Implications of microbial and non-microbial biostimulatory action on the quality of leafy and fruit vegetables

Recently the European Union decided to reshape the existing Fertilizers Regulations, under the new regulation that plant biostimulants “shall be a CE marked fertilising product stimulating plant nutrition processes independently of the product’s nutrient content with the sole aim of improving one or more of the following characteristics of the plant and the plant rhizophere or phyllosphere: nutrient use efficiency, abiotic tolerance and product quality”. Plant biostimulants can be divided in two major categories, the non-microbial biostimulants including bioactive substances such as humic acids, protein hydrolysates from vegetal and animal origin as well as seaweed extracts. The second category is the microbial biostimulants including arbuscular mycorrhiza fungi (AMF) and plant growth promoting rhizobacteria belonging to the genera Azotobacter, Azospirillum and Rhizobium spp. In the present paper several case studies on important greenhouse vegetables such as tomato, spinach, zucchini squash and cucumber were reported in order to demonstrate how the application of non-microbial and microbial plant biostimulants can modulate the quality of the product. The application of protein hydrolysate and seaweed extracts improve the fresh yield of both tomato and spinach but differently modulated the quality characteristics in particular lycopene, total ascorbic acid, phenols and nitrate. The application of microbial biostimulant in particular AMF containing Rhizophagus irregular and Funneliformis mosseae was able to improve the macro- and microminerals profile in zucchini squash and cucumber. The present paper concludes by proposing future research toward enhancing product quality of greenhouse vegetables by optimizing biostimulants application and composition.