high pH. RNA molecules have 2' hydroxyls that attack the 3' phosphodiester bond at high pH and degrade the RNA polymer to monoribonucleotides. DNA lacks 2' hydroxyl groups and is not hydrolyzed at high pH.
allele Any of several alternative forms of a gene. See also locus.
allometric A relationship between two variables of the form y = axb where a and b are constants such that lny = blnx + lna; b is termed the allometric coefficient. Of common use in morphogenesis studies, where organ dimensions and growth rates frequently can be fit by the relation.
alloplasm Cytoplasm from an alien species that has been transferred by backcrossing into a cultivated species. The alien cytoplasm's most common and readily observable effects are male sterility, female sterility, or reduced plant vigor. If the alien cytoplasm does not manifest itself, the cytoplasm is not considered alien.
allozyme Alternative form of an enzyme encoded by different alleles at a given locus. Compare isoenzyme.
amendment, physical Any substance (such as sand, calcined clay, peat, or sawdust) added to soil for the purpose of altering physical conditions.
amphidiploid A plant possessing the sum of the somatic chromosome number of two species.
amphiphloic Having both internal and external phloem.
amphivasal bundle Concentric vascular bundle in which the xylem surrounds the phloem, as in the rhizomes of some monocots. Compare amphicribral bundle.
amplification Process by which the copy number of a gene or plasmid is increased.
androdioecious Plants that have male and perfect flowers on separate plants. Compare andromonoecious.
andromonoecious A plant bearing bisexual or perfect flowers in addition to staminate flowers. Compare androdioecious.
aneuploid (1) Characterizing a cell in which the nucleus does not contain an exact multiple of the haploid number of chromosomes, one or more chromosomes being present in a greater or lesser number than the rest. The chromosomes may or may not show rearrangements. (2) Such a cell or organism. See also -ploid.
animal day One day's tenure upon a pasture by one animal. Usage: Not synonymous with animal-unit day.
animal month One month's tenure upon pasture by one animal. Usage: Not synonymous with animal-unit month.
animal unit One mature nonlactating bovine weighing 500 kg and fed at maintenance level, or, in other kinds of animals, the equivalent, expressed as (weight)0.75. Usage: The use of this term in a publication should be followed by a description in a standard format, including at least the following: kind (species and breed), class, sex, size, age, and physiological status of livestock. Abbr.: AU. Compare standard livestock unit.
anisogamy Sexual reproduction in which the female gamete is large and immobile and the male gamete is small and motile. Compare oogamy. See also autogamy.
anther culture The in vitro culturing, on a synthetic medium, of anthers containing microspores. The microspores may form haploid callus or develop directly into haploid plants. See also androgenesis.
anthesis The action or period of opening of a flower, especially the period when the stigma is favorable for the germination of pollen coming in contact with it. See also blooming; flowering.
anthoxanthins Water-soluble pigments, widely distributed in leaves, stems, and flowers. They are mostly colorless, but can modify other colors, especially anthocyanins.
antibiosis Physiological antagonism of one organism toward another, used chiefly in reference to such antagonism among fungi. See also resistance and related entries.
anticodon A specific sequence of three ribonucleotides in a tRNA molecule that base-pair with the three complementary nucleotides of the codon sequence within mRNA molecules. This codon-anticodon interaction occurs on ribosomes and specifies the insertion of amino acids in the correct order during elongation of polypeptides.
antioxidant An organic compound that accepts free radicals and thus prevents autoxidation of fats and oils. At very low concentrations in food, antioxidants not only act in retarding rancidity but protect the nutritional value or minimize the breakdown of vitamins and essential fatty acids.
antisense gene An engineered gene placed in inverted orientation relative to a promoter that when transcribed, produces a transcript complementary to the mRNA transcribed from the normal orientation of the gene.
antiserum Blood serum containing specific antibodies against an antigen.
apex cultureSee shoot-tip culture.
apoplast The continuum of nonliving plant cell-wall material that surrounds the symplast. It includes the tracheary elements of xylem and the free space, and is one of two parallel pathways for solute movement through plants (the other being symplast). Substances can move through the apoplast only down a thermodynamic potential gradient.
apparent free space A measure of free space gained from ion influx and efflux experiments; it is influenced by ion exchange of solutes with cell walls and thus can differ for each solute. Abbr.: AFS.
aroma A characteristic odor, as of a plant, feed, or food.
artificial turf A fabricated rug of fibers simulating turf. Usage: Astroturf is a trademark, not the general term.
aseptic Characterized by absence of contaminating fungi, bacteria, viruses, mycoplasms, and other microorganisms (e.g., in cultures).
asexual propagationSee vegetative propagation.
ash (1) The residue remaining after complete burning of combustible matter. (2) See cell-wall constituents.
assimilation (1) The incorporation of inorganic forms of chemical elements into organic forms termed assimilates. Thus, we have the assimilation of CO2 into photosynthates such as sugar and of NO3, NH4, and SO4 into amino acids. (2) In growth analysis, net assimilation rate is given a specific definition as the rate of change of crop biomass (i.e., crop growth rate, reflecting the net of photosynthetic and respiratory activities) per unit leaf area index. Net assimilation rates can also be defined for carbon, nitrogen, or other entities. (3) The formation of protoplasm.
atavism Recurrence of an ancestral character after an interval of several generations.
autecology The ecology of individual species, as opposed to that of communities.
autonomic movement Plant movement as a result of internal stimuli. Synonym: autogenic or spontaneous movement.
autonomousSee habituation.
autoradiography A method for determining the presence and location of radioactively labeled molecules by their effect in creating an image on a photographic emulsion, usually x-ray film, by activating the silver halide grains, which are then reduced to metallic silver when the film is developed. Autoradiography is commonly used to determine whether a radioactive probe molecule has hybridized to denatured DNA or RNA following Southern or northern transfers, respectively, or in colony hybridization procedures.
autotrophic Self-reliant. Compare heterotrophic.
auxotroph A mutant requiring specific compound(s) for its growth that are already present in the wildtype. Useful as a genetic marker and for complementation tests. adj. auxotrophic. Compare autotroph.
available assimilatesSee reserves.
available pastureUsage: Not a recommended term (pasture refers to a specific type of grazing management unit, not to that which is consumed). See forage mass; available forage.
avoidance Ability of an organism to prevent an injurious stress, pathogen, or predator from penetrating its tissues (e.g., drought avoidance may be achieved through restriction of water loss or by expansion of the root system to a greater supply of water). See also resistance.
axenic Without other living organisms. Used to refer to a pure culture, i.e., a culture uncontaminated by symbionts or parasites.