The Effect of Biostimulants on Horticultural Crops

1. Introduction

The growing human population increasingly demands more food, which is tied to the concept of food security. According to the Food and Agriculture Organization of the United Nations (https://www.fao.org/home/en [accessed 27 September 2024]), food security is based on four fundamental pillars: (1) Food availability: this ensures that there is enough food, both in quantity and quality, for the population, whether through local production or imports. (2) Access to food: this ensures that people have the economic resources to obtain adequate food for good nutrition. (3) Food utilization: this relates to people’s ability to properly use food, which includes aspects such as preparation, consumption, storage, and nutrition and plant health. (4) Stability: this ensures that people can continuously access safe and sufficient food, without interruptions due to factors such as economic crises, natural disasters, or armed conflicts [1].

In the case of horticultural products, including fruits and vegetables, they play a fundamental role due to their nutritional quality and their widespread use in global cuisine, thus generating jobs and wealth for countries [2]. However, the production of horticultural products can face various abiotic factors (such as soil salinity, drought, heavy metal contamination, and flooding) and biotic factors, including pests and pathogenic microorganisms. In both situations, losses can range from minimal to 100% [3]. On the other hand, horticultural production requires biostimulants, which can be of microbial origin or metabolites/substances that, when applied to plants or soil, stimulate multiple processes, such as nutrient absorption and resistance to abiotic stress, and make plant metabolism more efficient. Unlike fertilizers, which directly provide essential nutrients, biostimulants improve plant health and vigor indirectly [4].

2. Overview of Published Articles

In this Special Issue, six research articles were published, addressing relevant topics that analyze the effect of biostimulants on horticultural crops from different perspectives and on various economically important crops, such as Prunus armeniaca, Olea europaea L., Daucus carota L., Pelargonium peltatum L., Solanum lycopersicum L., and Allium cepa.

The work of Al-Saif and colleagues (2023) (Contribution 1) begins by justifying their research due to the overuse and excessive application of chemical fertilizers in apricot orchards, which has harmed the environment and soil health, as well as food security. Therefore, the authors aimed to use biostimulants such as humic acid (HA), brassinosteroids (Brs), and seaweed extract (SWE) at various concentrations in apricot trees (P. cv. Canino). Untreated trees served as a control group. The results showed that all treatments improved growth, fruit yield, and quality, with higher concentrations producing better results. The most effective concentrations were 2000 mg/L of HA, 2 mg/L of Brs, and 3000 mg/L of SWE. This study highlights the potential of biostimulants to improve the yield of fruit trees. Finally, the authors propose conducting further research to explore long-term effects and scalability.

Salinity is a global issue affecting large areas of arable land (approximately 20% of agricultural land), negatively impacting crop production and quality. To address this problem, the same research group (Contribution 2) evaluated the application of biostimulants like moringa leaf aqueous extract (MLE) at 2, 4, and 6%; seaweed extract (SWE) at 1000, 2000, and 3000 ppm; and their combinations on different parameters, including yield, fruit quality, oil content, and the nutritional status of olive trees (O. europaea). This experimental work was conducted under saline conditions in a private orchard in Wady El Natron, Beheira Governorate, Egypt, during the 2021–2022 seasons. The authors found that MLE and SWE, applied at concentrations of 6% and 3000 ppm, respectively, significantly improved various aspects of olive yield, such as an increase in chlorophyll content in the leaves, flower count, fruit yield, oil content, fruit firmness, total soluble solids (TSS), and macro- and micronutrient levels in the leaves. Interestingly, the combined use of MLE and SWE showed a synergistic effect compared to their individual application. This work, conducted in Egypt, where olive production is significant (230,543 tons harvested annually on 103,382 ha), suggests that the use of biostimulants can mitigate the stress caused by soil salinity.

Another study published in this Special Issue also evaluated the foliar application of humic acid (HA) as a biostimulant to increase salt tolerance in Ivy Geranium (P. peltatum) plants (Elhindi et al., 2023) (Contribution 3). The results revealed that HA (1000 mg/L) significantly improved plant growth, flowering capacity, nutrient status, proline accumulation, and chlorophyll index under both saline and non-saline irrigation conditions. Other concentrations of HA also improved various phytometric parameters in the same plant species. In conclusion, HA is an effective biostimulant that can be applied in the ornamental and flowering plant industry. It would be relevant to evaluate whether HA can biostimulate other ornamental plant species under other stress conditions associated with arid and semi-arid environments, such as drought.

Noor and colleagues (2023) (Contribution 4) evaluated the application of endophytic plant-growth-promoting bacteria like Bacillus sp. MN54, Enterobacter sp. MN17, Pantoea sp. MN34, and Burkholderia phytofirmans PsJN, which are good auxin producers, in carrot seed production and quality by altering the umbel order. In carrot seeds (D. carota), their quality can be affected by umbel position due to uneven ripening in different umbel orders. The bacterial strains evaluated were co-applied with L-tryptophan, an auxin precursor, through foliar application, improving all traits related to growth and yield, as well as various enzymatic activities in carrots. Notably, the MN17 + L-tryptophan and MN34 + L-tryptophan combinations effectively minimized the number of tertiary umbels by increasing the number of secondary umbels. This study highlights the potential application of beneficial microorganisms from various genera, along with auxin precursors, to improve carrot growth and quality under field conditions.

Biostimulants can vary widely, as highlighted by González-Hernández and colleagues (2023) (Contribution 5), who used compost tea (CT) from garden waste applied to tomato plants (S. lycopersicum) to promote growth. This study evaluated three physicochemical properties of CT prepared with different brewing processes, as well as its application immediately and after six months of storage. The results showed that the brewing process significantly impacted the chemical composition of the extracts, while long-term storage did not produce notable differences. Furthermore, the most energy-efficient CT was evaluated in pot and in vitro conditions, showing improvements in several growth parameters, including an increase in leaf number, plant height, and dry weight of tomato plants compared to the control and foliar treatments. Notably, CT application also stimulated root parameters like primary root length and lateral root count. In conclusion, CT may be a good alternative to stimulate phytometric parameters in tomato plants during their vegetative growth, either in aerial or root parts.

Another study evaluated three types of biostimulants—seaweed extracts, humic and fulvic acids, and the biocontrol fungus Trichoderma sp.—to improve biomass and the antioxidant status of onion (A. cepa) under different production methods (Contribution 6). After two years of evaluation, the results showed that drip-irrigated onions produced more biomass, while furrow-irrigated onions had a higher dry matter content. The treatments yielded multiple results, some positive and others negative, but notably, the seaweed extracts increased flavonoids and improved antioxidant activity in the drip irrigation treatment. This study exemplifies that biostimulants can be an important option in sustainable onion production under different irrigation systems, but further studies are needed for industrial-scale application.

Table 1. List of contributors and their main findings in this Special Issue, including the benefitting plants and biostimulants used. See text for abbreviations.

Plant	Biostimulant(s)	Beneficial Effects	Number of Contributor
Apricot (Prunus armeniaca)	Humic acid (HA), brassinosteroids (Brs), seaweed extract (SWE)	Improved fruit growth, yield, and quality with higher concentrations showing better results (2000 mg/L HA, 2 mg/L Brs, 3000 mg/L SWE).	1
Olive (Olea europaea)	Moringa leaf extract (MLE), seaweed extract (SWE)	Improved chlorophyll content, fruit yield, oil content, and macro/micronutrient levels under saline conditions. Synergistic effects with combined MLE and SWE application.	2
Geranium (Pelargonium peltatum)	Humic acid (HA)	Enhanced plant growth, nutrient status, chlorophyll content, and flowering ability, especially under saline irrigation.	3
Carrot (Daucus carota)	Bacillus sp., Enterobacter sp., Pantoea sp., Burkholderia phytofirmans, L-tryptophan	Improved growth, yield, and enzymatic activity. Altered umbel order to enhance seed quality.	4
Tomato (Solanum lycopersicum)	Compost tea (CT)	Improved growth parameters (number of leaves, plant height, root length, etc.). Different brewing methods impacted the extract composition.	5
Onion (Allium cepa)	Seaweed extracts, humic and fulvic acids, Trichoderma sp.	Varied results in biomass and antioxidant activity. Seaweed extracts improved flavonoid content and antioxidant activity under drip irrigation.	6

lists the contributors’ works in this special issue.

3. Conclusions

The studies published in this Special Issue highlight the vast potential of microbial biostimulants, such as bacteria belonging to the genera Bacillus, Enterobacter, Pantoea, and Burkholderia phytofirmans, as well as the biocontrol fungus Trichoderma, to improve growth and various phytometric and physiological parameters of horticultural crops. Likewise, other natural biostimulants, such as seaweed extracts, humic and fulvic acids, and L-tryptophan, also have great potential to improve the quality of fruit and ornamental plants [5,6]. Although much remains to be researched, the use of biostimulants can be a sustainable option for horticulture under different production systems.