AN APPARENT INCREASE IN SYMPLASTIC WATER CONTRIBUTES TO GREATER TURGOR IN MYCORRHIZAL ROOTS OF DROUGHTED ROSA PLANTS

Using psychrometric pressure‐volume analysis, root water relations following drought were characterized in Rosa hybrida L. plants colonized by the vesicular‐arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith. Measurements were also made on uncolonized plants of similar size and adequate phosphorus nutrition. Under well‐watered conditions mycorrhizal colonization resulted in lower solute concentrations in root symplasm, and hence lower root turgors. Following drought, however, mycorrhizal roots maintained greater turgor across a range of tissue hydration. This effect was apparently not due to increased osmotic adjustment (osmotic potentials at full turgor were similar in mycorrhizal and non‐mycorrhizal roots after drought) or to altered elasticity but to an increased partitioning of water into the symplast. Symplast osmolality at full turgor was similar in mycorrhizal and non‐mycorrhizal roots but, because of their higher symplastic water percentages, mycorrhizal roots contained a greater amount of osmotic (symplastic) solutes. Drought‐induced changes in osmotic potential were observed only in mycorrhizal roots, where a 0.4 MPa decrease (relative to well‐watered controls) brought the full turgor osmotic potential of mycorrhizal roots to the same level as that of non‐mycorrhizal roots under either watering treatment. Copyright © 1990, Wiley Blackwell. All rights reserved