habituation The ability of cells to grow in the absence of normally required and exogenously supplied growth factors as a result of shifts in metabolism, especially as in the case where cultured cells become autonomous (habituated) for the production of phytohormones. Usage: Habituated is the preferred term.
HAPSee hydroxyapatite.
hard seed Seed that is dormant due to a seed coat impervious to either water or oxygen.
hardiness Capability of an organism to withstand environmental stress. Synonym: stress tolerance. See tolerance.
head components Components of the inflorescence of grains and grass crops; generally, the grain rather than vegetative structures, but may include individual vegetative structures such as rachis, peduncle, and pedicel.
heading The developmental stage of a grass plant from initial emergence of the inflorescence from the boot until the inflorescence is fully exserted.
heat unitSee heat sum.
hemicellulose Polysaccharides that accompany cellulose and lignin in the cell walls of green plants; differs from cellulose in that it is soluble in alkali and with acid hydrolysis gives rise to uronic acid, xylose, galactose, and other carbohydrates as well as glucose.
herbage The biomass of herbaceous plants, other than separated grain, generally above ground but including edible roots and tubers.
heteroduplex A DNA duplex formed by hybridization of complementary regions of single strands derived from different parental double-stranded DNAs. If the single-strand sequences are not completely complementary, then single-stranded regions will be present in the heteroduplex molecule and can be visualized and mapped using electron microscopy.
heterogeneous Characterizing individuals with nonhomozygous allelic genetic constitutions, or populations of plants with individual plants of different genetic constitution. Usage: Not applied to a single locus, where the term is heterozygous. n. heterogeneity.
heteroploid Of a cell culture: comprised of cells possessing nuclei that contain chromosome numbers other than the diploid number. Thus, a heteroploid culture is one containing aneuploid cells. Usage: Not used to describe individual cells. See also mixoploid; -ploid.
high-intensity grazingUsage: Not an acceptable term (it is a relative concept, best described in terms of management and method). See grazing management; intensive grazing management.
high-velocity microprojectile transformation A method for introduction of DNA into plant cells. Tungsten or gold particles are coated with DNA and propelled at high speed through the target cell walls by means of an electrical or gunpowder discharge.
histochemistry The chemistry of cells and tissues.
holistic resource managementUsage: Not a recommended term.
homogenotization The replacement of one copy of a DNA sequence (usually a gene) with an altered copy by genetic and recombinant DNA techniques. The target gene is cloned and modified in some way, such as by inserting a transposable element containing a marker gene into the target gene. Introduction of the modified gene back into the target host cell may allow the modified gene to be exchanged for the target gene by in vivo genetic recombination. Selection for expression of the marker gene facilitates identification of modified cells.
homology (1) The extent to which two nucleic acid molecules have the same nucleotide sequence or two proteins have the same amino acid sequence. Homology can be determined by direct comparison of sequence data, or estimated by DNA-DNA or DNA-RNA hybridization. (2) The degree of identity between chromosomes or chromosome segments.
host A cell or organism used for supporting the replication of a plasmid, virus, or other form of foreign DNA, or for the production of cloned substances.
host 2 Organism infested by a parasite or pathogen.
host-vector system Combination of DNA-receiving cells (host) and DNA-transporting substance (vector) used for introducing foreign DNA into a cell.
hot-water insoluble nitrogen Fertilizer nitrogen not soluble in boiling-point hot water (100 °C; 212 °F); used to determine activity index of ureaforms. See nitrogen activity index.
hybrid released translation A technique to identify the protein encoded by a cloned DNA sequence, usually a cDNA. The cloned DNA is bound to a filter and hybridized to a mixture of mRNA. Only the hybridizing mRNA is used in subsequent in vitro translation reactions for analysis of the polypeptides formed.
hybrid variety First-generation (F1) progenies from a cross, produced through controlling the pollination, between (i) two inbred lines; (ii) two single crosses; (iii) a single cross and an inbred line; (iv) an inbred line or a single cross and an open-pollinated or a synthetic variety; or (v) two selected clones, seed lines, varieties, or species.
hybridization (1) The pairing of complementary DNA or RNA strands to give stable DNA-DNA or DNA-RNA duplexes. The efficiency of hybridization is a test of sequence homology. (2) Production of offspring, or hybrids, from genetically dissimilar parents. The process can be used to produce hybrid plants (by cross-breeding two different varieties) or hybridomas (hybrid cells formed by fusing two unlike cells, used in producing monoclonal antibodies). See also cell hybridization.
hydrocyanic acid HCN. A poison produced as a glucoside by several plant species, especially sorghums. Synonym: prussic acid.
hydroseeding Planting seed in a water mixture by pumping through a nozzle that sprays the mixture onto a seed bed. The water mixture may also contain addends such as fertilizer and mulches.
hydroxyapatite Ca5(PO4)3OH. A form of calcium phosphate that can bind nucleic acids. Under certain conditions it will bind double-stranded but not single-stranded DNA. It can therefore be used to fractionate DNA preparations and to determine the extent of hybridization of two classes of single-stranded molecules. Synonym: hydroxylapatite. Abbr.: HAP.
hypogeal germination A type of germination in which the cotyledons remain below the ground while the epicotyl grows and emerges above the ground.
hysteresis A phenomenon evidenced by the fact that at a given relative humidity, seeds or grain may reach two different equilibrium moisture contents: one by increasing the relative humidity from a low level, and another by decreasing the relative humidity from a high level.