The relation between turgor and tip growth inSaprolegnia ferax: Turgor is necessary,but not sufficient to explain apical extension rates

Linear growth rate ofSaprolegnia was reduced in direct proportion to increased osmotic pressure (II) of the medium, when sorbitol or PEG-400 was used as osmotica. However, increasing medium II reduced hyphal turgor only to a minimum positive level, which was maintained while extension rates continued to decline. TPA, a K⁺-channel agonist effective onSaprolegnia protoplasts, also caused dose-dependent linear growth rate reductions but did not substantially affect turgor. When turgor was compared with linear growth rate in the osmoticum experiments, there was a positive correlation only for hyphae growing faster than 12 μm/min; below this, there was a twofold range in extension rate despite essentially constant turgor. As well, TPA-treatments produced a twofold reduction in hyphal extension rate without substantially affecting turgor. Turgor should be consistent within a coenocyte, and is steady under constant growth conditions. However, under such conditions, we found average variations of fivefold in extension rate between hyphae, and twofold for hyphae over time. These results suggest that turgor is not the prime determinant of tip extension rate, and they are consistent with cytoskeletal regulation of that rate. Linear growth rates ofSaprolegnia colonies were similar on basal medium containing 1% (w/v) glucose, sorbitol, or PEG and only slightly faster than without added carbohydrate. Increasing medium II with glucose also reduced hyphal extension rate.