Arbuscular mycorrhizal fungi enhance growth of tomato under normal and drought conditions, via different water regulation mechanisms

Abstract Arbuscular mycorrhizal fungi (AMF) are soilborne microorganisms that establish a mutualistic symbiotic association with most of land plants. To investigate the effects of AMF symbiosis under different water status conditions, we grew AMF-inoculated and non-inoculated tomato plants in the greenhouse under two irrigation regimes, 70% and 30% of growth-substrate water holding capacity. Two different AMF inoculation strains, Funneliformis mosseae and Rhizophagus irregularis, were applied as single inocula. AMF colonization significantly enhanced plant vegetative growth by 40% and 50–60%, under normal and reduced irrigation respectively. In the presence of the AMF, phosphorous concentrations in the leaves were increased under both watering regimes while K, Ca, Mg, Zn, and Mn were also increased under limited watering to levels similar to those of non-stressed plants. Transpiration and stomatal conductance increased by an average 80% and 65% respectively in the presence of the AMF under full watering, but were kept stable and coupled to reduced leaf area-to-leaf biomass ratios and to increased metabolic water use efficiency under limited irrigation. This indicates a different mode of action induced by AMF colonizers, prioritizing water conservation in tomato plants under drought stress.