Parenchyma

One of the three main ground tissues (the others being Collenchyma and Sclerenchyma). These cells generally provide packing within a plant, forming tissues such as the pith and cortex. The cells can function as storage places for products of photosynthesis as well as for water. Some specialised parenchyma cells are formed within a leaf, the mesophyll layer, being called Chlorenchyma and include chloroplasts for photosynthesis. Other parenchyma cells within this layer are called the spongy mesophyll and function as a place for the exchange of plant and atmospheric gases.

  Parts Per Million

This is often used to indicate the concentration of elements within a plant or nutrients needed by a plant for adequate growth, for example, 30ppm of x-nutrient; or as a quantity of active ingredient within a pesticide above which there is considered an unacceptable risk, for example 80ppm of x. It is often referred to as PPM or ppm.

  Pathogen

An organism which causes a disease or disorder. Fungi are typical pathogens of turf. Pathogens might be specific a certain plant, or may only infect certain parts of a plant, root, leave or stem.

  Penetrant

A wetting agent which reduces surface tension and allows water to penetrate a soil and move through the soil profile. These types of wetting agent can help as a first stage of soil-water improvement, reducing some repellency conditions which contribute to water logging, and which is then followed up by the use of residual wetting agents which have a longer residence time within a soil, thereby also helping to retain water better, especially within sandy soils. A penetrant is a surfactant.

  Penetrant Wetting Agent

A substance which reduces surface tension and allows water to penetrate a soil and move through the soil profile.

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  Perennial

A plant that lives for more than two years. There are woody perennial and herbaceous perennials. Woody perennial These are trees and shrubs that have persistent vegetation that is above ground. Woody perennials can be either deciduous or evergreen. Examples include: Evergreen • Holm Oak (Quercus ilex). • Viburnum tinus. Deciduous • Spiraea japonica. • Hawthorn (Crataegus monogyna). Herbaceous perennials These plants often have their vegetation die down and over winter as an underground organ, such as a bulb, corm, rhizome or tuber, with examples including:

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  Permanent Wilting Point

The point at which a plant will wilt and not recover. The measure is usually given as minus 15 bar suction. This means a plant cannot exert enough suction to extract water from its surroundings, because water is held more strongly by the soil. Subsequently the plant will wilt and die.

  Pest Threshold

A threshold level above which unacceptable damage or harm will occur to a turfgrass surface. Setting a threshold for pest (which is a generic term for diseases, animal pests and weeds) infestation is part of the approach taken in Integrated Pest Management. This can help to reduce unnecessary usage of pesticides, but also to focus a grounds manager’s attention on making best use of physical resources and technical knowledge. Levels of pest infestation below the threshold can be tolerated, although this doesn’t mean that the extent of the infestation and damage is ignored, but actively mon

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  pH [2]

pH is a measure of the soil acidity and the scale ranges from 0 to 14. It is measured as the negative (decimal) logarithm of the hydrogen ion concentration of a solution. With the pH scale being logarithmic (base 10) each pH unit is 10 times the value of the previous value; thus pH 5 is 10 times more acidic than pH 6; whilst pH 5 is 100 times more acidic than pH 7. Turfgrasses will grow over a wide range of pH levels, although optimum growth will take place over narrower parameters. The more acidic soils with a pH of less than 5 will favour the fine fescues and browntop bent grasses.

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  Phenotype

The collective characteristics of a plant, or organism. This consists of genotype and morphology, and the influence of environmental factors (the latter being non-inheritable).