Influence of the Matric and Osmotic Components of Water Potential on Zoospore Discharge in Phytophthora

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Abstract
Mycelial disks from agar plate cultures of P. cryptogea and P. megasperma [plant pathogenic fungi] incubated in soil at -150 millibars (mb) matric potential (.psi.m) on tension plates formed abundant sporangia within 3-4 days. The effect of .psi.m on zoospore discharge was then determined by changing .psi.m from -150 mb, where sporangia failed to release zoospores, to 0, -1, -5, -10, or -25 mb .psi.m. Sporangia typically discharged large numbers of zoospores within 60-90 min in completely saturated soil (.psi.m = 0) and at -1 mb .psi.m. Discharge was impaired at -5 mb, greatly restricted at -10 mb and fully prevented at -25 mb .psi.m. Similar results were obtained with sporangia formed at -50, -150, or -300 mb. Use of different textured fractions of soil revealed that discharge is governed more by .psi.m than by soil water content. Shifts in temperature between 16-24.degree. C failed to induce zoospore discharge at limiting .psi.m values. The influence of osmotica on zoospore discharge was evaluated by removing mycelial disks bearing sporangia from soil at .psi.m = -150 mb and placing them in solutions of known solute potential (.psi.s). Zoospores were discharged in solutions of KCl and MgSO4 at .psi.s > -4.5 bars and in solutions of sucrose, NaCl, sea salts and polyethylene glycol (PEG) 300 with .psi.s values as low as -6 to -9 bars. Discharge in PEG 6000 occurred only in solutions of .psi.s > -1.3 bars. Zoospore discharge is extremely sensitive to the .psi.m component relative to the .psi.s component of soil water potential. Possible mechanisms of zoospore discharge are discussed.

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