Abiotic Stresses and Their Management in Vegetable Crop Production

Abstract

The stress concept, first proposed by Hans Selye in 1936, has also been applied to plants to describe adverse and environmental restrictions. The notion of plant stress, differs significantly from that of animals and humans. Due to ever fluctuating climatic circumstances and variables, the crop-environment interaction in horticultural crops leading to losses in yields and quality of produce occurs and thus climate change with respect to horticulture industry is attracting more attention. Abiotic stress is the leading cause of crop yield loss globally, lowering average yields by more than half for most main crop plants. Abiotic stressors are highly correlated and connected, causing morphological, biochemical, physiological and molecular changes in vegetable crops, leading in a significant profit drop. Water stress is the most common abiotic stress that causes significant losses in vegetable production, especially because it is often coupled by additional stresses like as salt, high temperatures, and nutritional deficits. Increased CO2 and temperature in the atmosphere, variation in amounts of precipitation causing more frequent droughts and floods, widespread runoffresulting in soil nutrient leaching and a loss in fresh-water availability are all contributing factors. Efforts to mitigate various pressures should be focused both throughout the growing season and after harvest. Stress-tolerant cultivars are being developed using a variety of methods, including traditional breeding and transgenic technology. Instead of genetic engineering, using vegetable breeding procedures or directed breeding is one the best options to improve stress tolerance in vegetables. Besides, post-harvest treatments, application of growth regulators, antioxidants, germplasm and in vitro selection, and modified environment packaging with different plastics may all help to improve tolerance and hence increase the shelf and nutritive life of vegetables.

1. Introduction

Crops are increasingly commonly subjected to abiotic stressors in today’s climate change scenarios. Abiotic stress, such as drought, salt, and severe temperatures, which typically cause main crop losses around the world, are predicted to produce a yield loss of more than 50% in agricultural crop plants [1]. We should also focus on increasing the food production and supplies by two-fold till 2050 so as to fulfill the requirement and demand of human population. This can be achieved by a basic comprehension of mechanisms underlying abiotic stresses. It is for this reason that development of stress-tolerant plants has received gotten a lot of attention in recent years for these reasons. During production, processing, storage, and distribution, harvested vegetables might be subjected to a variety of abiotic stressors. When there is a moderate or severe abiotic stress, quality losses nearly always occur at market [2, 3]. Moreover, there are abiotic stressorswhich ultimately decreases the defense mechanisms of plants and increases their susceptibility to infection by pathogens. Understanding the nature andorigins of abiotic stressors that impact vegetables is critical. In addition, increased understanding opens up possibilities for better control or resistance [3, 4]. As a result, as postharvest problems limit the storage and shelf-life potential of vegetables, understanding the effects of field abiotic stresses on postharvest stress susceptibility will become increasingly important [4]. Why there are studies in relation to advances in physiology, molecular biology, and genetics is because of the fact that our comprehension of plants’ responses to various stresses, as well as the basis for varietal tolerance variances is a big solution to the very big problem of abiotic stresses.

As the best way to solve these issues is to focus on both pre-harvest and post-harvest abiotic stress reduction, it’s crucial to understand the relationship between pre-harvest and post-harvest abiotic stresses that occur during vegetable crop production and handling, storage, and distribution, respectively.

2. Pre-harvest stresses during vegetable production

2.1 Temperature extremes

Plants are vulnerable to low temperature stress if the temperature dips below 15°C, and high temperature stress if the temperature rises over 45°C. Plants are affected by high temperature stress in a variety of ways, including physiology, biochemistry, and gene regulation mechanisms. High temperatures during the reproductive period of plants can increase senescence, diminish fruit set and lower yield. Furthermore, temperature stress makes the plant vulnerable to pests and other environmental issues besides limitingor preventing seed germination, depending on the species and stress level. Exposure to high temperatures throughout the growing season might also affect antioxidants in vegetable crops. Susceptibility to postharvest chilling injury can be exacerbatedif the preharvest temperature causes chilling induced harm in the field. As a result, the magnitude of the preharvest temperature extreme will determine whether the exposure has a favorable or negative impact on postharvest stress sensitivity.

2.2 Drought

We know that a third of the world’s population resides in areas that are having water-stress condition which may become more severe due to increasing carbon-dioxide concentrations in the atmosphere. The climatic changes are therefore more expected with severe droughts in furture. Water scarcity is expected to remain a major abiotic issue influencing worldwide crop output. Reduced canopy absorption of photosynthetically active sunlight, decreased radiation-use efficiency, and reduced harvest indexare all effects of soil moisture deficit on crop output. Drought circumstances during the development of vegetable crops are becoming increasingly common as a result of climate change patterns [5]. In root crops, field water deficiency (stress) has been found to have both positive and negative effects. Water stress prior to harvest (Irrigating to 25–75 percent of soil water field capacity) may weaken the cells, resulting in increased membrane leakage (cell damage) and, as a result, more weight loss in storage for root crops like carrots.In response to mechanical stresssuch as bruisingpotato cells undergo decompartmentationresulting in black spot conditions [6]. Water stress, especially during the tuber-forming stage, can make potatoes more susceptible to the black spot condition after harvest [6].

2.3 Light

Tomatoes grow smaller when cultivated in low light environments, such as early spring in northern latitudes [7], and because theratio of surface area to volume is higher in smaller fruits, vulnerability to postharvest desiccation stress increases [8]. It has been reported that when lettuce is cultivated under less intense lights, due to less number of photons, photosynthetic efficiency as well as quality traits like vitamin C is suboptimal and thus decreasing shelf life after harvest.

2.4 Salinity

Excessive quantities of soluble salts in the soil water (soil solution) are known as saline soil, and they can significantly affect plant growth, resulting in lower crop yields and even plant mortality in extreme cases. Salts are substances that dissolve into ions, such as NaCl, MgSO4, KNO3, and sodium bicarbonate. Electrical conductivity (ECe), exchangeable sodium percentage (ESP) or sodium adsorption ratio (SAR), and pH of soil paste (saturated) extractare used to calculate it. As a result, saline soils have saturated soil paste extracts with an ECe of more than 4 dSm⁻¹, an ESPof less than 15%, and a pH of less than 8.5 [9]. Tomato crop grown inhigh salinity generate smaller fruits with a greater soluble solids content. It is a matter of understanding that fruits having smaller size have more surface area than their volumes (Known as Surface-area to Volume ratios), making them more vulnerable to postharvest water loss (desiccation stress) [8].

2.5 Flooding stress

Crops are also subjected to severe physiological stress as a result of sudden inundation following heavy rainfall events. Plants must adapt to a distinct, but equally challenging, flooding environmentthat occurs in a more regular cycle of seasonal fluctuations in river levels and concomitant slow flooding of crop lands. Waterlogging is the term for soil condition when there is flooding that creates hypoxia which also affects stems causing wilting with other physiological conditions.

2.6 Plant nutrition

When plants are unable to complete the reproductive stage of their life cycle due to a shortage of mineral components, they are considered essential. In several crops, calcium supplementation during production has been associated to postharvest issues [10]. Calcium has been proposed as a possible signaling molecule involved in the development of abiotic stress cross tolerance [11]. As a result, the effect of preharvest calcium nutrition on postharvest stress resistance is likely to be multifaceted, and it will depend on whether the vegetable is also exposed to abiotic environmental difficulties. Preharvest nitrogen levels are frequently linked to poor postharvest vegetable quality. Excessive nitrogen fertilization causes large zinc and aluminum accumulations in cabbage, as well as nitrate-induced manganese deficiency [10]. Nitrogen fertilizer affects black spot susceptibility in potatoes. Nitrogen deficit or lower-than-recommended nitrogen treatment rates, on the other hand, will almost always result in higher vitamin C concentration in plants. Vitamin C concentration has been connected to storage life potential [12], which is likely due to the antioxidant nutrient’s usefulness in preventing oxidative damage, which leads to quality losses in storage. Potassium deficit in carrots is linked to increased weight loss during storage. At potassium levels below 1 mM in the soil media, weight loss was directly linked to increased membrane leakage (i.e., damaged cells) in carrot tissues.

3. Post-harvest stresses during handling, storage and distribution of vegetable crops

3.1 Temperature extremes

The relevance of temperature in determining a harvest index cannot be overstated. Many products, particularly those delivered by air or ocean container, face persistent postharvest temperature abuse during distribution [13]. Heat treatments limit respiration and ethylene generation, diminish protein synthesis, and accelerate protein degradation in the short term. Fruit vegetables, root and tuber crops are all susceptible to chilling [14]. The ability to produce flavor has been shown to be a sensitive early indicator of cold stress effects, andChilling injury is related with visible (surface pitting, interior browning) and texture (accelerated softening and development of mealiness) alterations.

3.2 Oxygen and carbon-dioxide level

The difficulty is worsened when processing products in modified atmosphere (MA) containers rather than controlled atmosphere (CA) systems, because temperature is generally not as easily managed in MA packages as produce goes through a distribution chain [4]. High CO₂ stress can cause a diverse array of physiological problems, such as black heart in potatoes and brown stain in lettuce [15]. Chilling stress, ethylene-induced diseases, and vulnerability to pathogenic attack can all be influenced by high CO₂ levels [15].

Low oxygen levels are known to cause stress-induced changes in metabolism and metabolite accumulations [16], but acute low oxygen injury does not show up until the tissue is re-aerated and an uncontrolled oxygen burst (consisting of hydrogen peroxide and other radicals) occurs, causing lipid peroxidation, protein denaturation, and membrane injury [17]. Varied vegetables have different low O₂ stress thresholds depending on architecture, temperature, physiological age, the presence of supplementary gases (e.g., CO₂, CO, SO₂), and the duration of exposure.

3.3 Mechanical injury

Injury from impact is linked to product loading for transportation, incidents during transportation, unloading, and throughout the packaging and processing lines. Cuts can produce brief increases in respiration, ethylene synthesis, phenolics generation, and cell degradation near the injury site [18]. The severity of the reaction to cutting is highly dependent on tissue properties, the maturity of the vegetable of interest, the coarseness or sharpness of the cutting object employed, and the cutting temperature.

Fresh-cut vegetable products have the highest rate of cut injuries. Many vegetables suffer from cut injuries during the harvesting process, which are more severe in machine-harvested produce than in hand-harvested produce. Cut-edge browning or blackening is the most typical symptom of cutting-related diseases, however yellowing in green tissues and whitening on carrots can also occur [18]. The degree and amount of bruising received is influenced by maturity, tissue or cellular orientation at the region of the injury, water potential, form of the object imparting the bruising force, energy and angle of impact, and product temperature. Internal black patches appear in potato tubers as a result of impact trauma.

3.4 Desiccation

Water loss in vegetable tissues causes degradation, which is a significant problem inpostharvest processing and distribution [19]. Water stress can cause rapid senescence, which manifests as tissue weakening, membrane degradation, and yellowing in addition to wilting [19]. The vapor pressure deficit, which is the connection that explains the difference in water activity of the vegetable and the water activity of the atmosphere surrounding it [19], is thedriving factor for water loss. The more of a vapor pressure deficit there is, the more water is lost. For limiting water loss in any vegetable, there are a few postharvest handling principles to follow:

1. Delays in chilling will result in extended exposure to higher vapor pressure deficit circumstances, therefore cooling after harvest should be done as soon as possible.

2. When a warm product is placed in a cool area, it loses water faster than a cool product, hence rapid precooling before storage is crucial; and

3. Water loss can be minimized by storing products at the coolest storage temperature and with the highest relative humidity achievable.

4. Abiotic stress response Mechanismsat the biochemical and molecular levels

Acute and sub-acute reactions to abiotic stresses exist; acute responses reflect circumstances where cell death is a direct resultof the stress, whereassub-acute responses represent cases where the stress induces adaptive changes in biochemistry and gene expression [6]. Many reactive oxygen species (ROS), especially hydrogen peroxide, operate as signaling molecules that cause biochemical changes in gene expression. Abiotic stresses disrupt vegetable cellular homeostasis, resulting in increased production of reactive oxygen species (ROS) in the apoplast, mitochondria, peroxisomes, cytoplasm, chloroplasts, and endoplasmic reticulum [20]. The cell’s ability to cope at first will be largely determined by its endogenous free radical scavenging capacity [20]. When free radical production surpasses endogenous scavenging capability, ROS interact with sensors whose full natureis unknown, triggeringmitogen activated protein kinase (MAPK) cascade events and up-regulating transcription factors and calcium/calmodulin kinases directly. The MAPK cascade reactionactivates a number oftranscription factors that enable de novo ROS formation, ROS scavenging systems, heat shock protein accumulation, and NADPH supply modulation in the cell [20]. Some MAPK cascade pathways have also been connected to ethylene production particularly, which is likely why ethylene production appears to be intrinsic to most stress reactions. Heat shock proteins (HSPs) accumulation, whichismediated by transcription factor activation downstream of the MAPK cascade [20], has been shown to improve long-term stress resistance in afflicted tissues [21]. Under normal, stress-free conditions, HSPs are thought to be an essential element inprotein folding, assembly, translocation, and degradation [22]. HSPs have also been linked to protein stabilization, membrane stability, and protein refolding under stress. As a result, HSPs are assumed to play akey role in protecting plant tissues from stress by ensuring cellular homeostasis. Metabolic Responses to Abiotic Stress Signaling, physiological regulation, and defense responses are all heavily influenced by metabolism when the environment is hostile and plant development is harmed. Abiotic stressors alter the production, concentration, transport, and storage of primary and secondary metabolites in a feedback loop.

5. Drought stress and plant metabolomics

Water deficiency stress causes a variety of physiological and biochemical changes in plants, including cell development and photosynthesisarrest, as well as increased respiration. Thegenome’s expression is modified extensively, activating and suppressing a wide range of genes with various roles. Abscisic acid (ABA) accumulates inhydric-stressed plant tissues and increases stomatal closure, which reduces transpiration. Plants use this technique to reduce water loss and reduce stress injury.

6. Temperature extremes and plant metabolomics

In sensitive vegetables, chilling stress causes the formation of lipid peroxidation products, superoxide anions, and hydrogen peroxide, as well as a reduction in flavor volatile synthesis. It interferes with ethylene metabolism [23], causing the softening process to speed up. Cell wall metabolism is also altered by chilling stress, with up-regulation of cell wall breakdown enzymes as pectin methyl esterase and endopolygalacturonase. Heat stress causes metabolic changesthat lead to the buildup of heat shock proteins, which are known to give long-term stress resistance in heat-exposed animals [22]. It can alsostop lycopene from being produced and accumulated.

7. Wounding stress and plant metabolomics

The upregulation of phenylalanine ammonia lyase (PAL) by wounding stress causes phenolic buildup [24]. The upregulation of PAL was linked to the synthesis of ethylene by wounds. Theinitial reaction to wounding stress is characterized by a progressive accumulation of ACC synthase, ACC, and ethylene synthesis in tomatoes, which can last up to 2 hours, butethylene production reduces if the sliced tomatoes are kept for longer periods of time. ACC synthase and ACC, on the other hand, continue to accumulate, showing that there is a capability to generate ethylene, but ACC to ethylene transition is prevented or inhibited. Other metabolites, such asisocoumarin in carrots [25], anthocyanins in red-pigmented lettuce midribs [25], methanethiol, allylisothiocyanates, and dimethyl disulfide in cabbage [26], and six-carbon aldehydes and alcohols in cut peppers [27], rise in response to wounding stress.

8. Oxidative stress and plant metabolomics

When exposed to anaerobic or anoxic environments, the body goes through two separate phases. The first phase is characterized by a metabolic shift caused by a limitation of the principal electron acceptor, molecular O2, in the mitochondrial electron transport chain. ATP levels, pyruvate dehydroxylase activity, and cytoplasmic pH have all decreased as a result of this impairment [15]. The activity ofpyruvate decarboxylase, alcohol dehydrogenase, and lactate dehydrogenase also rise under these conditions [15]. Anaerobic respiration is induced, and acetaldehyde, ethanol, ethyl acetate, and/or lactateaccumulate. Damage to the mitochondrial electron transport chain also causes electron leakage in the cells, resulting in the production of superoxide anions and hydrogen peroxide, which are destroyed by existing cellular antioxidant systems. During an anoxic or hypoxic incident, ascorbate and glutathione levels can also rise. Any or all of these alterations in vegetable tissues are signs of oxygen stress and have an impact on qualitative qualities. However, it isnot until the second phase, when the vegetable is returned to greater O₂ atmospheres, that true tissue harm occurs. When cells are exposedto aerobic environments, rates of oxygen radical production in the impaired electron transport chainincrease, resulting in largeaccumulations of superoxide anion, hydrogen peroxide, and hydroxyl radical that cannot be fully decomposed by existing antioxidant protection systems, resulting in membrane damage, enzymatic browning, and cell death [17].

9. Salinity stress and plant metabolomics

Salt stress causes the production of abscisic acid, which is transferred to guard cells and seals stomata, resulting in impaired photosynthesis, photoinhibition, and oxidative damage. This results in an instantaneous halt tocell expansion, which manifests as slowed plant growth, rapid development, and senescence. Plants use strategies including reduced photosynthesis, stomatal conductance, and transpiration rates to cope with salt stress. Because sodium ion has the same chemical structure as potassium ion, it competes with potassium uptake and suppresses it. Potassium deficiency inhibits growth since it is involved in the capacitance of a wide range of enzyme activities, as well as controlling membrane potential and cell turgor.

10. Desiccation stress and plant metabolomics

Under extreme handling conditions, desiccation stress increased the osmotic potential of carrots, which is a function of free sugars in the roots. (i.e., at 13°C). The elevation in osmotic potential in response to water loss was most likely explained by increased polysaccharide hydrolyzing enzyme activity in response to stress. As a result, enzymes such polygalacturonase and pectin esterase may become more active, resulting in a loss of cell wall integrity and a rise in soluble sugars. This could account for at least some of the loss of stiffness observed in carrots as they lose water.

11. Management of Abiotic stresses

11.1 Abiotic stress tolerance and the role of plant growth regulators

Applications of growth regulators may also improve stress resistance, particularly invegetables that are prone to rapid senescence in response to stress. As a result, anti-ethylene products such amino vinyl glycine (AVG) and 1-methylcyclopropene (1-MCP) may help to extend storage or shelf life if ethylene synthesis in reaction to stress is a major problem. Bell peppers and zucchini squash can benefit fromother growth hormones, such as methyl jasmonate (which increases leaf senescence). In some crops, abscisic acid has been shown to decrease chilling-induced damage. Other growth regulators (e.g. 2, 4-D) have been proposed for use in avoiding senescence in leafy vegetables, but their practical utility is limited.

11.2 Postharvest treatments to enhance stress resistance

A variety of postharvest treatments have been tested to improve vegetable abiotic stress tolerance [3]. Temperature manipulation (including intermittent warming), extreme atmospheres (high O₂, CO₂, and low O₂), growth regulators, anti-transpirants, antioxidant dips, growth regulators, nitric oxide, and ethanol haveall been put to the test [3]. In fresh-cut items, hot or warm water treatments have been demonstrated to reduce cutting-induced damage. Treatments like these can also be utilized to prevent chilling harm by inducing heat shock proteins [21]. In cut and packed lettuce, a warm water treatment has also been demonstrated to lower irradiation susceptibility [28]. Gradual cooling (2°C per day) has been reported to lower the tomato chilling injury susceptibility, most likely by enabling the intrinsic stress resistance systems to mature before true chilling conditions [29]. Atmospheric treatments, such as modified or controlled atmospheres, have been demonstrated to aid in the reduction ofchilling injury in a variety of vegetable crops.

11.3 Use of molecular probes for marker-assisted breeding

As there are many genes and proteins linked to stress tolerance in plants, applying the stress of interest and doing quantitative trait loci (QTL) analysis is the best way to find stress tolerant lines. This method canbe employed with intact plants and/or harvested plant parts, with the plant component of interest in the breeding improvement strategybeing used in most cases [30]. The method necessitates the examination of adaptive changes in QTL expression rather than constitutive expression. To distinguish between resistant and susceptible lines, a stress protocol must be created to which the target vegetable will be exposed. However, because the stress response is complicated, successful use of QTLs will necessitate an interdisciplinary effort that integratesbiochemistry, gene mapping, and phenotyping activities to allow for reliable interpretation and successful application of adaptive QTLs for stress resistance selection.

11.4 Molecular engineering

Because of two major factors, molecular engineering for stress resistance in vegetables is limited:

• Due to the general complexity of the stress response network, single gene insertions are unlikely to modify stress resistance, and.

• Many key vegetable crops have yet to be successfully transformed, and ways to do so have yet to be devised. Insertion of anti-freeze genes to defend against low temperatureharm is one area where progress has been made. Future developments, on the other hand, will demand a deeper molecular knowledge of the stress response network and regulatory points.

11.5 Germplasm selection

Germplasm selection and cultivarsdeveloped through breeding programmes will be more resistant to postharvest stress and so have improved storage capacity. Hodges et al. [12] were able to establish that variations in the balance of antioxidant systems in the tissues caused larger accumulations of ROS, notably hydrogen peroxide, in a cultivar that was more prone to yellowing. They theorized that greater ROS levels were directly responsible forthe yellowing of the chlorophyll in spinach leaves. In vitro selection isa technique in which plant cells from a target vegetable are tissue-cultured and subjected to a stressor, with the surviving cells used to regenerate new plants with higher stress resistance [31]. It is a far less expensive technique than molecular engineering, and laboratories may be built up practically everywhere in the globe with basic utilities and utilizing low-cost technology. This method has proven to be particularly effective in regenerating germplasm from a variety of crop plants that can be regenerated using tissue culture techniques.

11.6 Postharvest handling

Simple changes to postharvest handling methods can occasionally result in a significant reduction in stress exposure, allowing for longer storage and/or shelf life. Because most produce is refrigerated as a required step to minimize rotting and protect food safety, avoiding low temperature stress is typically impossible. While quick cooling is normally suggested to maintain quality, delaying or gradually chilling sensitive crops to allow them to acclimate to storage and handling conditions may be beneficial. Slow cooling of tomatoes from 12–4°C at a rate of 2°C per day has recently been proven to decrease chilling harm when stored at the lower temperature [19]. In many circumstances, modified environment packaging is thought tohelp minimize moisture loss in fresh-cut and whole vegetables by controlling humidity surrounding the product. Many vegetables have improved their shelf life and quality by using plastic film packaging or wraps to prevent desiccation [19]. Anti-transpiration coatings have also been demonstrated to be useful in preserving quality by reducingwater loss. A single stress resistance-enhancing treatment may notprovide enough resistance to all postharvest stresses [3]. To obtain optimal levels of resistance, it may be beneficial to explore using a combination of two or more stress tolerance increasing treatments.

12. Conclusion

Drought, excessive watering, severe temperatures, salt, and mineral toxicityall have a negative impact on thegrowth, development, yield, and quality of vegetables on and off the farm until they reach the customer. Furthermore, climate change has introduced new environmental variablesthat may influence the vulnerability of vegetables to postharvest stress. Crop management can have a substantial impact on stress susceptibility. Adapting horticulture crops to changing surroundings could be the single most essential action we can take to prevent climate change’s negative consequences. The management steps must be followed from the field circumstances to the point where the product reaches the consumer. While many crops are being bred for stress resistance to helpthem adapt to climate change, it is unclear if in the field breeding for stress resistance will also transmit stress resistance qualities to the harvestedcomponent. To properly evaluate the benefits that abiotic stress during production may offer for postharvest abioticchallenges, it’s critical tounderstand the basis of molecular and biochemical response networks to diverse stresses faced in the field and throughout the postharvest continuum. Theuse directed plant breeding to improve the toleranceto stress in vegetablesshould probably be the focus of attention. Temperature modulation, usage of growth regulators, anti-transpirants, antioxidants, and other sorts of postharvest management can improve tolerance and hence extend the keeping quality of vegetables. The use of various plastic sheets to generate tailored environment packaging is one of the best promising technologies. As a result, new vegetable types that are resistant to abiotic stressors are urgently needed to assure food security and safety for many years to come.