Growth and rates of uptake of nitrogen and phosphorus (inflow) in Allium cepa L. were measured in three experiments. Effects of mycorrhizal infection (Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe) and N and P fertilization were investigated. The experiments were carried out in a naturally-lit glasshouse, so that seasonal variations in solar radiation influenced experimental results.
In all experiments, a large positive growth response to mycorrhizal infection was observed when soil P was low. However, at high soil P smaller growth responses to infection were observed, as expected. Infection was associated with increased inflow of P, at all levels of soil P, even when non-mycorrhizal plants grew as well as or better than mycorrhizal plants. Lower P inflow was observed when infection was low and also at low irradiance in both mycorrhizal and non-mycorrhizal plants. The results show that mycorrhizal fungi increase the rate at which P is absorbed from soil, even under conditions which precluded a positive growth response to infection.
In plants grown in spring and summer (two experiments), mycorrhizal infection was also associated with increased N inflow. N inflows in both mycorrhizal and non-mycorrhizal plants were lower in a third experiment in late winter and mycorrhizal infection had little or no effect on N inflow in this experiment. Increased N supply increased the inflow of N, concentrations of N in plant tissues and plant growth except in plants that were severely P limited, and there was no evidence that mycorrhizal infection alleviated N stress in low N plants.
The results are discussed in relation to the environmental factors limiting plant growth, in particular nutrient uptake via and carbohydrate use by the fungal symbiont.