Physiological and growth responses of young tomato seedlings to drip-irrigation containing two low doses of the arbuscular mycorrhizal fungus Glomus iranicum var. tenuihypharum sp. nova By F. FERNÁNDEZ 1 , J. VICENTE-SÁNCHEZ 1 , J. F. MAESTRE-VALERO 2 * , A. J. BERNABÉ 1 , E. NICOLÁS 2 , F. PEDRERO 2 and J. J. ALARCÓN 2 1 Symborg S.L., Campus de Espinardo, 7-Edificio CEEIM, 30100 Espinardo, Murcia, Spain 2 Department of Irrigation, CEBAS-CSIC, Campus de Espinardo, 30100, Espinardo, Murcia, Spain (e-mail: jmaestre@cebas.csic.es) (Accepted 3 June 2014) SUMMARY Two different doses of an arbuscular mycorrhizal fungus (AMF; Glomus iranicum var. tenuihypharum sp. nova) were applied through a drip irrigation system to evaluate their effects on the physiological, nutritional, and agronomic performance of tomato plants.Trials were conducted in south-eastern Spain under controlled greenhouse conditions from September – December 2012. The high rate of AMF colonisation at both doses applied improved plant growth performance, leading to significant increases in leaf macro- (N, P, K, Ca and Mg) and micro- (Fe, Cu, Zn, Mn and B) nutrient concentrations. The AMF present in colonised plants not only exerted some control over the rate of transpiration (stomatal control), but also maintained a higher rate of net photosynthesis and hence improved the intrinsic water use efficiency (computed from the ratio of the rate of net photosynthesis:stomatal conductance). In addition, a close relationship was found between the respiratory activity of the AMF and the rate of net photosynthesis, leaf N, P, Fe, and K concentrations, and fruit yield (expressed as the product of the average number of fruit  the average fruit weight in each treatment). In summary, the application of AMF was effective at improving the performance (i.e., the growth, nutrition, and yield) of tomato plants cultivated under an intensive fertigation regime. S ustainable intensive agricultural production demands the development of new, inexpensive and environmentally-friendly strategies to improve competitiveness. Among such strategies, establishing mutually beneficial associations between plants and microorganisms in the rhizosphere has been shown to be an appropriate technique (Aroca et al., 2013). Perhaps, the most widely studied plant–microorganism associations are those involving arbuscular mycorrhizal fungi (AMF; Smith and Read, 2008), since AMF are ubiquitous in terrestrial ecosystems and may establish symbiotic relationships with more than 85% of plant species of agricultural interest (Ruiz-Lozano and Azcón, 1996). AMF play important roles in: (i) increasing plant root hydraulic conductivity (Aroca et al., 2007); (ii) increasing plant growth and nutrient uptake (Hamel and Plenchette, 2007); and (iii) improving plant water relations (Sánchez-Blanco et al., 2004), among other effects. Therefore, in order to promote sustainable agricultural systems, AMF should always be considered because of their ability to colonise root systems extensively and to participate actively in water and nutrient uptake (Sánchez-Blanco et al., 2004; Rivera et al., 2007). Moreover, there is evidence that AMF are significantly influenced by the physico-chemical properties of the soil (Gryndler et al., 2009) as the soil not only provides AMF with mineral nutrients, but also constitutes the chemical and physical environment in which both the fungus and the plant co-exist. Research has shown that AMF work best within a specific range of soil conditions, therefore only AMF species that have adapted to the specific soil conditions that favour symbiosis with a given plant should be used. AMF have been applied to different cultivation systems (Hamel and Plenchette, 2007) usually by placing an inoculum directly in the substrate, near the roots (Abdel-Latef, 2011; Aroca et al., 2013) which is a laborious task under commercial-scale field conditions. For instance, Subramanian et al. (2006) concluded that the AMF, Glomus intraradices improved the nutritional and water status and growth of tomato plants and, as a result, fruit production and quality. In a study on the effect of G. mosseae on the agronomic and physiological performance of tomato plants, Abdel-Latef (2011) obtained similar results, and concluded that AMF inoculation significantly increased leaf P concentrations and fruit yields. However, to our knowledge, there have been few studies in which AMF inoculation was applied via a drip irrigation system (Vicente-Sánchez et al., 2014). If successful, this would make the application of AMF much easier. Previous studies agree that, despite the successful laboratory results obtained with certain strains of AMF, scale-up of their use for agriculture has been slow, probably due to the relative ineffectiveness of the inoculation process (Ryan and Graham, 2002), or other factors such as the technical difficulty involved in their application, their compatibility, field carrying capacity, abundance, and priority effects (i.e., the influence of *Author for correspondence. Journal of Horticultural Science & Biotechnology (2014) 89 (6) 679–685