Protein hydrolysates as biostimulants in horticulture

In recent years, new strategies have been proposed in order to improve the sustainability of production systems for horticultural crops. A promising tool would be the use of substances and/or microorganisms defined also as ‘biostimulants’ able to enhance crop quality parameters, nutrient efficiency and abiotic stress tolerance. Protein hydrolysates (PHs) are an important group of plant biostimulants based on a mixture of peptides and amino acids that have received increasing attention in the recent years due to their positive effects on crop performances. PHs are mainly produced by enzymatic and/or chemical hydrolysis of proteins from animal- or plant-derived raw materials. The current review gives an overview of the biostimulant properties of PHs on productivity and product quality of horticultural crops, in par- ticular fruit trees, vegetables, flower crops and ornamentals. After a brief introduction on PHs as plant biostimulants, this review focuses on the classification and chemical composition of PHs according to the source of proteins and method of protein hydrolysis. The plant uptake and transport of amino acids and peptides and the effects of PHs on primary and secondary metabolism as well as the biochemical and physiological processes conferring tolerance to abiotic stress are also covered. The review concludes by proposing several perspectives for future research aiming to understand the mode of action of PHs based on their composition and also to define the suitable time and dose of application.