Arbuscular mycorrhizae are important for phosphorus uptake and root biomass, and exudation for nitrogen uptake in tomato plants grown under variable water conditions

Aims: Mycorrhiza and rhizodeposition are important for nutrient and water uptake but their role under variable water conditions remains unclear. We investigated how mycorrhiza and rhizodeposition contributed to plant uptake of water, nitrogen (N) and phosphorus (P) in response to water availability and variability. 

Methods: Two tomato genotypes (a mycorrhiza-defective tomato mutant, rmc, and its mycorrhizal wild-type progenitor, Solanum lycopersicum cv. Rio Grande 76R were grown in a greenhouse under different watering conditions (wet, medium, and dry conditions, and dry-rewet conditions). 

Results: We found that non-mycorrhizal plants were as successful as mycorrhizal plants in terms of N and water uptake under both reduced and variable water availability. However, we observed lower water use efficiency and shoot N recovery in mycorrhizal plants that could be driven by mycorrhizal requirements for water and N. Mycorrhizal plants were more extensively colonised and were more successful in taking up P under dry conditions compared to non-mycorrhizal plants. On the other hand, greater specific root exudation in non-mycorrhizal plants across all watering conditions resulted in relatively greater uptake of N than of P. 

Conclusions: A flexible carbon allocation towards mycorrhiza and root exudation may help plants in maintaining a balanced uptake of N and P under variable water conditions.