paddock A grazing area that is a subdivision of a grazing management unit, and is enclosed and separated from other areas by a fence or barrier. Compare pasture.
PAGESee polyacrylamide gel electrophoresis.
pantropical Growing throughout the warmer regions of the world. Used especially of the ranges of widespread weeds.
partial digest Treatment of a DNA sample with a restriction endonuclease for a limited period so that only a portion of the target sites in any individual molecule are cleaved. Partial digests are often performed with four base pair cutters to give a collection of overlapping DNA fragments for use in the construction of a gene bank. Synonym: incomplete digest. Compare complete digest.
partitioning The process by which assimilates are distributed among competing sink tissues and activities.
passage number The number of times the cells in the culture have been subcultured. Usage: In descriptions of this process, the ratio or dilution of the cells and interval lengths should be stated, so that the relative cultural age can be ascertained.
pasture (1) A type of grazing management unit enclosed and separated from other areas by fencing or other barriers and devoted to the production of forage for harvest primarily by grazing. Compare paddock. (2) See forage.
pasture carrying capacitySee carrying capacity.
pasture, introducedSee under native pasture. Compare naturalized pasture.
pasture, supplemental A crop used to provide grazing for supplemental use, usually during periods of low pasture production.
pasture, temporary A field of crop or forage plants grazed for only a short period, usually not more than one crop season.
path coefficient A measure of the importance of a given path of influence from cause to effect, it is the ratio of the standard deviation of the effect (with all causes kept constant except the one in question, the variability of which is kept unchanged) to the total standard deviation. This is the standard partial regression coefficient in a prediction equation in which all independent variables are causes of the effect and all relevant variables are included.
pathogen-free Part II.
PCRSee polymerase chain reaction.
pedigree (1) A system of plant breeding in which detailed records are kept of all plants in segregating generations following hybridization. (2) A record, using standardized notation, showing the line descent for a variety.
pelleted seedSee coated seed.
perennial Persisting for several years, usually with new growth from a perennating part.
period of occupation The length of time that a given land area is occupied, whether by one animal group, or by two or more animal groups in succession. Usage: Period of occupation may involve several different groups moving through in sequence, as in first-last grazing or in migrations). It differs from grazing period in that grazing may or may not be involved (e.g., feeding hay on pasture in winter when ice prevents grazing). Compare period of stay. See also forward creep grazing.
perloline A plant alkaloid found to interfere with cellulose digestion by rumen microorganisms; commonly associated with tall fescue.
persistent (1) Retaining its place, shape, or structure; remaining attached after the growth period; not deciduous; designating fruits, flowers, leaves, etc., that remain on a plant after frost. (2) Of a disease: difficult to control.
phage Short form (and usual form) of bacteriophage.
phenotype Part II.
photomorphogenesis A special case of morphogenesis in which light, by virtue of absorption of particular wavelengths, promotes development of tissues or organs. Photomorphogenetic pigment systems commonly recognized are: (i) phytochrome; (ii) a blue-absorbing high-energy system perhaps related to phytochrome; and (iii) a riboflavin-carotenoid system active in phototropism.
photosensitization A noncontagious disease in animals resulting from the abnormal reaction of light-colored skin to sunlight after a photodynamic agent has been absorbed through the system. Grazing certain kinds of vegetation or ingesting certain molds under specific conditions causes photosensitization.
physical adaptationSee adaptation.
phytohormone Plant hormone.
pistilloidy The conversion of any organ of a flower into carpels, often stamens into pistils or pistillike structures.
plant breeder Person or organization actively engaged in the breeding and maintenance of varieties of plants.
plant nutrientsSee nutrient, plant.
plant regeneration Part II.
planting density The rate at which seed or vegetative propagules are placed in a field or nursery planting.
plasmid An autonomous, self-replicating extrachromosomal DNA that is usually circular but may be linear. Plasmid sizes vary from <1 kb to >300 kb and copy numbers per cell range from 1 to >100. Plasmids often have genes for specific functions, such as antibiotic resistance, but may also be lost from the host cell. Plasmid cloning vectors have been constructed for desirable features such as antibiotic resistance genes, multiple copies, single restriction endonuclease sites, and strong promoters. Usage: Plasmid names usually (but not always) start with a lowercase p, followed by two or three capital letters and a number (e.g., pBR322, pMON631).
plastid One of various double-membraned organelles in plant-cell cytoplasm, including chloroplasts and chromoplasts.
plugging Vegetative propagation of turfgrasses by plugs or small pieces of sod. Used for (i) establishing vegetatively propagated turfgrasses or (ii) repairing damaged areas.
pollen culture The in vitro culturing of pollen grains to obtain haploid callus or haploid plantlets directly from the pollen grains. Pollen culture differs from microspore cultures by the stage of development in gametogenesis cycle.
pollination The transfer of pollen from the anther to the stigma of a flower.
poly(A) Synthetic or biosynthetic polynucleotide composed of adenlyate residues.
poly(G) Synthetic polynucleotide composed of guanylate residues.
polyacrylamide gels Gels made by cross-linking acrylamide with N'N'-methylene-bis-acrylamide. Polyacrylamide gels are used for the electrophoretic separation of proteins and RNA molecules, but DNA molecules usually are too large to migrate far in polyacrylamide.
polyembryony The condition in which a seed has more than one embryo.
polymerase chain reaction An in vitro procedure to amplify the number of copies of a specific DNA sequence often found in rare abundance in a complex DNA source. Two single-stranded synthetic oligonucleotides of defined sequence are required to prime synthesis of the source DNA sequence located between its sites of homology to the primers. Primers are annealed to denatured source DNA, and DNA polymerase from Thermus aquaticus is added to initiate synthesis of two new complementary DNA strands by extension from the primers. The reaction mixture is alternately heated (to produce single-stranded templates for the next round of synthesis) and then cooled (to allow synthesis to proceed). After the first few cycles, most new strands end at the primer from the previous synthesis. A sequence of a specific length and base sequence can be amplified many orders of magnitude and used for cloning, sequence analysis, and other molecular biology procedures. Abbr.: PCR.
population density (1) The number of cells per unit area or volume of a culture vessel. (2) The number of cells per unit volume of medium in a suspension culture.
population density 2 The number of units (plants) per given area (or unit of row); i.e., plant density.
population doubling time The interval calculated during the logarithmic phase of growth (in which, for example, 1.0 x 106 cells increase to 2.0 x 106 cells). The average population doubling time may be calculated from the time it takes for the number of cells, at the point of inoculation, to increase to the number at the point of termination or harvesting of the culture. Usage: Not synonymous with cell generation time. Synonym: cell doubling time.
prebloom The stage or period immediately preceding blooming.
precleaning The process for removing the bulk of foreign materials grossly different in size from the crop seed.
preference-follower grazingUsage: Not a recommended term. See first-last grazing.
preservative An additive used to protect against decay, discoloration, or spoilage.
Pribnow boxSee TATA box.
primer A short oligonucleotide that is paired with one strand of DNA and provides a free 3' hydroxyl group required for initiation of DNA synthesis by DNA polymerase. An RNA primer is used for the initiation of DNA replication in vivo. In contrast, RNA polymerase can initiate synthesis de novo and does not require a 3' primer.
primordium Part II.
probe (1) A defined nucleic acid sequence that has been labeled with a radioactive isotope or fluorescent dye. It is used to identify specific DNA or RNA molecules that have the complementary sequence. (2) To perform hybridization to detect a specific gene or transcript.
prokaryote A cell or organism lacking a true nucleus separated from the cytoplasm by a discrete membrane (e.g., bacteria, cyanobacteria).
promoter A regulatory region of DNA near one of the two ends (usually the 5' end) of the coding sequence of a gene or operon that binds RNA polymerase and directs the enzyme to the correct transcriptional start site.
protected variety A plant variety that is released and granted a certificate of plant variety protection under the legal statutes of the USA or some other country. The owner of a protected variety has the right during the term of the protection to exclude others from selling the variety, offering it for sale, reproducing it, importing it, exporting it, or using it in producing a hybrid or different variety. See also varietal protection.
protein fraction Solubility of proteins, as originally termed by Osborne, who designed a sequential extraction scheme based on water, dilute salt, 70% (v/v) alcohol, and dilute acid or alkali.
protein quality (1) Balance of essential amino acids in protein. (2) Biological availability of protein. In general, for cereals the first limiting amino acid and the one that affects protein quality the most is lysine.
protoplast A membrane-bound cell formed after the cell wall is removed from a microbial or plant cell by the action of pectinases and cellulases. Protoplasts are used to produce hybrid cells via fusion and to facilitate transformation. Compare spheroplast.
prototrophSee autotroph.
proximate analysis Analytical system that includes the determination of ash, crude fiber, crude protein, ether extract, moisture (dry matter), and nitrogen-free extract.
pseudogene A DNA sequence similar to a known functional gene, but which is not itself functionally expressed.
pseudothatch The upper surface layer above a thatch, made up of relatively nondecomposed leaf remnants and clippings.
pubescence A general term for hairs or trichomes.
puffiness Spongelike condition of turf, resulting in an irregular surface.
pure live seed Percentage of pure germinating seed: (pure seed percentage x germination percentage)/100.
put-and-take stocking The use of variable animal numbers during a grazing period or grazing season, with a periodic adjustment in animal numbers in an attempt to maintain desired sward management criteria (e.g., a desired quantity of forage, degree of defoliation, or grazing pressure).