Use of a mixture of sand and water-absorbent synthetic polymer as substrate for the xenic culturing of plant-parasitic nematodes in the laboratory

Abstract

Xenic culturing of plant-parasitic nematodes on host plants was successful in sterilised natural soils in the laboratory, particularly in native soil, in which large populations of selected nematode species were observed in situ (Mountain, 1960; Wallace, 1963). However, there are cases when native soil cannot be used. For example, this would not be possible in our laboratory in France for the culturing of tropical nematode species, because importation of large quantities of tropical soils is prohibited for regulatory reasons and because shipping would be too expensive anyway. Moreover, tropical and temperate soils have quite different physical and chemical properties (Duchaufour, 1995) and it would not be possible to find in France soils similar to the native tropical soils. Moreover, most of the soils that are available in gardening stores are too rich in clay and organic matter to be easily handled in the laboratory. As an alternative to native tropical soil, we tried to use pure silica sand. This substrate looked very promising because it could be obtained from quarries for the glass industry and it has several convenient features: unlimited and inexpensive supply, fine granulometry from 100 to 300 pm, dryness and cleanliness, which made sterilisation unnecessary. The addition of a mineral nutritive solution, such as Hoagland’s solution, provided the mineral requirements for host plant growth. Nevertheless, during a culturing period of several weeks or months, leaves became chlorotic and the growth of the host plant was poor, compared with the growth observed on sterilised native soil. This was mainly due to the fact that watering caused the sand progressively to reach maximal compactness (Sloane, 1984). This probably reduced aeration and prohibited penetration of the hardened substrate by the roots of many host plants. Besides, water retention of pure sand is low and, when the plant has been growing for some time, water vapour loss becomes high and the plants need watering up to several times daily. Thus, pure sand was found convenient for short term (10-15 days) experiments only, e.g., determination of the invasion rate of infective J2 of Heterodera spp. (Reversat & Merny, 1973). In this paper we report the use of a mixture of pure silica sand and water-absorbent synthetic polymer as substrate for the successful culturing of some tropical nematodes in our laboratory. The addition of this polymer to the silica sand balanced the drawbacks of pure sand mentioned above: progressively acquired compactness and low water content. Such water absorbents have been developed by the chemical industry since the seventies for various purposes, particularly as soil conditioners in agriculture for dry areas (De Boodt, 1990; Rognon, 1995). Most of them are based on an acrylic polymer of high molecular weight, able to gelify by rapidly absorbing up to 400 times its own weight of deionized water. In commercial packages (e.g., Graind’eauB in France, AgrosokeB in UK, Hydro Kristall@ in Germany, etc.), these absorbents are conditioned as granules 1 to 2 mm in size. After absorption of deionized water, they form pieces of gel up to 10 mm in size, which could not be mixed homogeneously with sand. Consequently, the pieces of gel were first converted into a paste by sieving under pressure. For this purpose, we used a 100 ml plastic syringe with the bottom end cut out and replaced by a screen made of stainless steel wire gauze, with an aperture of 0.25 mm, solidly melted into the plastic wall of the syringe. The syringe was filled with pieces of gel and the pressure of the plunger forced the gel to pass through the screen apertures. This resulted in filaments of 0.25 mm by up to 10 mm. Since these filaments were not found to be convenient, they were put back in the same syringe and processed in the same manner to form a pasty gel. This pasty gel (consisting of 200 g of water and 2 g of