Maize (Zea mays L.) response to the biostimulant Methylobacterium symbioticum under various nitrogen fertilizer rates

Climate change, declining soil quality, and the overreliance on synthetic agrochemicals have intensified the need to develop more sustainable agricultural strategies that support crop productivity while enhancing resilience. In this context, biostimulants have emerged as a promising tool to improve nutrient utilization in plants. However, it is needed to confirm if these products can effectively stimulate plant growth and enhance nutrient availability under field conditions. The objective of this work was to test the effect of the bacteria Methylobacterium symbioticum as a biostimulant on the yield and N uptake of maize ( Zea mays L.) under various N fertilizer rates, and to assess whether optical sensors and metabolic indicators of plant performance (i.e. photosynthetic pigments, tocopherols and amino acids) were affected by its application. Two field experiments were conducted in consecutive years in Central Spain, where maize that received different amounts of N fertilizer was inoculated or not with M. symbioticum . The yield and N uptake were determined at harvest for each treatment combination and repetition. Additionally, measurements with optical sensors (Dualex® and spectroradiometer) were taken at three growth stages. Leaf samples taken at eight unfolded leaves and flowering were analyzed for amino acids concentration, while pigment and tocopherol concentrations were analyzed at flowering. Treating maize with M. symbioticum increased both yield and crop N uptake when N fertilizer was not applied; no differences were observed when N fertilizer was applied. On average, the inoculated non-fertilized treatment achieved 59 % of the yield obtained in the well- fertilized, whereas the non-inoculated achieved 43 %. The yield increase was accompanied by a higher crop N uptake. This positive response of inoculated plants was not detected by measuring chlorophyll concentration with the optical sensors or with biochemical determination of total chlorophyll a+b concentration. However, leaves from plants inoculated with M. symbioticum exhibited higher light reflectance in the near-infrared (NIR) region, lower chlorophyll a/b ratio and increased total free amino acid content (particularly tryptophan, proline, and GABA). Overall, the presence of M. symbioticum enhanced indicators associated with plant performance that improved agronomic crop response under strong N-limited conditions. • M. symbioticum tested as biostimulant to boost maize yield and nitrogen uptake. • Field trials showed yield gains without fertilizer when maize was inoculated. • Whith fertilizer no difference in yield or N uptake observed when maize was inoculated. • Optical sensors did not detect increase in leaf chorophyll from M. symbioticum treatments. • Inoculated plants showed higher reflectance in the near infrared and amino acid content.