Introduction

Plants are often grown in sub-optimal conditions, and plant stress has a significant adverse effect on global agriculture. Therefore, future food security will be dependent on better plant growth and higher productivity, especially as the human population expands and the demand for food increases (Molotoks et al. 2021). It has been shown that climate change-driven abiotic stresses causes a loss of food productivity, compromising the future of food security, with the cost of lost produce estimated to be more than US$170 billion per annum on a global scale (Razzaq et al. 2021). A challenge is to develop easy and cheap solutions that mitigate against plant stress and lead to better agricultural outputs, enhancing plant growth and productivity.

Hydrogen is the most abundant and lightest chemical element, constituting ~ 75% of the mass of the universe. At standard pressure and temperature, hydrogen gas (H2) is highly flammable, colourless, tasteless and odourless. Atomic hydrogen (H) is rare in the Earth's atmosphere and hydrogen is more typically found as a diatomic molecule (H2) that is electrochemically neutral and non-polar, which has previously been thought of as a relatively physiologically inert molecule (Zeng et al. 2014). H2 emission from plant tissues was reported many years ago. Recently, H2 has been found to play vital roles in biomedical fields, where it has been described as having anti-apoptotic, and anti-oxidant effects (Russell et al. 2020). Furthermore, the metabolism of H2 in different organisms including bacteria, green algae, and higher plants, has also been widely reported (Renwick et al. 1964; Melis and Happe 2001; Bothe et al. 2010; Russell et al. 2020). Although the production of H2 in higher plants remains somewhat unclear (Russell et al. 2020), a growing understanding of its physical and bio-regulatory roles hydrogen gas has recently gained attention in both plant and animal research. H2 is becoming recognised as an integral signaling molecule that is likely to have influential roles in a broad array of adaptive and developmental responses. In plants, H2 is known to combine a regulatory role in the control of gene expression and in signal transduction, mediating the management of numerous stress responses (Cui et al. 2020; Wang et al. 2020a, b), including cold stress (Xu et al. 2017), metal stress (Zhao et al. 2017; Wu et al. 2019, 2020a; Cui et al. 2020; Fan et al. 2020), UV stress (Xie et al. 2015; Zhang et al. 2018), high light stress (Zhang et al. 2015), and salinity stress (Xie et al. 2012; Wu et al. 2020b). Therefore, a greater understanding of how H2 acts in cells, and how it leads to better stress tolerance will be important for its future use.

Biological generation of H2 in plants

Plant cells may be exposed to H2 in different ways, either via endogenously production or by being applied exogenously. Endogenous production of H2 occurs via the activity of hydrogenase enzymes (Vignais et al. 2001; Russell et al. 2020) [(FeFe)-hydrogenases] or nitrogenase enzymes (Fig. 1). The metallo-protein complexes possess the potential of generating or removing H2 via catalysing both forward and backwards reactions. The presence of phytohormones such as auxin, abscisic acid, jasmonate and ethylene can enhance endogenous production of H2 in plants (Lin et al. 2014; Liu et al. 2016; Cao et al. 2017). To illustrate, Legume-rhizobia contributes to H2 generation inside root nodules during biological nitrogen fixation process as an obligate by-product of the nitrogen fixing enzyme nitrogenase (Golding and Dong 2010). In this process of H2 production, H2 comes out of the root nodules leading to elevation of H2 around the root surface (Porte et al. 2020). Increases in H2 in the rhizosphere leads to beneficial impacts for subsequent plant growth resulting in 15–48% biomass increase in plants, a phenomenon termed as the hydrogen fertilization effect (Dong et al. 2003). Furthermore, H2 production is also increased when plants are exposed to abiotic stress conditions such as drought and salt stress, suggesting this particular gasotransmitter may be important in stress signaling (Zeng et al. 2013).

Fig. 1
figure 1

Abiotic stresses—highlights the potential causes of abiotic stress, with a focus on the particular vulnerability of commercial crops. Cellular effects of abiotic stress include elevation of ionic, osmotic and oxidative stress indicators that, if exposed for prolonged periods of time, can lead to reduced growth potential. H2 application and stress relief—suggests possible application methods of molecular hydrogen and illustrates the positive cellular effects attributed to enhanced levels of cellular H2, and how these can relate to increased growth potential

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H2 delivery to plants

Although H2 treatments to economically important crops are reported to have beneficial impacts in both yield and quality (Hu et al. 2021), H2 is a gas and so is not easy to administer. As a gas, H2 is lighter than air, therefore under field conditions it is not pragmatic to apply in gaseous form. Furthermore, H2 is highly explosive and is therefore not suitable to use at any significant concentrations due to safety and storage issues.

A more practical approach is the use of saturated forms of H2 such as hydrogen rich water (HRW) (Fig. 2). This can be used to apply H2 directly onto plants, as used by Wu et al. (2020a). Briefly, HRW is prepared by pumping/bubbling H2 into distilled water (or watering media) following H2 gas production by hydrogen or oxy-hydrogen generators. Subsequently, this HRW can be diluted to make desired concentration for treatments. Other methods of HRW production include the mixing of magnesium-based tablets in the water, although these will leave behind by-products in the media. HRW can then be applied in spray form or as a soil drench to agricultural crop plants. If plants or plant cells are being grown in hydroponics or culture media, H2 can be bubbled directly into the media. However, it needs to be borne in mind that H2 is not very soluble (Wilhelm et al. 1977) and will readily diffuse into the atmosphere where, H2 will be lost from the environment of the plant and can potentially build up to flammable levels if ventilation is limited, in laboratory conditions for example.

Fig. 2
figure 2

Illustrates the known cellular effects of H2 enrichment inferred from evidence delineated across both plants and animal species. From top left, running clockwise—the increase in activity of transcription factor (Nrf2) and downstream enzymes with anti-inflammatory properties (e.g. HO-1). Inhibition of multiple signalling pathways that activate the stress response (e.g. p38/MAPK) are detailed. The altered gene-expression profile described in the nucleus (Increased anti-oxidant profile, CAT, SOD e.g./Reduced pro-inflammatory profile NFkB e.g.). The reduction in oxidative degeneration to DNA and lipid membranes is also described. Furthermore, the reduced activity of caspase c, and resulting decline in apoptosis. Additionally, increased adenosine triphosphate (ATP) production and improved membrane potential of the mitochondria have been detailed. Finally, enhanced photosynthesis and energy metabolism is also shown. Thiobarbituric acid reactive substances (TBARS), MDA, lipoxygenases (LOX), ascorbic acid (AsA), gibberellic acid (GA3), indole acetic acid (IAA), glutathione (GSH), heme oxygenase 1 (HO-1), hydroxyl group (•OH), peroxynitrite (ONOO-), (created in BioRender.com)

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H2 can be useful as a treatment for post-harvest storage. Here, fresh produce can be easily dipped, sprayed or misted with HRW. Moreover, under storage conditions the use of H2 in gas form is also possible. Safety measures, again, are necessary on the use of large scale HRW or H2 in gaseous form. HRW application has been shown to improve the vase life and quality of cut rose and lily flowers by decreasing leaf stomatal size and reducing oxidative damage by elevating ROS scavenging antioxidants such as ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) while reducing malondialdehyde (MDA) and electrolyte leakage (Ren et al. 2017).

The use of H2 gas and HRW are relatively easy but not without problems, as discussed above. The future of H2 in agriculture will need to look to more advanced solutions if the gas is to be widely used. Advances in development of solid hydrogen-storage materials may offer ways to improve the production and storage of H2. When considering safety, sustainable hydrogen supply of solid-state storage, portable, and large hydrogen contents (Hirscher et al. 2020). Magnesium hydride (MgH2) is also a promising low cost, abundantly available donor (Grochala and Edwards 2004), showing potential to be used as a H2 source material in agriculture (Li et al. 2020a). Another potential method of H2 delivery to agriculture is the use of nanotechnology that provides the opportunity for sustained delivery of H2 via using hydrogen-releasing nanomaterials such as nanocapsule (AB@hMSN) by encapsulating ammonia borane (AB) into hollow mesoporous silica nanoparticles (hMSN) (Wang et al. 2021). In addition, the use of loaded hydrogen nanomaterials has the advantage of increased residence time in liquids such as water (Wang et al. 2021). However, the release of by-products into the environment has to be considered before adopting any new technologies. In this regard, future studies should focus on the use of nanomaterial-based hydrogen release and their impacts on crops.

Physiological effects

Effect of H2 on leaf stomata

Water uptake and transpiration are vital for plant growth and survival, but at the same time, climate change is leading to increased uncertainty of water availability for plants, with expanding areas of drought leading to declined agricultural output in some regions (Cook et al. 2018). On the other hand, other regions will be at more risk from flooding, a major climate-related calamity (Hirabayashi et al. 2013). Hydrogen treatment can mitigate drought stress (Zeng et al. 2013; Hancock and Russell 2021), whilst hydrogen is also known to be released from flooded soils (Piché-Choquette and Constant 2019), and, therefore, the role of H2 in controlling the plants transpiration stream will be important to understand.

Stomata are tiny holes present on the aerial plant surfaces, especially the leaves that play vital role in regulating gas exchange rate and transpiration rates and are crucial for plant growth and survival. Studies have demonstrated that H2 may have a crucial role in regulation of stomatal aperture via interaction with phytohormones (Liu et al. 2016). For example, in A. thaliana grown under drought stress conditions HRW application enhanced endogenous H2 production, along with reduction in stomatal aperture, leading to drought tolerance (Xie et al. 2014). Zhang et al. (2020) tested the introgression of a Chlamydomonas reinhardtii hydrogenase gene (CrHYD1) for H2 biosynthesis in Arabidopsis thaliana and demonstrated that increased H2 resulted in closure of stomata under osmotic stress leading to osmotic stress tolerance. From these examples, it is clear that H2 has a critical role in maintaining the stomatal closure particularly under drought/osmotic stress conditions, although the mechanism and pathways involved have yet to be fully delineated.

Drought is a major abiotic stress/challenge affecting agriculture productivity worldwide. Drought stress negatively affects physiological and biochemical mechanisms, and ultimately leads to plant growth reduction and diminishing agricultural productivity (Abideen et al. 2020; Zulfiqar et al. 2021). Chen et al. (2017b) reported that carbon monoxide and hydrogen gas (HRW as a source) co-ordinately improved growth, particularly during root formation, as well as other important traits such as chlorophyll content, relative water content and chlorophyll fluorescence characteristics, under drought-stress conditions. Furthermore, these compounds jointly increased the activities of SOD, POD, CAT, APX, proteins, water soluble carbohydrate, and proline content (also an antioxidant). These factors alleviated the drought induced oxidative stress, evidenced by decreases in hydrogen peroxide (H2O2), thiobarbituric acid reactive substances (TBARS), and superoxide radical (O2·−) levels (Chen et al. 2017b). Moreover, exogenous H2 (HRW) efficiently can lead to regulation of the stomatal aperture resulting in improved drought stress tolerance (Xie et al. 2014), as mentioned above (Jin et al. 2016) also demonstrated that under drought stress condition, H2 promptly increased H2O2 signaling and altered the apoplastic pH of alfalfa leaves by influencing an abscisic acid (ABA)-based mechanism. The evidence, therefore, points to H2-based treatment as being a useful adjunct for enhanced of drought tolerance in the future (Table 1).

Table 1 Role of H2 applied as HRW against different climate change induced abiotic stresses in plants
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Effect of H2 on root development

The development of a good root system for the uptake of water and nutrients, as well as to ground the plants at its location, is vital for optimal plant growth and productivity. H2 has been found to be influential in root growth (Lin et al. 2014; Zhu et al. 2016; Wu et al. 2020b).

Studies have demonstrated that H2-induced root development is related to its influence on the endogenous level of plant hormones. In a recent study, Wu et al. (2020b) reported that HRW-treated mung bean seedlings enhanced the endogenous level of indole acetic acid (IAA) and gibberellic acid (GA3) resulting in enhanced hypocotyl and root length. Augmentation of the endogenous levels of phytohormones is evidenced by up-regulation of related genes (Lin et al. 2014). Reports show that HRW seed treatment could induce phytohormone signalling pathways in response to environmental stresses (Zeng et al. 2013; Liu et al. 2016). In cucumber explants, Zhu et al. (2016) also demonstrated that the adventitious rooting-related target genes were upregulated in response to hydrogen-rich water (HRW). Additionally, it was found that HRW treatment upregulated the expression of cell cycle-related genes including A type cyclin (CycA), B type cyclin (CycB), cyclin-dependent kinase A (CDKA) and cyclin-dependent kinase B (CDKB) during adventitious rooting (Zhu et al. 2016). Similar to other accepted gasotransmitters such as NO, the endogenous regulatory functions of H2 can be imitated through the exogenous treatment with hydrogen-based products/compounds.

The interaction of H2 with other gaseous signals, such as nitric oxide (NO) and hydrogen sulfide (H2S), is important to understand, as they may be working together or antagonistically. It was recently reported that NO is involved in the H2-induced root formation (Li et al. 2020a). The combination of these gaseous molecules effectively regulates gene expression of plasma membrane H+-ATPase and 14-3-3 proteins, which are integral for the proper growth, development and response to stresses. H2 has also been reported to play a role in root formation via interacting with the NO and hemeoxygenase-1/carbon monoxide pathways in plants (Lin et al. 2014; Zhu et al. 2016).

An alternative study of adventitious rooting describes H2 as regulating target genes related to auxin signaling and root development such as CsDNAJ-1, CsCPDK1/5, CsCDC6, CsAUX228-like, and CsAUX22D-like through CO pathways. Such results confirm that H2 promotes rooting by increasing the content of NO and the activities of NO synthase-like enzymes and nitrate reductase (Zhu et al. 2016). H2 was also shown to activate the cell cycle and up-regulate cell cycle-related and rooting-related genes via the NO pathway (Zhu et al. 2016). Therefore, it is likely H2 has a vital role in root formation through the interaction with endogenous plant signaling molecules, and their related downstream target genes.

Effect of H2 on pre- and post-harvest senescence

The distance between food production sites and subsequent consumption sites is going to significantly increase due to continuous shift of population towards urban areas, a factor that becomes of increasing importance when considering the international distribution of food. Post-harvest losses in the supply chain of fresh horticulture commodities ahead of arrival to the consumer are estimated to be between 13 and 38% (Duan et al. 2020). The high market value of cut flowers in international markets compels major increases in the production of floricultural crops globally, particularly in developing countries, generating billions of US-dollars economically through worldwide trade and industry (Chandler and Brugliera 2011). Thus, minimizing post-harvest losses during storage and transportation, whilst also retaining the quality standard, is a great challenge for industries in pursuit of financial success. Therefore, further research in this area is needed if enhancing the post-harvest life of horticultural and floricultural produce and consequently reducing post-harvest losses is to be realised.

Of particular importance to the horticulture industries are the studies which describe the application of H2 as enhancing the post-harvest longevity of “a variety of” crops (Su et al. 2019a; Li et al. 2020b). H2 application has been reported to enhance the post-harvest life of many commodities including fruits, vegetables and floriculture crops (Fig. 3). For instance, Hu et al. (2021) demonstrated that pre-harvest HRW treatment to daylily flowers not only enhanced the daily yield of budding flowers, but also reduce the chilling injury induced by elevated ROS levels and membrane oxidation. Whilst, HRW treated daylily buds generate less browning in storage conditions (Hu et al. 2021).

Fig. 3
figure 3

Demonstrates the possible methods of treating horticultural produce for enhancing post-harvest longevity

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Treatment with HRW also improved the ornamental and aesthetics traits and enhanced the vase life rose (Rosa hybrid) and cut lily (Lilium spp.) flowers (Ren et al. 2017). Here, H2 maintained membrane stability and water balance, whilst enhancing antioxidant activities and reducing oxidative damage and stomata size (Ren et al. 2017). It is generally observed that H2 content decreases during senescence and therefore improving endogenous H2 level during the senescence process might decrease the deterioration of produce (Hu et al. 2018). To illustrate, Su et al. (2019a) demonstrated that altering endogenous H2, by applying exogenous HRW, can extend vase life of lisianthus cut flowers by retaining redox homeostasis through enhancing endogenous antioxidant potential. Moreover, H2 is reported to improve vase quality and extend the life of cut roses by inhibiting endogenous ethylene biosynthesis and mitigating ethylene signal transduction during senescence (Wang et al. 2020a). Recently, Li et al. (2020b) used magnesium hydride (MgH2) as a source of H2 in the vase solution, evaluating its role in prolonging the vase life of cut carnation flowers. An increase in H2-induced H2S was observed along with enhancement in the longevity of cut flowers via H2S signaling, re-establishing redox homeostasis and decreasing the transcripts of representative senescence-associated genes, including DcbGal and DcGST1 (Li et al. 2020b). A proteomic study revealed that HRW and NO (from sodium nitroprusside) application improved the post-harvest freshness of cut lilies, possibly via AtpA protein and activity of ATPase, as well as through regulating photosynthesis and describes the positive role of NO signaling in the H2-induced enhancement in post-harvest of cut flowers (Huo et al. 2018).

As well as flowers, H2 treatment of fruit can also be beneficial. It has been reported that H2 can make a significant impact in the post-harvest preservation of kiwifruit (Hu et al. 2014). Here, H2 treatment delayed ripening and senescence via constraining the respiration intensity, lowering the incidence of rot, decreasing lipid peroxidation levels and enhancing SOD activity, post-harvest. Furthermore, H2 treatment was shown to prolong the post-harvest life of kiwifruit via restricting endogenous ethylene (Hu et al. 2018), a gaseous plant hormone known to be instrumental in the ripening process (Wei et al. 2021).

In tomato, during post-harvest treatment, H2 was shown to not only reduce senescence and extend post-harvest life, but also to reduce the nitrite content, a substance that is harmful to human health (Zhang et al. 2019). In the edible mushroom Hypsizygus marmoreus, H2 application in the form of HRW during post-harvest, improved quality through decreasing oxidative stress, depicted by lowered relative electrolyte leakage rate and MDA content and anti-superoxide-radical (O2·−) activity (Chen et al. 2017a). The application of H2 concertedly enhanced the activities of antioxidants including SOD, CAT, APX and glutathione reductase (GR) through inducing their gene expression levels (Chen et al. 2017b). Together, these reports (Chen et al. 2017a; Hu et al. 2018) demonstrate that H2 is able to reduce endogenous ethylene production and prolong the longevity and shelf life of floriculture and horticulture crops by augmenting antioxidant activities and suppressing ethylene biosynthesis genes. However, further research is needed to ascertain the specific treatment regimens of H2 that will be effective for individual species and genotypes of plants, particularly as phytotoxic reactions are highly variable between individual horticultural commodities. The authors suggest for floral longevity treatment of HRW as a vase solution, while for fruits and vegetables, fumigation at different intervals may be beneficial. As well as molecular hydrogen, other important signalling molecules such as hydrogen sulphide (H2S) have been proposed as the post-harvest treatment in horticulture (Zulfiqar and Hancock 2020), although toxicity needs to be considered for their use, which does not seem to be an issue for H2.

Seed germination

Seed priming is a process used to enhance seed germination, a process which inevitably leads to various desirable traits such as enhanced photosynthesis and tolerance to abiotic stresses (Zulfiqar 2021).

There are numerous seed priming methods available and many researchers have described the use of both chemical and non-chemical compounds effective for boosting seed germination, with varied responses being reported (Zulfiqar 2021). Importantly H2 has also shown its potential as a seed priming agent. For example, Xu et al. (2013) demonstrated that HRW treatment to rice improved seed germination under salt stress condition, with the authors reporting activation of α/β-amylase activity resulting in the accelerated formation of reducing sugar and total soluble sugar content. Here, HRW treatment was also noted to trigger elevated antioxidant enzyme activity (SOD, CAT, APX) and decreases in oxidative stress markers (thiobarbituric acid reactive substances) (Xu et al. 2013).

H2 and abiotic stress tolerance in plants

Metal stress can be alleviated with H2

Plants depend on soil to acquire nutrients for their proper growth and development, although soils are often contaminated through anthropogenic activity (Okereafor et al. 2020). Increasing contamination of productive land has become a great concern for agriculture productivity (Okereafor et al. 2020). Productive soils can gain metal pollutants via solid or liquid fuel burning, industrial effluents, mining activities, sewage waste disposal, urban runoff, agrochemicals runoff and domestic garbage disposal in rivers and canals, for example (Hou et al. 2020). Few (potentially toxic) heavy metals including Cu, Co, Fe, Ni, Se, and Zn are vital elements for plants, but these are known to become toxic as soon as their excess accumulation occurs in soil solution. Accordingly, non-essential elements including arsenate (As), cadmium (Cd), and caesium (Cs) can threaten crop productivity should they accumulate in the soil, even in minute amounts (Okereafor et al. 2020).

Soil contamination with toxic heavy metals increases subsequent uptake in plants and their accumulation in plant tissues not only causes lowered crop productivity, but there is a concomitant risk to animal and human health (Couto et al. 2018). At the cellular level, elevated quantities of heavy metals can impose damage through numerous mechanisms. The most common of these is the production of reactive oxygen species (ROS) that can induce oxidative stress, although other consequences include inactivation of biomolecules through displacement of essential metal ions, or blocking essential functional groups, have also been described (Stohs and Bagchi 1995). ROS at normal physiological levels play an essential role in plant physiology (Hasanuzzaman et al. 2020), however, enhanced generation disrupts homeostatic cellular functions (Jalmi et al. 2018), through oxidation of vital biomolecules including DNA, RNA, lipids, proteins and enzymes etc. (Dumanović et al. 2020). Fan et al. (2020) described that H2 application alleviates the Cu toxicity in Daphnia magna by depressing Cu bioaccumulation and decreasing oxidative stress.

Redox active transition metals such as Fe and Cu, can generate ROS directly through redox reactions, as with the Fenton Reaction, for example; in contrast, other metals like Pb, Cd, Ni, Al, Mn, and Zn generate ROS by indirect mechanisms. The indirect mechanisms of ROS production include heightened ROS production within the mitochondria, stimulation of ROS-producing enzymes such as NADPH oxidases, or by displacing essential cations from the binding sites of functional enzymes and inhibiting their activities (Shahid et al. 2014; Stork and Li 2016).

Existing literature reveals positive influences of H2 on alleviating metal accrued stress (Fig. 4). In cucumber, under cadmium stress, HRW has been demonstrated to promote adventitious rooting and reduce the content of deleterious compounds such as hydrogen peroxide (H2O2), MDA, superoxide radical (O2·−) and thiobarbituric acid reactive substances (TBARS), all of which are indicators of oxidative stress (Wang et al 2019). Furthermore, decreases in ascorbic acid (AsA), glutathione (GSH), lipoxygenase (LOX) activity, relative electrical conductivity (REC), AsA/docosahexaenoic acid (DHA) ratio, and GSH/oxidized glutathione (GSSG) ratio were observed as well, indicating a reduction in stress-associated biomarker activity. Concomitant increases in beneficial biomolecules GSSG and DHA, content under cadmium stress were also noted (Wang et al. 2019), evidencing that H2 possesses the ability to induce adventitious rooting under Cd stress by decreasing oxidative damage. In alfalfa seedlings, application of HRW alleviated the mercury (Hg) toxicity reducing adverse effects of stunted growth as a result of Hg accumulation, through avoiding oxidative stress and maintaining redox homeostasis (Cui et al. 2014). HRW has also been demonstrated to regulate genetic expression related to glutathione and sulfur metabolism. Here, increased GSH (a key antioxidant that regulates the intracellular redox state of the cell) (Schafer and Buettner 2001) metabolism resulted in enhanced Cd tolerance via Cd chelation and activating antioxidation pathways (Cui et al. 2020). An earlier proteomics study revealed that H2 (HRW) eliminated cadmium toxicity through various mechanisms including altering genetic expression related to the reduction of oxidative damage, maintaining nutrient balance and by enhancing sulfur compound metabolism (Dai et al. 2017).

Fig. 4
figure 4

Depicts the possible mechanisms of H2 during photosynthesis, describing the reduction of electrophilic ROS/RNS and the reversible activity of Fe–Fe hydrogenases. The cellular excess hydrogen produced in this manner is also hypothesised to enhance salubrious H2-oxidising bacteria. (created in BioRender.com)

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H2 application is also reported to lower uptake of heavy metals and hence reduce toxicity in multiple plant species. For example, a study of the cadmium accumulation in Pak choi (Brassica chinensis) (Wu et al. 2019) observed that HRW applications repress the expression of cadmium absorption transporters (BcIRT1 and BcZIP2) on exposure to increased Cd concentrations. It has also been reported that H2 application mitigates the toxic effects of cadmium in Brassica campestris by upregulating the expression and ultimately the activity of nitrate reductase (NR) (Su et al. 2019b). Here, pre-treatment with HRW induced lower ROS, enhanced AsA content, increased activity of POD and SOD in seedling roots. Proteomic analysis revealed altered proteins related to antioxidants and oxidation–reduction processes in response to HRW treatment. Furthermore, mitigation of cadmium stress in Brassica campestris, through H2 application was dependent on endogenous NO (Su et al. 2019b). A supporting study (Wu et al. 2015) reported alleviation of cadmium (Cd) toxicity in Chinese cabbage (Brassica campestris spp. chinensis L.). Molecular evidence showed that Cd-induced up-regulation of iron-regulated transporter1 (IRT1) and natural resistance associated macrophage protein genes, responsible for Cd absorption, was blocked, while expression of the HMA3 gene, responsible for Cd sequestration into the root vacuoles, was substantially improved by HRW. Furthermore, it was demonstrated that the Cd-protective effect related to H2 may be associated with its control of plasma membrane-based NADPH oxidase encoded by respiratory burst oxidase homolog D (RbohD), which activates upstream of IRT1 and adjusts root Cd uptake at both the functional and transcriptional levels (Wu et al. 2020a). Chen et al. (2014) reported improvement in the aluminium induced inhibition of root growth through decreasing the endogenous NO level in alfalfa (Chen et al. 2014).

Work such as this shows that there is potential for H2-based treatments to be used to mitigate against metal stresses, which may be seen in the future as increased anthropogenic activities and associated climate changes become more significant (Table 1).

Salinity stress

Salinity stress is a major environmental constraint globally. About 45 million hectares of productive, irrigated land have been estimated to be affected by salinity stress worldwide. This figure is increasing day-by-day (Munns and Tester 2008) largely due to climate change, especially in coastal areas (Hadley 2009). Salinity negatively affects plant productivity by causing imbalance in cellular osmotic and ionic equilibria. Major hostile effects of salinity include increased osmotic stress, specific ion toxicity, nutrient-acquisition and homeostasis/deficiencies, increased cell-turgor loss, and stress induced increased ROS causing oxidative stress (Flowers 2004; Munns and Tester 2008). The role of H2 in improving salinity stress tolerance has been reported in many crops including Hordeum vulgar (Wu et al. 2020c), Oryza sativa (Xu et al. 2013) and Medicago sativa (Wang et al. 2012).

Exogenous HRW application on Hordeum vulgar under salinity stress showed that H2 increased the rate of Na+ extrusion from roots, a mechanism mediated by salt-overly-sensitive SOS1-like Na+/H+ exchangers in the root epidermis. Furthermore, H2 application enhanced root K+ retention by preventing NaCl-induced membrane depolarization and reducing sensitivity of K+ efflux channels to ROS, as detected by electrophysiological studies using non-invasive ion flux measuring MIFE techniques (Wu et al. 2020c). It has also been reported that endogenous H2 levels are enhanced in response to salt stress where H2 pre-treatment of A. thaliana modulated genes/proteins of the zinc-finger transcription factor ZAT10/12 and related antioxidant defence enzymes, resulting in reduction of oxidative stress (Xie et al. 2012). Here, H2 pre-treatment was noted to regulate ion homeostasis by regulating the H+ pump and antiporters responsible for Na+ exclusion and compartmentalization. The same study suggested that APX genes such as cAPX1, and salt overly sensitive1 (SOS1) protein gene, SOS1, might be the target genes of H2 signaling (Xie et al. 2012). H2 may also enhance total, isozymatic activities or corresponding transcripts of antioxidant enzymes, and reduced oxidative damage under salt stress during rice seed germination (Xu et al. 2013). More recently, a study on the role of CrHYD1 transgenic A. thaliana under salinity stress reported that enhanced endogenous H2 regulated the redox and ion homeostasis via interaction with melatonin (Su et al. 2021). It is clear, therefore, that the generation and accumulation of H2 has an influence on stress-induced signaling pathways that can help mitigate the effects of high salt (Table 1), a factor which may become more significant as salinity continues to have an impact in the future (Munns and Tester, 2008).

UV and high light stress

Ultraviolet (UV) radiation of 280–400 nm is a minute portion of the solar energy that is able to modulate plant physiology upon reaching the terrestrial ecosystems (Paul and Gwynn-Jones 2003). Under open field conditions, plants are exposed to direct excess UV radiation, which can affect plant vitality and defence responses. Exposure to UV triggers alterations in fundamental cellular processes including production of ROS, DNA repair mechanisms and by causing damage to cellular structures (Jenkins 2009; Hideg et al. 2013; Li et al. 2013). Plants use their natural defence systems comprising of various antioxidant proteins and peptides, and osmoprotectants, to counteract the negative impact of ROS as a result of abiotic stresses (Zulfiqar and Ashraf 2021; Zulfiqar et al. 2020). Studies have demonstrated that H2 application can induce tolerance to UV stress through modulation in the antioxidant defence system in plants (Xie et al. 2015; Zhang et al. 2015). HRW has been shown to confer tolerance to UVB-induced oxidative damage partially through the manipulation of (iso) flavonoid metabolism in Medicago sativa L. (Xie et al. 2015). Whilst studying the impact of light stress on maize seedlings, it was observed that HRW treated plants showed great tolerance to photo-oxidation by maintaining high levels of antioxidant activities, including SOD, CAT, APX, and GR (Zhang et al. 2015).

HRW significantly blocked UVA-induced accumulation of H2O2 and O2·– and increased anthocyanin production (Su et al. 2014). Interestingly, it has been demonstrated that incremental inositol 1,4,5-trisphosphate/calcium (InsP3-dependent cytosolic Ca2+) contributes to H2-promoted anthocyanin biosynthesis under UVA irradiation in radish sprouts (Zhang et al. 2018).

Genetic approach

Modern breeding tools can enhance the development of new and refined cultivars which demonstrate higher tolerance toward environmental stresses, thereby achieving productivity goals within the agricultural and horticultural food industries. Classic breeding programs are time consuming and laborious, with very little success in developing desired traits in the intended crop, however with the advancement in molecular biology technology and improved techniques, promising opportunities are on the way to develop new cultivars inserted with the specific genes including osmolyte genes for abiotic stress resistance (Zulfiqar et al. 2020). Regarding H2, genetic studies have revealed introgression of H2-related genes into plants can enhance abiotic stress tolerance. For instance, the physiological roles of H2 biosynthesis via expressing hydrogenase1 gene (CrHYD1) in A. thaliana demonstrates a contributing link among osmotic tolerance, endogenous H2 level, and CrHYD1 expression in transgenic lines (Zhang et al. 2020). However, introgression of CrHYD1 from Chlamydomonas reinhardtii enhanced the salinity tolerance through enhancing endogenous production of H2 and melatonin, depicting the crucial role of H2 in response to salinity (Su et al. 2020). Results from such studies strongly suggest that future research should also focus on H2-based genetic improvements of crops to further understand the role of H2 in enhancing the stress tolerance in plants.

Possible mechanisms behind H2 responses

The evidence from numerous studies, as discussed above, suggests that H2 has a profound, and positive effect on the growth of plants, and their tolerance to stress; be that heavy metals (Cui et al. 2014; Dai et al. 2017; Wang et al. 2019), or salinity (Wang et al. 2012; Wu et al. 2020c; Xu et al. 2013). However, the molecular basis of these effects is far from certain. It is unlikely that H2 is perceived by a classical receptor-type mechanism, as it is so small. It is also relatively inert, so reactions with thiol groups, as seen with other reactive biological gases including H2O2 (Ulrich and Jakob 2019), H2S (Kumar and Banerjee 2021) and NO (Gupta et al. 2020), are unlikely.

Many empirical investigations (Xu et al. 2013; Ren et al. 2017; Chen et al. 2017b) report beneficial effects of H2 on antioxidant levels in cells, but how this is brought about is also unclear. It has been reported that H2 can scavenge both hydroxyl radicals (·OH) and peroxynitrite (ONOO−) (Ohsawa et al. 2007), although this has been brought into doubt when the reaction kinetics of H2 and these distinct molecules were considered (Penders et al. 2014). It appears, therefore, that the direct scavenging of such reactive signaling molecules by H2 can be ruled out. Hancock et al. (2021) have suggested that as the mid-point potential of the H2 couple is − 414 mV (relative to a Standard Hydrogen Electrode (SHE)), that iron ions, specifically Fe3+, may be the target. If this is the case, many significant and relevant hemeprosthetic groups could be targets, including those in mitochondria, chloroplasts and the respiratory burst oxidase homologues (RBOHs) (Hancock et al. 2021). This would lead to the reduction of Fe3+ to Fe2+, altering the activity of such heme-based enzymes, potentially account for some effects seen. Furthermore, Fe3+ has a role in hydroxyl radical formation (Fong et al. 1976) whilst Fe2+ has been shown to have pro-oxidant activity (Mozuraityte et al. 2006). The molecular basis of H2 action in cells has recently been reviewed (Hancock and Russell 2021; Hancock et al. 2021).

Due to the physical characteristics of molecular hydrogen, an alternative mechanism of action may be through the spin states of H2. The H2 molecule can exist in two states, that is, para and ortho spin states (Shagam et al. 2015). It has been of the posited (Hancock and Hancock 2018) that this may allow an interaction with other signaling molecules in cells, such as NO. However, to date, there is no evidence that this is the case and is another aspect of hydrogen chemistry that is worthy of further investigation.

Finally, another mechanism of H2 action may be through the alteration of the expression and/or activity of the enzyme heme oxygenase (HO-1). This enzyme aids in catabolism of heme (Wilks 2002). H2 is reported to enhance the abiotic stress tolerance and adventitious root development via the modulation of HO-1 gene expression, a Nrf-2 regulated gene, that has key role in preventing both hypoxia and vascular inflammation through enhancing antioxidant, antiapoptotic, antiproliferative and immunomodulatory pathways in animals, and is likely to have a similar effect in plants (Li et al. 2012). HO-1 may be part of vital signalling systems involved in the response to cellular stress in plants (Jin et al. 2013; Lin et al. 2014) and therefore its role in mediating H2 effects would seem appropriate.

Although research findings are demonstrating the positive influence of H2 usage in agriculture to manage the stress conditions, still much more work needs to be carried out as it is very unclear how H2 actually works in the cell and what exactly the molecular targets are. This needs to be a focus for H2-based research moving forward.

Conclusions and future perspectives

As a result of abiotic stresses, plants generally demonstrate retarded growth responses. Such stresses are only likely to be exacerbated by increased human activity and climate change. Therefore, mitigating treatments which are easy and safe to use would be of significance for future sustainable food security. H2 application in the form of HRW has been suggested as such a beneficial remedy in agriculture. H2 has been shown to mitigate against a range of abiotic stresses, including heavy metal tolerance (Dai et al. 2017; Wu et al. 2019; Wang et al. 2019; Dumanović et al. 2020; Fan et al. 2020; Cui et al. 2020), increased salinity (Wang et al. 2012; Xu et al. 2013; Wu et al. 2020c; Su et al. 2021) and UV exposure (Xie et al. 2015; Zhang et al. 2015; Su et al. 2014), as well as improving the post-harvest storage of flowers and fruits.

However, there are a few caveats which need to be the focus of future research. H2 treatment is not always easy to use, with HRW being the most obvious and easiest, but the loss of H2 from the liquid phase to the atmosphere will limit the longevity of such treatments and require regular administration. Using H2 in the gaseous form is both dangerous (owing to flammability) and time limited as it would easily escape ground level atmospheres. Due to the modernity of molecular hydrogen research, most of the studies related to H2 intervention for abiotic stress-induced negative impacts on crop plants are laboratory based and yield evaluations have not been completed, especially under field conditions where the severity of these stresses is more prevalent. Therefore the cost–benefit analysis of the viability of H2 applications within industry, to both crop or produce, at pre-harvest and post-harvest stage respectively, should also be considered in future studies.

H2 research for agriculture and horticulture is at its infancy stage and currently production costs are relatively high. For instance, green hydrogen production (by electrolysis) can range between $1.25 and $10.90 per Kg dependent on the generation method used (Calise et al. 2019). However, H2 is likely to become cost effective as it is adopted by other industries, for example within the transport and energy sectors. Within agriculture and horticulture various possibilities could be evaluated to reduce labor cost for H2 application. In this regard, H2 application via irrigation systems such as drip irrigation could be an alternative to reduce labor costs and ensure uniform application. Therefore, the future progress in H2- based mitigation of abiotic stresses and post-harvest benefits are totally reliant on the cost-effective technology of H2 production and H2 delivery on an agricultural scale, which is likely to be more attractive in the future.

In conclusion, H2 treatments can be beneficial in many stages of agriculture, from seed germination (Xu et al. 2013), root growth (Lin et al. 2014; Zhu et al. 2016; Wu et al. 2020b), post-harvest storage (Jiang et al. 2021). Current and growing research literature supports the evidence that H2 has positive effects on plants and appears to have no harmful biological effects. Future work on H2 treatments and a comprehensive understanding of the molecular basis of how H2 interacts in physiological systems are paramount and will require further research investment if this natural and effective compound is to be utilized to enhance food production on a commercial basis.