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Abstract
The water relations of mycorrhizal onions (Allium cepa L.) were compared with those of non-mycorrhizal controls grown under low and high soil phosphorus conditions. Mycorrhizal plants had higher leaf water potentials, higher transpiration rates, higher hydraulic conductivities and lower leaf resistances than did non-mycorrhizal plants grown in low soil phosphorus conditions. When controls were grown under high soil phosphorus conditions, all 4 parameters were not different from those of mycorrhizal plants. The magnitude of the effect of mycorrhizal fungi on the water relations of the host may, in part, be a function of phosphorus nutrition. The differences in leaf water potentials, transpiration rates and leaf resistances are considered to be the result of the differences found in hydraulic conductivities.
Abstract
Vesicular-arbuscular (VA) mycorrhizal colonization of onion (Allium cepa L.) roots was found at high levels in commercially used soils only when P concentrations were below 30 µg/cm3. Soils sampled were muck soils with varying amounts of organic matter and sand and marl soils, and the P range was 10-250 µg/cm3. Onions were also seeded in 2 fields of Houghton muck soil, at 4 P levels, with and without inoculum of Glomus etunicatus Becker & Gerdemann. High colonization occurred when soil P concentrations were below 15-20 µg/cm3. High yields and high colonization past midseason were achieved simultaneously upon the addition of 30 kg/ha P. In a separate experiment, onions were grown at 3 P levels and 2 watering regimes. The P concentrations associated with limited mycorrhizal root colonization shifted from 15 to 30 µg/cm3 with a decrease in water availability. The results demonstrate the need for control of soil water conditions, as well as P, when attempting to utilize VA mycorrhizae.
Abstract
Seeds of onion (Allium cepa, L.) were sown on 2 muck soils that were high and low in available phosphorus and which contained an indigenous population of mycorrhizal spores (Glomus sp.). Treatments were 4 levels of P (0, 30, 97, and 193 kg/ha) and inoculum of the mycorrhizal fungus Glomus etunicatus Becker & Gerdemann. In the soil that was low in available P (3 kg/ha) bulb weight increased with added P. Root infection by the mycorrhizal fungus and mycorrhizal spore numbers in the soil were negatively correlated with added P. Bulb weight and mycorrhizal spore number at harvest increased when mycorrhizal inoculum was added to the soil. In the soil that was high in available P (97 kg/ha) bulb weight, root infection, and spore numbers were not influenced by added P or added mycorrhizal inoculum. Root infection data from both soils suggested a threshold level of soil P below which mycorrhizal infection was high and above which infection was low. The levels of P commonly added to muck soils may negate any usefulness of mycorrhizae but addition of P might be reduced if mycorrhizal spore numbers were increased through inputs of mycorrhizal inoculum or cultural practices.