Fungal Growth Responses to Osmotic as Compared to Matric Water Potential

The osmotic and matric water potential characteristics of Phytophthora cinnamomi and Alternaria tenuis, fungi of widely different tolerance to water stress, were evaluated using agar media of varying nutritional status and three texturally different soils. The water potential of the agar media was controlled osmotically with KCl or sucrose. For the osmotic soil system, soil water content was maintained approximately constant and the desired water potential range was achieved by adding KCl solutions of different concentrations to air-dry soil samples at a level equivalent to the water content of the field moist soils; for the matric system, decreases in water potential were achieved by adding decreasing volumes of water to air-dry soil according to the moisture retention characteristics of the soils. Fungal growth was determined by measuring changes in colony diameter with time. Osmotic water potentials for optimum fungal growth in agar and soil systems were similar to the water potentials for optimum growth in soils of matric-controlled water potential. However, with decreasing water potential both fungi were much less tolerant to matric than to osmotic stress, emphasizing that fungal response to decreasing soil water cannot be explained solely on a water potential basis, but must include consideration also of changes in other water-related soil properties such as solute transport. Fungal responses to matric and osmotic water potential differences were not related to soil texture. From a practical standpoint, the matric potentials at which growth extinction occurred were numerically one-half to two-thirds of the corresponding osmotic potentials at which growth ceased. If this relationship holds for other fungi and other soils, fungal growth responses to water potential obtained using simple agar systems of osmotic-controlled water potential should provide useful approximations of the matric water potential relations of fungi in soil, thereby facilitating prediction of the maximum soil water potential range within which specific fungi may develop competitively in natural soils.