The Effect of VA Mycorrhizal Infection and Phosphorus Status on Sunflower Hydraulic and Stomatal Properties

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Koide, R. 1985. The effect of VA mycorrhizal infection and phosphorus status on sunflower hydraulic and stomatal properties.—J. exp. Bot. 36: 1087–1098. Mycorrhizal (M) and non-mycorrhizal (NM) sunflower plants were grown in a soil of low phosphorus availability (with and without phosphorus amendment) and in a soil of moderate phosphorus availability (without phosphorus amendment). Using the Ohm's law analogy and measured leaf water potentials, stem water potentials, and transpiration rates, hydraulic resistances were calculated for the whole plant, leaf, and below leaf components. Mycorrhizal infection (as high as 89%) was shown to have no effect on the intrinsic hydraulic properties of the soil/plant system over a wide range of transpiration rates in either soil when M and NM plants of equivalent root length were compared. When grown in the soil of moderate phosphorus availability, calculated hydraulic resistances under given environmental conditions were the same for M and NM plants, as were stomatal resistances and transpiration rates. When grown in the soil of low phosphorus availability, calculated values of hydraulic resistance were lower for M plants than for NM plants under given sets of environmental conditions. These differences in calculated hydraulic resistance were not due to a difference in the intrinsic hydraulic properties of M and NM plants. The differences were evident because stomatal resistances were lower and transpiration rates higher for M plants and because hydraulic resistance varied inversely with transpiration rate. When plants of significantly greater root length were compared to plants of lesser root length, the calculated hydraulic resistances under given environmental conditions were much lower for the plants of greater root length. This difference was largely due to a difference in the intrinsic hydraulic properties between large and small plants, and not because of differences in transpiration rate. The elevated transpiration rates exhibited by M plants were attributed to an enhanced phosphorus status. Short term phosphorus amendments made to phosphorus-deficient NM plants improved transpiration; transpiration rates were similar for M and NM plants before NM plants became phosphorus-deficient, and phosphorus-amended M and NM plants had similar transpiration rates. The data are discussed in relation to other reports of mycorrhizal influence on hydraulic and stomatal resistances. Possible mechanisms for the influence of infection on stomatal resistance are also briefly discussed.