Summary Vesicular‐arbuscular (VA) mycorrhizal infection of red clover grown in phosphate deficient soils enhanced the concentration of P in the tissues, stimulated growth of root and shoot but reduced the root/shoot ratio. Addition of phosphate to well below the optimum level also stimulated growth and enhanced P status of non‐mycorrhizal plants, but their yields and P concentrations were much smaller than those of mycorrhizal plants and their root/shoot ratios were unaffected. The hydraulic conductivities of the root systems were very much higher in mycorrhizal than in uninfected plants. This could be partly attributed to the greater lengths and diameters and hence total surface areas of the mycorrhizal roots. Per unit length of root, the conductivities of the mycorrhizal roots were still two to three times higher, suggesting that this was mainly due to hyphal growth in the soil. When soil water was not limiting, the lower root resistances combined with larger leaf surface areas and possibly also lower leaf diffusion resistances resulted in very much higher transpiration rates and fluxes in mycorrhizal than in non‐mycorrhizal plants. Under conditions of water stress, however, the leaf diffusion resistances of mycorrhizal plants were higher and transpiration fluxes lower compared with uninfected plants but, because of their higher total water demands, they wilted more rapidly. Mycorrhizal plants were able to extract soil moisture down to lower water potentials than non‐mycorrhizal plants (difference about 1 MPa) but, possibly because of their lower leaf water potentials and higher root conductivities, they recovered turgor more rapidly than non‐mycorrhizal plants when soil water was restored. Thus, the mycorrhizal habit is an advantage to the host plant in times of moisture stress.

SUMMARYResults from experiments on clover (Trifoliumpratense L.) and leek (Alliumporrum L.) revealed that removal of extraradical hyphae from vesicular-arbuscular mycorrhizal (VAM) plants had a detrimental effect on transpiration flux. The advantage of comparisons between VAM plants with and without extraradical hyphae rather than between VAM and non-mycorrhizal plants is discussed in relation to this type of investigation. It is suggested that extraradical hyphae increase the root absorptive-surface area, independently of an enhanced nutrient status.

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