Günter Neumann ^1,2* Fahim Nawaz ³ Markus Weinmann ² Vicent Arbona ⁴ Raffaella Balestrini ⁵ Chiara Pagliarani ⁶ Miguel Gonzalez-Guzman ⁴

• ¹ Institute of Crop Science, Faculty of Agricultural Sciences, University of Hohenheim, Stuttgart, Germany
• ² Institute of Crop Science, Dept. Nutritional Crop Physiology, Faculty of Agricultural Sciences, University of Hohenheim, Stuttgart Germany, 70599 Stuttgart, Germany
• ³ Research School of Biology, The Australian National University, 2601 Canberra, ACT, Australia, 26201 Canberra, Australia
• ⁴ Laboratori d’Ecofisiologia i Biotecnologia, Departament de Ciències Agràries i del Medi Natural, Universitat Jaume I, Castellón de la Plana, Spain, Castellón de la Plana, Spain
• ⁵ Institute of Biosciences and Bioresources, National Research Council of Italy, Bari, Italy, Bari, Italy
• ⁶ Institute for Sustainable Plant Protection, National Research Council (CNR), Turin, Piedmont, Italy

As an initial overview, two review papers addressed various biotechnological approaches currently investigated in this context, including adaptive modification strategies for crops, modification of soil properties, and exploiting interactions with plant-beneficial microbes in different production systems (Melini et al.; Badiyal et al.).The high variability of responses frequently observed under field conditions suggests a strong impact of environmental factors that can determine the beneficial functions of the respective adaptation strategies. This aspect is addressed by a multilevel approach, starting with three examples of investigations on the modes of action of various non-microbial BS to mitigate environmental stress under controlled conditions. The investigated stress responses comprised protective effects of the plant compound salvianolic acid on osmotic stress in maize and soybean (Kazerooni et al.) as well as mitigation of cold stress and salinity by applications of seaweed extracts and protein hydrolysates in tomato (Borella et al.; Zhang et al.). Metabolomics, transcriptomics, and the analysis of various physiological stress indicators revealed an improved oxidative stress defense as a common mode of action, but also differential effects depending on the type of applied BS products.At the next level, eight studies presented lab-to-field approaches to test the performance of microbial or non-microbial BS and management practices in different crops under variable environmental conditions. For better understanding of the critical factors interfering with the beneficial effects, it is more insightful to examine not only successful applications but also experiments that failed to produce the expected results. This last aspect was found in three experiments with applications of microbial consortia partially combined with micronutrients, seaweed extracts, and chitosan conducted with maize in Switzerland (Symanczik et al.) The third level consists of meta-analyses, which encompass a broad range of studies. This alternative approach offers large-scale insights into potential environmental factors influencing the performance of BS. Recently, various meta-studies have been conducted summarizing research achievements on the different groups of biostimulants (Schütz et al., 2018;Herrmann et al.,2022;Li et al., 2022). However, in a meta-analysis based on already published data, the interpretation of the results may be affected by the so-called "publication bias", as mainly positive results are usually considered for publication. Conversely, the present RT provides a meta-analysis covering more than 140 pot and field experiments and 107 treatments with microbial and non-microbial BS applied as single products or as product combinations (Nkebiwe et al.). The data set derives from an EU-funded project (BIOFECTOR), investigating the performance of BS in European agriculture. It covers all data generated within the project over five years and is therefore not affected by a publication bias. Accordingly, the reported beneficial BS effects on plant performance with an average growth/yield increase of 9.3% in 945 observations (Nkebiwe et al.) were generally smaller than those reported by meta-studies based on published data (Schütz et al. 2018;Herrmann et al., 2022;Li et al., 2022).A common outcome of all recently published meta-analyses is an apparent dependence of BS performance on various geo-climatic factors. Two meta-studies covering microbial (Schütz et al., 2018) and non-microbial BS (Li et al., 2022) suggested better performance of BS applications under arid and semiarid or subtropical/tropical climates as compared with more temperate climate conditions. Additionally, three meta-studies on microbial and non-microbial BS (Schütz et al., 2018;Li et al., 2022;Nkebiwe et al.) consistently showed a declining efficiency of BS applications with increasing soil organic matter. Both factors are closely correlated. Temperate climates often pose fewer challenges for crop production because they experience less extreme conditions in temperature, precipitation, soil pH, or salinity. Consequently, there is a reduced need for protective measures such as the application of biostimulants (BS). Moreover, soil organic carbon levels are frequently higher in temperate climates, often associated with higher levels of humic substances, higher fertility, better water-holding capacity, higher microbial activity and diversity, and a higher abundance of beneficial soil biota (Oldfield et al., 2019;Hoffland et al., 2020;Gerke, 2022). This may indicate a higher buffering capacity against the impact of environmental stress factors. In the respective soils, the effects of external BS applications may be at least partially replaced by higher levels of humic substances and native beneficial microbes with similar functions. Accordingly, also in this RT, the absence of beneficial yield effects after BS application was restricted to field experiments conducted under temperate climate conditions in Germany (Behr et al.; Goebel et al.) and Switzerland (Symanczik et al.), while the remaining studies showing positive effects were performed under tropical, subtropical or Mediterranean climates (Thanh Tam et al.; Cirillo et al; Jalal et al.; Wang et al.).For microbial inoculants, Symanczik et al. highlighted the importance of root colonization and rhizosphere competence for the establishment of beneficial effects, which was sufficient in controlled greenhouse studies during the early growth of maize but rapidly declined under field conditions. This is in line with the meta-analysis carried out by Nkebiwe et al., showing better field performance after BS application in crops maintained in a protected nursery before transplanting to the field compared with BS inoculation performed directly under field conditions.Improved performance of microbial inoculants in combination with manure-based organic fertilizers in comparison with mineral fertilization was reported by Behr et al., similar to various previously published studies (Thonar et al., 2017;Mpanga et al., 2018;Bradacova et al., 2019) and the meta-analysis by Nkebiwe et al. in this RT . The application of organic fertilizers with easily available carbon sources might improve the carbon supply of fastgrowing copiotrophics inoculants as well as indigenous plant growth-promoting microorganisms and support the establishment of a beneficial microbial community (Behr et al.). Furthermore, the high availability of N and P in manure-based fertilizers may serve as a starter fertilization for the host plant, facilitating root growth and the establishment of microbial inoculants in the rhizosphere (Bittman et al., 2006;Chekanai et al., 2018).All the meta-studies cited here highlighted genotypic differences at the plant species level as key factors influencing BS) interactions with host plants. These differences may stem from variations in compatibility, as well as differences in growing conditions (Nkebiwe et al.), the severity and timing of imposed stress conditions, and/or variability in stress tolerance of different plant varieties (Mahmood et al., 2022). Seven studies of this RT used BS combinations (Behr et al.; Cirillo et al.; Goebel et al.; Jalal et al.; Mendes et al.; Symanczik et al.; Zhang et al.), frequently employed as a strategy to provide higher flexibility under variable environmental conditions (Nuti et al., 2015;Sekar et al., 2016;Furlan et al., 2019). This was confirmed by the meta-study by Hermann et al. (2022). However, the benefits of BS combinations were preferentially observed under stress conditions (Bradacova et al., 2019;Nkebiwe et al.), and increased the propability of beneficial effets but not necessarily the absolute effect size (Bradacova et al., 2019;Mamun et al., 2024).Three studies pointed out the importance of interactions of microbial inoculants with native soil-microbial communities for the expression of beneficial BS effects in different crop species (Behr et al.; Cirillo et al.; Mendes et al.) as an aspect that deserves particular attention in future BS research, together with the impact on different genotypes inside a species. Finally, methodological difficulties related to the efficiency testing of BS-assisted strategies and green-biotechnological approaches were addressed by Mendes et al., Neuhoff et al., and Sun et al. Collectively, the articles included in this RT offer diverse examples for critical evaluation and characterization of conditions promoting the development of integrated plant production systems supported by environmentally friendly approaches based on BS applications and other green biotechnological strategies.