## 6 FPHs as Biostimulants
### Modes of Action of FPH as Biostimulants  
FPH contain a range of amino acids and of these, glutamic acid is a primary component. A myriad of studies have reported that a supply of isolated or combined amino acids is beneficial in the vegetative phase of different plants like beet, lettuce, and tomatoes [196]. Histidine and ornithine are normally absent from FPHs but they can also contain phytohormones, carbohydrates, and minerals. FPHs usually do not contain phenolics like seaweed or plant-based biostimulants. Peptides may have hormonal activities and can help to modulate biochemical processes in plants via biotic and abiotic stimulus [201]. Examples of peptides used as biostimulants include polypeptide families like the systems and the peptide NOD40, which can promote root nodulation in plants including rice and soybean) [202]. The mechanism of action of FPHs is due to amino acids and peptides mimicking the action of natural peptides with hormone activities in the plants. Fat free amino acids like aspartic acid, hydroxyproline, threonine, serine, glutamic acid, proline, glycine, alanine, methionine, isoleucine, leucine, tyrosine, melatonin, organic matter, short-chain peptides, and proteins are considered the active, biostimulants agents in FPHs. These compounds can improve the utilisation of nutrients by the plants and induce morphological changes in root architecture [203]. Other bioactivities include anti-drought effects and stimulation of beneficial microbes as well as increased antioxidant benefits. These benefits improve the biology of the plant and improve root growth and development, flowering and improved fruit setting, and reduced fruit drop [203]. The biostimulant market in the EU is valued at USD 2.91 billion. Table 5 details commercial examples of biostimulants available currently, including those derived from animal protein sources.  
The composition of FPHs impact the biostimulant impact on plants as well as where the FPH is applied (i.e., either soil or to leaves or roots). Results that follow application of biostimulants to plants are species and product specific [204]. Leaf permeability and penetration of the biostimulant are important factors that determine the efficacy of biostimulants. It is known, for example, that foliar application of FPHs increases amino acid and peptide availability by the plant as this form of application reduces competition with soil microbes for the amino acids and peptides present in biostimulants products.  
A Amino-acid-based biostimulants from FPHs have shown promising benefits and this market is likely to grow in the coming years. The synthesis of amino acids by plants requires a lot of energy consumption and application of amino-acids in FPHs via foliar application allows the plant to save energy by regulation of nitrogen in roots that increases the speed of production during transplantation or climate stress conditions like drought. Amino acids also bind to or chelate metal ions and use of micronutrients with amino acids accelerates absorption of micronutrients and their transport within the plant, which results in increased growth. Nitrogen containing compounds have a positive effect on the production of amino acids.  
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photosynthetic activity in plants and proline and glutamic acid are known to play a positive role in this activity which can result in greater yields of fruits from plants. Glutamic acid is abundant in FPHs [205].