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The enhanced growth of plants infected by vesicular–arbuscular (VA) mycorrhizal fungi results primarily from improved uptake of soil phosphate. Extra phosphate reaches the root through the fungal hyphae, which tap the soluble P in soil beyond the phosphate-depletion zone near the root surface. This mechanism can explain the many corrrelations between root geometry and mycotrophy and other nutritional effects of VA mycorrhizae such as increased uptake of zinc and copper ions. Recently VA mycorrhizae have been shown to increase the levels of chlorophyll and some hormones in plants and to alleviate water stress. Legumes are now receiving considerable attention because VA mycorrhiza affects nitrogen fixation in them indirectly by its action on P uptake. In this review the physiology of the VA mycorrhizal symbiosis is discussed in categories reflecting successive stages in its formation and function: (i) activation of the VA mycorrhizal propagules; (ii) penetration and initial infection of the host plant; (iii) spread of infection in roots; (iv) response of the plant; the components and mechanisms of VA mycorrhizal systems; (v) benefits to the fungus; carbon sinks; and (vi) imbalances in the symbiosis. It is suggested that studies on the physiological complexities of VA mycorrhizal associations should take more account of the biological diversity of VA mycorrhizal fungi and the wide range of host–endophyte–soil specificities.

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Light and temperature greatly influenced the development of vesicular-arbuscular mycorrhiza and growth of onions in a phosphate-deficient soil. There were more large arbuscules and host growth was stimulated more with 25,000 lux than with 13,000 lux at 23° C and in a 14-23° C diurnal cycle. At 14° C and 13,000 lux mycorrhiza caused no growth stimulation even in three low-phosphate soils. At 18° C infection was much sparser in a 6 h daylength than in 12 and 18 h. Mycorrhizal plants kept in daylengths of 6 h at 13,000 lux were 3.5 times heavier than their non-inoculated controls. The effect of infection increased in longer daylengths and higher light intensities to 14.2 times the weight of controls with 18 h at 25,000 lux. The addition of soluble phosphate stimulated growth to the same extent as mycorrhizal inoculation did in the highest light conditions but phosphate stimulated growth more than did mycorrhiza under intermediate light conditions. The amounts of soluble carbohydrate in the roots of plants given phosphate and in those that were mycorrhizal did not differ significantly, but there was more soluble carbohydrate in plants growing in most light. Plants both with and without mycorrhiza contained much glucose, fructose, sucrose and an unidentified sugar with a low RF value, but there was no indication of fungal carbohydrates such as trehalose and mannitol.

Summary Mycorrhizal infections formed by different endophytes were examined in 10 crop species grown separately and in pairs in sterilized and unsterile soils. No infection was observed in cabbage, kale, rape or swede (in the supposedly non‐mycorrhizal family Cruciferae) and only traces were seen in sugar beet (supposedly non‐mycorrhizal Chenopodiaceae) when these plants were grown alone. However, slight (< 5 %) infection (cortical mycelium and vesicles, but no arbuscules) developed in some when a mycorrhizal host plant was present and there were many clumps of endophyte mycelium on their root surfaces, usually attached to entry points which had often aborted. Glomus fasciculatus‘E3’ was a more infective endophyte than Gigaspora margarita. Infection was usually well developed in the host plants barley, lettuce, maize, potato and onion. It was depressed only in a few pairs but no more by the presence of a ‘non‐host’ plant than by a host plant. The results suggest that the barriers to mycorrhizal infection in ‘non‐hosts’ are intrinsic and more probably related to characteristics of the root cortex or epidermis than to any infection‐inhibiting factors that might be released in root exudates.