VIEW ARTICLE

Physiology and Biochemistry

Some Effects of Water Potential on Growth, Turgor, and Respiration of Phytophthora cryptogea and Fusarium moniliforme. D. M. Woods, Department of Plant Pathology, University of California, Davis 95616, Present address: California Department of Food and Agriculture, 3288 Meadowview Road, Sacramento 95832; J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 76:1248-1254. Accepted for publication 12 May 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1248.

Mycelial growth, turgor, and respiration of Phytophthora cryptogea and Fusarium moniliforme were evaluated in media adjusted to various constant water potentials (φ) by adding solutes. Growth by P. cryptogea in standing liquid cultures was reduced by 50% at -9 to -31 bars φ, whereas a 50% reduction in the growth of F. moniliforme occurred at -26 to -200 bars, the exact values depending on the medium and solutes used to vary φ. Growth measured as fresh weight was generally decreased more by decreases in φ than was growth in dry weight. Whereas P. cryptogea grew at lower φ values in a complex liquid medium of high nutritional content than in a defined medium, F. moniliforme gave the opposite results. Use of agar rather than liquid medium extended the φ range over which P. cryptogea grew. Mycelial turgor in P. cryptogea, as estimated with thermocouple psychrometers, gradually increased from 12 to 25 bars as medium φ was decreased from -5 to -24 bars, even though the same decreases in φ reduced growth. F. moniliforme maintained turgor pressures that averaged 15 bars over the entire φ range of -5 to -200 bars that allowed measurable growth. Respiration rates were significantly higher for mycelial mats grown at low φ values, and respiration rates of P. cryptogea increased proportionately more than did those of F. moniliforme as φ decreased. The results suggest that the metabolic costs of growth at low φ values influence growth rate more than does turgor pressure and are more limiting for P. cryptogea than for F. moniliforme.