A review of the foliar application of individual amino acids as biostimulants in plants.

Enhancing crop yields in an environmentally sustainable manner is crucial in the agricultural sector, given the challenges posed by population growth, climate change, and the need to reduce pesticide usage. Biostimulants, such as amino acids, offer natural and stable solutions to improve yield, growth, and resistance to external factors. They achieve this by reducing the impact of abiotic stress and stimulating plant defence mechanisms. As biostimulants, amino acids' simplicity, cost-effectiveness, and environmental friendliness have garnered recognition whilst offering natural and stable solutions to improve crop yield and resistance to external factors. Mixtures containing multiple amino acids are commonly used in agriculture, but the effects of individual components within these mixtures are less understood. This review summarises current knowledge on the effects of the foliar application of the 20 most common naturally occurring amino acids. The similarities in observed effects, as well as proposed modes of action, have been discussed. Notably, distinct differences in the modes of action of these amino acids have been observed, with two key themes emerging: firstly, an enhancement in the rate of photosynthesis through the promotion of chlorophyll synthesis or improved electron transfer in Photosystem II (PSII); secondly, protection against oxidative stress, either by increasing antioxidant synthesis or directly acting as antioxidants. For some amino acids, the mode of action after foliar application is unclear, or the effects are minor, highlighting the need for further research into their specific mechanisms of action. Supplementary Information: The online version contains supplementary material available at 10.1007/s44279-025-00222-7.