Practical Applications of Plant Biostimulants in Greenhouse Vegetable Crop Production

Abstract

The research interest on plant biostimulant applications in vegetable crop production is gradually increasing and several reports highlight the beneficial effects that such products may have not only on crop performance but also on the quality of the final product. Moreover, numerous products with biostimulatory activity are being developed which need further evaluation under variable growing conditions and different crops. Plant hydrolysates which contain amino acids and peptides have been acclaimed with several positive effects on crop performance of diverse horticultural crops, while macro-algae are also considered effective biostimulants on plants grown under stress conditions. A recent study evaluated the use of protein hydrolysates and brown macro-algae (Ascophyllum nodosum and Ecklonia maxima) as innovative and cost effective approaches for sustainable vegetable production. The present editorial provides an overview of the main findings of that study, while discussing the practical applications that biostimulants may have in the greenhouse production of vegetable crops, aiming to increase the yield and the quality of the final produce and improve crop tolerance to abiotic stressors.

Greenhouse vegetable production is an intensive and technology oriented cultivation system which can ensure the availability of out-of season products throughout the year regardless of the environmental conditions, while increasing the quality of the final product at the same time. However, this cropping system requires high energy and natural resources inputs which need to be aligned with the concept of sustainable agriculture and ongoing climate change on the one hand, and the increasing consumers’ demands for high quality products and food security on the other hand [1,2]. Therefore, modern horticulture needs to reinvent itself and explore new pathways that will sustain natural resources, increase crop productivity within finite agricultural land and finally enhance the quality of the final produce, according to market trends and consumer preferences.

The application of plant biostimulants is an innovative environmental friendly approach towards sustainable crop production which faces several limitations such as water scarcity, depletion of natural resources, environmental stressors and climate change [3,4,5,6]. The use of biostimulants is indicated in high added value crops such as vegetable crops and intensified cropping systems due to the high cost for most of the commercially available products [7]. The wide range of compounds that can be used as biostimulants dictates the various mechanisms involved in the observed effects, which in several cases remain unknown [3,4,8]. So far, the proposed mechanisms include the stimulatory effects on the activity of enzymes involved in various biosynthetic processes, the hormone-like activity of biostimulant constituents and the improvement of nutrient uptake of plants. According to Colla et al. [9] the well-established effects of protein hydrolysates are associated with primary and secondary metabolism biosynthetic products that may induce tolerance against abiotic stressors and promote vegetative growth and yield of horticultural crops. On the other hand, macro-algae are relatively new biostimulants which contain various bioactive (polysaccharides and polyphenols) and phytohormone-like components that may alleviate negative stress effects and promote crop productivity [10,11,12]. Similarly, Halpern et al. [13] provided a comprehensive literature review regarding the effects of various biostimulants on nutrient uptake from plants, focusing on humic substances, amino acids, seaweed extracts and plant growth promoting bacteria and identified numerous mechanisms and selectivity of specific biostimulants to specific nutrients.

In the study of Rouphael et al. [14] three different commercially available biostimulant products were tested in spinach plants grown in soil under protected conditions, namely a protein hydrolysate derived from legumes (Trainer^®), extract of seaweed Ecklonia maxima (Kelpak^®) and a mixture vegetal oils and herbal and seaweed Ascophyllum nodosum extracts (Amalgerol^®). All the products were applied through foliage spraying in spinach plants (Donkey F1). The results of the study indicated the positive effects of all the used products in comparison to the untreated plants on fresh yield, which were attributed to the formation of bigger leaves and to higher chlorophyll content and better photosynthetic activity. The recorded yield increase was partly associated with the up-regulation of plant phytohormones biosynthesis which enhanced biomass production and yield, as well as with changes in root architecture that stimulated nutrient uptake. The authors also identified a crop specific activity for the tested products since the recorded improvement of yield was profoundly higher in spinach than other crops evaluated in previous studies. Protein and mineral composition (K, Mg and Na) were also positively affected by the biostimulant action, whereas nitrates content was higher in plants treated with seaweed extracts which could be considered a negative parameter. The effects on protein and macronutrient content are mostly attributed to the improvement of nutrient uptake by plants due to the presence of signaling molecules in biostimulant products, the changes in root architecture and the up-regulation of genes involved in macronutrient transportation. On the other hand, the variable response to the tested products in regards to nitrate content, indicates that foliar spraying with protein hydrolysates increases nitrate assimilation and could be proved as a cost effective and eco-friendly means to improve the quality of spinach leaves under intensified cropping systems where high nitrogen inputs are implemented. Beneficial effects were also observed on ascorbic acid and total phenolic compounds content in plants treated with biostimulants. This finding could be directly linked with phytochemical homeostasis through the increased expression of genes involved in the biosynthesis of these compounds [3,15], or indirectly through the increase in K and Mg contents which may induce the biosynthesis of antioxidants [16].

Although this study evaluated the biostimulatory effect of the tested compounds under optimal conditions, several other studies have highlighted the beneficial effects of biostimulants on horticultural crops grown under abiotic stressors through the increase in stress tolerance and the improvement of the quality of the final product [17,18,19]. This aspect is of great importance considering the adverse impacts of frequent weather extremities and gradual soil degradation on crop performance that modern agriculture has to face; thus, alternative cost effective control measures are always welcome by the farmers and the agro-industry sector. On the other hand, the quality of the final product should not be overlooked, since the current market trends identify a significant shift of consumer preferences towards highly nutritious and health beneficial food products. Petropoulos et al. [18] studied the effect of four biostimulant products containing arbuscular mycorrhizal fungi, saprophytic fungi, amino acids, Bacillus subtilis etc., on common bean plants grown under water stress and reported a significant increase in phenolic compounds content for the treated plants, while Kocira et al. [20] suggested a significant increase in phenolic compounds and anthocyanins when common bean plants were treated with an extract of Ecklonia maxima seaweeds. Biostimulants application also had positive effects on greenhouse-grown tomato plants through the improvement of crop performance and the nutritional parameters of fruit and—despite the increased production cost—the net revenue of farmers was significantly improved [21]. Tomato crops may also benefit from biostimulant application when plants are grown under stress conditions, since products such as Megafol^® and Viva^® have proved to be effective in alleviating drought and nitrogen deprivation stress, respectively [22,23]. In the case of leafy vegetable crops, the use of Ascophyllum nodosum alleviated the negative effects of potassium deprivation in lettuce plants, while the application of biostimulants and biofertilizers had a variable effect on the chemical profile and bioactive properties of spinach plants grown under water-stress [17].

Despite the recent prolific scientific research on biostimulants, further studies are needed to reveal the underlying mechanisms of biostimulant activity, as well as to identify the best suited biostimulants for specific crops and growing conditions, especially suboptimal ones, and moreover suggest the optimal doses and the proper developmental stage of the plant for biostimulant application. Another issue to be addressed is the variability in composition of naturally derived biostimulants, which needs to be standardized in order to be commercially available. Finally, biostimulant manufacturers’ efforts need to concentrate on providing less costly products on the market that will make their application more attractive to farmers, especially those who operate in small-scale farms.