Plant-derived protein hydrolysates (PHs) have gained prominence as plant biostimulants because of their potential to increase the germination, productivity and quality of a wide range of horticultural and agronomic crops. Application of PHs can also alleviate the negative effects of abiotic plant stress due to salinity, drought and heavy metals. Recent studies aimed at uncovering the mechanisms regulating these beneficial effects indicate that PHs could be directly affecting plants by stimulating carbon and nitrogen metabolism, and interfering with hormonal activity. Indirect effects could also play a role as PHs could enhance nutrient availability in plant growth substrates, and increase nutrient uptake and nutrient-use efficiency in plants. Moreover, the beneficial effects of PHs also could be due to the stimulation of plant microbiomes. Plants are colonized by an abundant and diverse assortment of microbial taxa that can help plants acquire nutrients and water and withstand biotic and abiotic stress. The substrates provided by PHs, such as amino acids, could provide an ideal food source for these plant-associated microbes. Indeed, recent studies have provided evidence that plant microbiomes are modified by the application of PHs, supporting the hypothesis that PHs might be acting, at least in part, via changes in the composition and activity of these microbial communities. Application of PHs has great potential to meet the twin challenges of a feeding a growing population while minimizing agriculture’s impact on human health and the environment. However, to fully realize the potential of PHs, further studies are required to shed light on the mechanisms conferring the beneficial effects of these products, as well as identify product formulations and application methods that optimize benefits under a range of agro-ecological conditions.

The aim of this study was to evaluate the biostimulant action (hormone like activity, nitrogen uptake, and growth stimulation) of a plant-derived protein hydrolysate by means of two laboratory bioassays: a corn (Zea mays L.) coleoptile elongation rate test (experiment 1), a rooting test on tomato cuttings (experiment 2); and two greenhouse experiments: a dwarf pea (Pisum sativum L.) growth test (experiment 3), and a tomato (Solanum lycopersicum L.) nitrogen uptake trial (experiment 4). Protein hydrolysate treatments of corn caused an increase in coleoptile elongation rate when compared to the control, in a dose-dependent fashion, with no significant differences between the four concentrations tested (0.375, 0.75, 1.5, and 3.0 ml/L), and inodole-3-acetic acid (IAA) treatment. The auxin-like effect of the protein hydrolysate on corn has been also observed in the rooting experiment of tomato cuttings. The shoot, root dry weight, root length, and root area were significantly higher by 21%, 35%, 24%, and 26%, respectively in tomato treated plants with the protein hydrolysate at 6 ml/L than untreated plants. In experiment 3, the application of the protein hydrolysate at all doses (0.375, 0.75, 1.5, and 3.0 ml/L) significantly increased the shoot length of the giberellin (GA)-deficient dwarf pea plants by an average value of 33% in comparison with the control treatment. Increasing the concentration of the protein hydrolysate from 0 to 10 ml/L increased the total dry biomass, SPAD index, and leaf nitrogen content by 20.5%, 15% and 21.5%, respectively. Thus the application of plant-derived protein hydrolysate containing amino acids and small peptides elicited a hormone-like activity, enhanced nitrogen uptake and consequently crop performances.

The current research aimed 1) at evaluating the effects of three biostimulants (legume-derived protein hydrolysate, PH; plant and seaweed extract, PE and SWE) on yield performance and nutritional quality, mineral profiling, antioxidant activities, lycopene, total phenols and ascorbic acid of greenhouse tomato ( Solanum lycopersicum L.) under soil culture and 2) to assess the economic profitability of biostimulant applications. Plants were sprayed four times during the growing cycle with a solution containing 1, 3, and 3 mL·L ⁻¹ of PE, SWE, and PH, respectively. Foliar applications of biostimulants improved the early and total marketable yield of fresh tomato. The increase of total yield by PE, SWE, and PH was 11.7%, 6.6% and 7.0%, respectively, in comparison with untreated plants. Legume-derived PH increased lycopene, total soluble solids, and K and Mg contents, thereby increasing the nutritional value of the fruits. The applications of SWE, and to a lesser degree PH, enhanced the Ca concentration in the fruit tissue. Our findings indicated that the three tested biostimulants, although they increased the total production cost, improved the nutrient status and yield performance of the crop to a level resulting in net economic benefits.