The combined effects of a growing population and climate change require improvements in crop production to meet the global food demand. Together with genetic advances and breeding techniques, improving agricultural product inputs to crops presents an opportunity to minimise losses and increase plant yield, helping achieve the goal of food security. This thesis explores the effects of amino acid addition to plants and their potential usefulness as biostimulants, functioning in a way that affects plant physiology and biological processes regardless of nutrient content. Special attention is paid to pyroglutamic acid (PGA), a scarcely studied non-proteic amino acid that is used in commercial agricultural products.
The effects of a wide range of amino acids were screened in vitro in Arabidopsis thaliana plants, as well as in wheat in controlled environment, greenhouse and field conditions. Results underline the differences in responses between different species and conditions. In Arabidopsis, high PGA concentration in solid media led to stress and toxicity symptoms. Seedling root growth varied between amino acids and led to different responses related to salt stress. Responses to amino acids were also altered in the Arabidopsis mutant oxp1, in which there is a disruption of the only known route of PGA conversion to glutamate (Glu). In greenhouse-grown wheat, some growth parameters appeared to change under the effect of PGA after two and four months of growth, particularly under limited watering conditions, but neither PGA nor any other trialled amino acids altered yield parameters in non-stressed greenhouse plants grown to full development. In the field, yield-related physiological parameters differed after the application of different amino acids, as well as leaf protein content and nitrogen assimilation enzymes glutamine synthetase (GS) and glutamate dehydrogenase (GDH) during grain filling stage. A metagenomic analysis of soil bacterial populations revealed differences in the soils corresponding to different amino acid treatments, indicating that part of the effect of amino acid biostimulants may be driven by effects on rhizosphere microbial communities.
Overall, this thesis evidences the contrasts in the effects of amino acid application under different conditions, with clearer effects observed in plants under specific stresses or under more complex field conditions compared with controlled conditions. The data presented enhances the known characteristics of amino acids as biostimulants with potential to improve plant stress responses.