valence

number of homologous or partially homologous chromosomes that pair during meiosis.

value

Value is a measure of the lightness or darkness of a color. It is given a numerical value equal to the square root of the percentage of incident light reflected by the sample being described. The value scale ranges from 0 to 10, with 0 representing pure black and 10 representing pure white. Munsell color notations use value along with hue and chroma to describe soil colors.

van der Waals forces

Attraction between nonpolar molecules resulting the motion of electrons. The movement of electrons on one molecule can be influenced by the movement of electron on a nearby molecule. The presence of the electrons of an adjacent molecule may push a molecule's own electrons to one side, resulting in a partial positive charge on one side of the molecule and partial negative charge on the other.

vapor pressure deficit (VPD)

It is a raw number of the amount of vapor needed to reach saturation. i.e., you could have 50% relative humidity at 40°F and 80°F. But the VPD at 80°F is much higher than at 40°F.

variables

Variables describe some measurable attribute such as yield or color.

variant

Seeds or plants that (i) are distinct within the variety but occur naturally in the variety, (ii) are stable and predictable with a degree of reliability comparable to other varieties of the same kind, within recognized tolerances, when the variety is reproduced or reconstituted, and (iii) were originally a part of the variety as released. Usage: Variants are not the same as off-types.

variate

A measurement made on a variable.

Variation

Observable or measurable differences among individuals.

varietal protection

Legal statutes that give to the developers or owners of a variety the exclusive right to control the seed production and marketing of that variety. See also protected variety.

variety

See cultivar.

vascular bundle or vein

An elongated strand containing xylem and phloem, the conducting tissues. Influx of water and minerals is through the nonliving xylem vessels which generally have the larger diameter. Export of carbon and nitrogen assimilates occurs through the living phloem sieve tubes.

vascular cambium

a growing region of the plant that produces parallel rows of new vascular cells.

vectors

An organism capable of transmitting pathogens from one host to another.

vegetative propagation

Asexual propagation using pieces of vegetation (e.g., sprigs or sod pieces). Compare vegetative reproduction. See also Part II.

vein

the thread of vascular of tissue through a leaf.

venation

the pattern of veins.

ventral

referring to the lower side. Compare to dorsal.

vermiculite

Vermiculite occurs in rocks and soils that have high contents of magnesium. It has Mg2+ in the octahedral sheet instead of Al3+. The lattice is partially expanded, and two layers of water molecules occupy the space between "sandwiches" composed of two tetrahedral sheets and one octahedral sheet. Mg2+ ions in these water layers offset the excess negative charges resulting from Al3+ replacing one-fourth of the Si4+ in the tetrahedral sheets. These Mg2+ ions are exchangeable, so vermiculite has a high cation-exchange capacity.

vernal dormancy

quiescence in the spring.

vernalization

low temperature requirement to promote flowering

vertical

in biological systems, vertical refers to the orientation of organisms in relation to one another. This may be hierarchical or spatial in relation to a vertical axis of orientation

virulence

1) qualitative, differential ability to cause disease in a specific host genotype; 2) degree of pathogenicity; relative or quantitative ability to cause disease in a specific host genotype or species (see aggressiveness).

virulent

able to cause disease in a specific host genotype; lacking an avirulence gene corresponding to a given host resistance gene.

viviparous

Giving birth to active young, which undergo growth and development inside the mother. With plants, germination takes place while the new plant is still attached to the parental one.

volatilization

The escape of gaseous materials from a soil to the atmosphere is called volatilization. Volatilization of ammonia can occur from alkaline soils. Another type of volatilization occurs in large amounts in wet soils. Certain anaerobic microbes produce volatile materials (mostly nitrogen gas, partially reduced oxides of nitrogen, hydrogen sulfide, and methane) under wet conditions. Nutrient deficiencies can result from these gases escaping to the atmosphere.

volatilize

transformation from a liquid form to a gaseous form.

volcanic ash

The deposit of dusty material originating from a volcanic eruption is called volcanic ash and is usually thin but extensive. Volcanic ash particles are small, angular, and easily weathered, and soils formed from volcanic ash generally have high clay contents.

voltinity

the number of generations that occur in one year.

volumetric heat capacity

the measure of the amount of heat stored in a unit volume of soil when the soil's temperature is raised by a unit temperature. For a mixture of materials in soil, the volumetric heat capacity is the sum of the volume fraction of each component times the specific heat capacity of each component. Units are cal cm-3 C-1.

volunteer plants

Plants that occur (in a population) not as a result of current seeding (of the crop under consideration) but resulting from seeds or propagative vegetative parts growing uncontrolled from previous seeding or from plants escaped from cultivation that have been scattered by natural means.