  Citation: Jiménez-Arias, D.; Morales-Sierra, S.; Borges, A.A.; Herrera, A.J.; Luis, J.C. New Biostimulants Screening Method for Crop Seedlings under Water Deficit Stress. Agronomy 2022, 12, 728. https://doi.org/10.3390/ agronomy12030728 Academic Editors: Miguel A. A. Pinheiro De Carvalho, Jan Slaski, Carla Gouveia and Carla Ragonezi Received: 14 February 2022 Accepted: 15 March 2022 Published: 17 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agronomy Article New Biostimulants Screening Method for Crop Seedlings under Water Deficit Stress David Jiménez-Arias 1,2, * , Sarai Morales-Sierra 3 , Andrés A. Borges 2 , Antonio J. Herrera 4 and Juan C. Luis 3, * 1 Investigador Principal Convidado, ISOPlexis, Centro de Agricultura Sustentável e Tecnologia Alimentar, Campus Universitário da Penteada, 9020-105 Funchal, Madeira, Portugal 2 Chemical Plant Defence Activators Group, Department of Agrobiology, IPNA-CSIC, Avenida Astrofísico Francisco Sánchez 3, P.O. Box 195, 38206 La Laguna, Tenerife, Canary Islands, Spain; aborges@ipna.csic.es 3 Grupo de Biología Vegetal Aplicada (GBVA), Departamento de Botánica, Ecología y Fisiología Vegetal-Facultad de Farmacia, Universidad de La Laguna, Avenida. Astrofísico Francisco Sánchez s/n, 38071 La Laguna, Tenerife, Canary Islands, Spain; smorales@ull.edu.es 4 Natural Product Synthesis Group, Department of Chemistry of Bioactive Natural and Synthetic Products, IPNA-CSIC, Avenida. Astrofísico Francisco Sánchez 3, P.O. Box 195, 38206 La Laguna, Tenerife, Canary Islands, Spain; ajherrera@ipna.csic.es * Correspondence: david.j.a1983@gmail.com (D.J.-A.); jcluis@ull.edu.es (J.C.L.) Abstract: Biostimulants can be used in many crops growing under water deficit conditions at the seedling stage. This study used tomato, Solanum lycopersicum L., seedlings growing in commer- cial 150-cell trays as an experimental setup to reproduce mild drought stress effects. The method showed significant reductions in seedling growth and RGR (25%) after a seven-day experiment. Gas exchange parameters (Pn, Gs and E) had significantly lower values (30–50%) than the con- trol seedlings. Stress-related metabolite, ABA, exhibited a significant accumulation in the tomato seedlings (24 h), consistent with SINCED2 gene expression. Proline levels were twice as high in the water-deficit treated seedlings, remaining at this level until the end of the experiment. However, total carbohydrates were significantly lower in water-deficit treated seedlings. Qualitative and quantita- tive analysis suggested that using the variable ‘seedling biomass accumulation’ could simplify the methodology. Twelve different biostimulants were assayed, implementing this simplification, and all of them showed higher biomass accumulation in the treated seedlings than in the non-treated ones under water deficit. Among them, putrescine, spermine and spermidine were the most effective. The method is adjustable to different biostimulant volumes (1, 3 and 5 mL; 1 mM BABA), with no significant differences between the treatments. Keywords: biostimulant; evaluation; abiotic stress; growth promoters; model system; drought 1. Introduction Crop productivity is highly dependent on irrigation management and water quality. Consequently, global climate change is a threat to crop production. In the words of M. Mizutori—UN special representative for disaster risk reduction—“Drought is a hidden global crisis, at risk of becoming the next pandemic if countries do not take action on water and land management and at the same time tackle the climate emergency” [1]. Indeed, drought losses reached USD 124 billion during the 1998–2017 period and were suffered by more than 1.5 billion people [2]. With the increasing global population, estimated to reach up to 9.8 billion by 2050, the proportion of undernourished people will increase every year. It is expected to be more than 3 billion by the end of this century [3]. Therefore, a substantial increase in food production is needed, raising more than one concern about the use and availability of water [4]. Future climate predictions point out water shortage as one of the leading global concerns in coming years, severely affecting agricultural systems, crop yield and product quality [5]. Irrigation water needs will increase by more than 50% Agronomy 2022, 12, 728. https://doi.org/10.3390/agronomy12030728 https://www.mdpi.com/journal/agronomy