phenomenon is called allelopathy. The term allelopathy was coined by Molisch in 1937 to refer to biochemical interactions between all types of plants, including both inhibitory and stimulatory interactions. Allelopathy is a well known fact in ecosystems and specially in desert ecosystems (Friedman 1987). Plants with allelopathic potential are often adult perennials, members of the Compositae or Labiatae, that are capable of reducing germination and/or growth of various annuals or of their own seedlings (autotoxicity). Variable ecological conditions may trigger allelopathy in one habitat and abrogate it in another. Volatile compounds accumulate in the soil during the long dry summer and in the winter, when germination commences, they will be released into the soil microsphere, where they inhibit growth of other plants.

Allelochemicals have already been shown to impose numerous impacts in cultivated and natural ecosystems. Many types of chemicals are involved in allelopathy, including essential oils, which contain a great variation of compounds most of them belonging to terpenes or terpenoids.

Plant essential oils can have, besides allelochemical properties, many other biological functions such as attracting or repelling insects, antimicrobial activity or making particular plants or part of plants unattractive as food for animals.

S-carvone from the essential oil of caraway seed has recently been developed as a crop protection product on potatoes, because of its interesting allelochemical properties as a sprout growth suppressant and antifungal agent on pathogens causing storage diseases.

12.2. POTATO STORAGE 12.2.1. Storage Conditions

The potato is the world's fourth most important food crop after the leading three rice, maize and wheat. The demand for potatoes is relative stable in Western Europe and the USA. In Eastern Europe we can expect dramatic increases in productivity per unit area if current know-how and technological expertise are applied. Furthermore, an increasing proportion of this global crop is being produced in the less-developed countries, now approximately 1/ 3 of the total, and rising. The proportion of the crop utilized in processed products like French fries, chips (crisps) etc. is worldwide increasing.

A potato tuber is in fact a modified stem with a shortened (and broadened) axis. The apex and the eyes can be distinguished on the tubers. The eyes often contain not only a main bud but also a few small lateral buds (Figure 1). As many as 20 or more buds may be present on a tuber. Control of spout growth from these buds is very important when tubers have to be stored for a long time. Potato storage is essential in all parts of the world where the crop can be grown for only parts of the year. After storage the potatoes should be firm and unwilted. Potato tubers cease to be firm when evaporative loss from an initially turgit tuber reaches about 5%. Moisture losses are much greater in potatoes that have sprouted. Apart from direct weight loss in the form of sprouts, Burton (1955) calculated that extra weight losses occurred due to the fact that the epidermis of the sprout is about 100 times more permeable to water as the surface of the tuber.

Figure 1 Scanning electron micrograph of a single potato eye, showing the original axillary bud of the leaf scale and two second order axillary buds of its two leaf primordia. (By courtesy of Chris van der Schoot, ATO-DLO)

Storage at low temperature can control sprouting for a long time, but causes a marked increase in reducing sugars, particularly when stored at temperatures below 6°C. Below 10°C reducing sugars are formed to an increasing degree (Burton 1965a). The reducing sugar content is very important where potatoes are processed into fried products. On frying, the potato products darken owing to the reaction between the reducing aldehyde groups of the sugars glucose and fructose and the amino groups of the free amino acids: this is known as the Maillard reaction. This darkening may be so intense as to greatly reduce the value of the end product. Of all the factors that determine the amount of reducing sugars during storage and hence the suitability for processing, cultivar and storage temperature are probably the most important.

In practice storage temperatures are generally maintained at 6-8°C. To keep the temperature of the potatoes during storage at 6-8°C, regular cooling is needed, because the respiration of the tubers during storage results in a permanent uptake of oxygen and a release of carbon dioxide, water and energy in the form of heat. Cooling with outside air is an efficient and relative inexpensive method, which is often used in potato storage. Outside air can be used to cool the potatoes only during periods in which the ambient temperature regularly falls below the required storage temperature. Consequently during such, sometimes short, periods high ventilation rates are used to be able to cool down the potatoes as quick as possible (Rastovski 1987).

In order to be able to control sprout growth during longterm storage at these temperatures, chemical sprout growth suppressants are widely used. In general, the high ventilation rates of the cooling systems make regular reapplication of sprout suppressants during storage necessary.

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