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- W -

w/v  Weight per volume; the weight of a constituent in 100 cm3 of solution, expressed as a percentage.

walking  A method for cloning large regions of a chromosome. Starting from a known site, a gene library is screened for clones that hybridize to DNA probes taken from the ends of the first clone. These clones are then isolated, and their ends used to screen the library again. These clones are then isolated and their ends used, and so on. See gene walking; chromosome walking.

wall pressure  Pressure that a cell wall exerts against the turgor of the cell contents. Wall pressure is equal and opposite to the turgor potential.

wash-out  The loss of the slower growing micro-organism when two organisms are being grown together.

water potential  Refers to the difference between the activity of water molecules in pure distilled water at atmospheric pressure and 30°C (standard conditions), and the activity of water molecules in any other system. The activity of these water molecules may be greater (positive) or less (negative) than the activity of the water molecules under standard conditions.

water soaked  See vitrified.

water stress  The condition when plants are unable to absorb enough water to replace that lost by transpiration. The results may be wilting, cessation of growth, or even death of the plant or plant parts.

wavelength  The distance between two corresponding points on any two consecutive waves. For visible light it is very small and is generally measured in nanometres.

wax (A.S. weax, wax)  Esters of alcohol higher than glycerol, which are insoluble in water and difficult to hydrolyze; wax forms protective waterproof layers on leaves, stems, fruits, animal fur and integuments of insects.

weed  Simply any plant growing where it is not wanted. In agriculture, used for a plant which has good colonizing capability in a disturbed environment, and can usually compete with a cultivated species therein. Weeds are typically considered as unwanted, economically useless or pest species.

weediness   In agriculture, the ability of a plant to colonize a disturbed habitat and compete with cultivated species.

western blot  A technique in which protein is transferred from an electrophoretic gel to a cellulose or nylon support membrane following electrophoresis. A particular protein molecule can then be identified by probing the blot with a radiolabelled antibody which binds only the specific protein to which the antibody was prepared. Useful, for example, for measurement of levels of production of a specific protein in a particular tissue or at particular developmental stage. cf blot; northern blot; Southern blot.

wet weight  The gross weight of a product with its full water content, or weight of fully hydrated tissue. See fresh weight.

wetting agent  A substance that improves surface contact by reducing the surface tension of a liquid: e.g., Triton X-10TM added to disinfecting solutions promotes the disinfestation process. See detergent; surfactant.

wild type  An organism as found in nature; the dominant allele usually found in nature, and from which mutations produce other dominants or recessives alleles.

wilt  Drooping of stems and foliage due to loss of water and decreased turgidity of cells. May be caused by water stress or by disease.

wilting point (WP)  The moisture content of soil at which plants start to wilt, but not to the extent that they fail to recover when placed in a humid atmosphere. cf. permanent wilting point.

wobble hypothesis  An explanation of how one tRNA may recognize more than one codon. The first two bases of the mRNA codon and anticodon pair properly, but the third base in the anticodon has some wobble that permits it to pair with more than one base.

WP  See wilting point.

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