Effects of P nutrition on growth and photosynthetic activity of tomato plants inoculated or not with AM fungi

Arbuscular mycorrhizal (AM) fungi colonize plant roots, improving mineral nutrition and promoting photosynthesis. Phosphorus (P) has a key role in plant physiology, affecting the photosynthetic process and being involved in sugar/carbon metabolism. The aim of this work was to investigate the effects of the arbuscular mycorrhizal symbiosis and P nutrition on the growth parameters and photosynthetic activity of tomato plants grown in controlled conditions. Plants were maintained in a growth chamber for 50 days and watered three times a week with a Long Ashton nutrient solution at three different P levels (32, 96 and 288 μM, respectively). At harvest, mycorrhizal colonization, biomass production, P and photosynthetic pigment concentrations were measured. Moreover, the photosynthetic efficiency relating to the activity of the two photosystems and the biochemical analysis of proteins extracted from thylakoid membranes were also performed. Results showed that inoculation did not affect growth parameters. AM symbiosis was strongly inhibited at the highest P level. Plant biomass production was positively correlated with increasing level of P. The analysis of chlorophyll fluorescence in inoculated plants highlighted that Y(I), Y(II), ETR(I), ETR(II) varied proportionally to the AM colonization and inversely proportionally to the P supply, whether this effect on NPQ and ETR occurs by a modulation of the xanthophyll cycle, remains to be established.