Physiological and Nutraceutical Quality of Green and Red Pigmented Lettuce in Response to NaCl Concentration in Two Successive Harvests

Plant extract improves quality traits of green and red lettuce cultivars

The use of biostimulants in agriculture has been rapidly increasing in recent years, because of their positive effects on crop yield, product quality, and tolerance to abiotic stresses. In the present study the efficacy of multiple applications of a plant-derived biostimulant, obtained from the aqueous maceration of borage (Borago officinalis, L.) flowers on two lettuce cultivars, namely a green (Lactuca sativa L. cv. Expertise RZ) and red (Lactuca sativa L. cv. Codex RZ) Salanova® was evaluated. The treatment was applied at 10 mL L-1 as foliar spray three times, once a week starting from two weeks after transplant. Control plants were treated with water. Non-destructive measurements (pigments, leaf nitrogen index, chlorophyll a fluorescence) were taken during plant growth after at each treatment application. At the end of the experiment, destructive analyses were performed to assess qualitative traits. The research work was focused on the evaluation of physiological parameter changes during plant growth, and on primary and secondary metabolism. Foliar applications did not affect the accumulation of total sugars (4.56 mg g-1 in Expertise, 3.5 mg g-1 in Codex) in either cultivar. However, the lettuce head weight was negatively affected by the extract application in red cultivar (-10 g/plant), while no changes were observed in the green lettuce. The nitrogen-flavonol index (NFI) increased after the third application of borage extract in green cultivar (+67 %), suggesting an improvement of nitrogen nutrition status or a reduced stress condition. A different response resulted in term of maximum quantum efficiency of PSII (FV/FM), performance index (PI), nitrate, and anthocyanin accumulation in leaves. The FV/FM ratio significantly increased in green cultivar after the first application (from 0.80 to 0.84) and at harvest (from 0.79 to 0.84). The PI showed a slight but not significant increase at the same time points. On the contrary, the PI was significantly higher in red cultivar after the third application (+9.4 %). Interestingly, the borage extract induced a significant decrease of nitrate accumulation in lettuce leaves of the red cultivar (from 4149.7 to 2711.6 mg/kg, -34 %). At the same time a positive variation of anthocyanin content was observed in red lettuce (+24.7 %). The application of biostimulant products might improve the quality of some lettuce varieties as regards the accumulation of metabolites useful for the plant to overcome stress conditions and fundamental in human healthy diet, increasing the leaf concentration of Ca, Na, and Mg.

Mitigation of salt stress in lettuce by a biostimulant that protects the root absorption zone and improves biochemical responses

Horticultural crops constantly face abiotic stress factors such as salinity, which have intensified in recent years due to accelerated climate change, significantly affecting their yields and profitability. Under these conditions, it has become necessary to implement effective and sustainable solutions to guarantee agricultural productivity and food security. The influence of BALOX®, a biostimulant of plant origin, was tested on the responses to salinity of Lactuca sativa L. var. longifolia plants exposed to salt concentrations up to 150 mM NaCl, evaluating different biometric and biochemical properties after 25 days of treatment. Control plants were cultivated under the same conditions but without the biostimulant treatment. An in situ analysis of root characteristics using a non-destructive, real-time method was also performed. The salt stress treatments inhibited plant growth, reduced chlorophyll and carotenoid contents, and increased the concentrations of Na+ and Cl- in roots and leaves while reducing those of Ca2+. BALOX® application had a positive effect because it stimulated plant growth and the level of Ca2+ and photosynthetic pigments. In addition, it reduced the content of Na+ and Cl- in the presence and the absence of salt. The biostimulant also reduced the salt-induced accumulation of stress biomarkers, such as proline, malondialdehyde (MDA), and hydrogen peroxide (H2O2). Therefore, BALOX® appears to significantly reduce osmotic, ionic and oxidative stress levels in salt-treated plants. Furthermore, the analysis of the salt treatments' and the biostimulant's direct effects on roots indicated that BALOX®'s primary mechanism of action probably involves improving plant nutrition, even under severe salt stress conditions, by protecting and stimulating the root absorption zone.

Cucumber grafting on indigenous cucurbit landraces confers salt tolerance and improves fruit yield by enhancing morpho-physio-biochemical and ionic attributes

Pakistan is the 8th most climate-affected country in the globe along with a semi-arid to arid climate, thereby the crops require higher irrigation from underground water. Moreover,  ~ 70% of pumped groundwater in irrigated agriculture is brackish and a major cause of secondary salinization. Cucumber (Cucumis sativus L.) is an important vegetable crop with an annual growth rate of about 3.3% in Pakistan. However, it is a relatively salt-sensitive crop. Therefore, a dire need for an alternate environment-friendly technology like grafting for managing salinity stress in cucumber by utilizing the indigenous cucurbit landraces. In this regard, a non-perforated pot-based study was carried out in a lath house to explore indigenous cucurbit landraces; bottle gourd (Lagenaria siceraria) (cv. Faisalabad Round), pumpkin (Cucurbit pepo. L) (cv. Local Desi Special), sponge gourd (Luffa aegyptiaca) (cv. Local) and ridge gourd (Luffa acutangula) (cv. Desi Special) as rootstocks for inducing salinity tolerance in cucumber (cv. Yahla F1). Four different salts (NaCl) treatments; T0 Control (2.4 dSm-1), T1 (4 dSm-1), T2 (6 dSm-1) and T3 (8 dSm-1) were applied. The grafted cucumber plants were transplanted into the already-induced salinity pots (12-inch). Different morpho-physio-biochemical, antioxidants, ionic, and yield attributes were recorded. The results illustrate that increasing salinity negatively affected the growing cucumber plants. However, grafted cucumber plants showed higher salt tolerance relative to non-grafted ones. Indigenous bottle gourd landrace (cv. Faisalabad Round) exhibited higher salt tolerance compared to non-grafted cucumber plants due to higher up-regulation of morpho-physio-biochemical, ionic, and yield attributes that was also confirmed by principal component analysis (PCA). Shoot and root biomass, chlorophylls contents (a and b), activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POX) enzymes, antioxidants scavenging activity (ASA), ionic (↑ K and Ca, ↓ Na), and yield-related attributes were found maximum in cucumber plants grafted onto indigenous bottle gourd landrace. Hence, the indigenous bottle gourd landrace 'cv. Faisalabad round' may be utilized as a rootstock for cucumber under a mild pot-based saline environment. However, indigenous bottle gourd landrace 'cv. Faisalabad round' may further be evaluated as rootstocks in moderate saline field conditions for possible developing hybrid rootstock and, subsequently, sustainable cucumber production.

Effect of mixed light emitting diode spectrum on antioxidants content and antioxidant activity of red lettuce grown in a closed soilless system

BACKGROUND

Light spectra have been demonstrated to result in different levels of comfort or stress, which affect plant growth and the availability of health-promoting compounds in ways that sometimes contradict one another. To determine the optimal light conditions, it is necessary to weigh the vegetable's mass against the amount of nutrients it contains, as vegetables tend to grow poorly in environments where nutrient synthesis is optimal. This study investigates the effects of varying light conditions on the growth of red lettuce and its occurring nutrients in terms of productivities, which were determined by multiplying the total weight of the harvested vegetables by their nutrient content, particularly phenolics. Three different light-emitting diode (LED) spectral mixes, including blue, green, and red, which were all supplemented by white, denoted as BW, GW, and RW, respectively, as well as the standard white as the control, were equipped in grow tents with soilless cultivation systems for such purposes.

RESULTS

Results demonstrated that the biomass and fiber content did not differ substantially across treatments. This could be due to the use of a modest amount of broad-spectrum white LEDs, which could help retain the lettuce's core qualities. However, the concentrations of total phenolics and antioxidant capacity in lettuce grown with the BW treatment were the highest (1.3 and 1.4-fold higher than those obtained from the control, respectively), with chlorogenic acid accumulation (8.4 ± 1.5 mg g- 1 DW) being particularly notable. Meanwhile, the study observed a high glutathione reductase (GR) activity in the plant achieved from the RW treatment, which in this study was deemed the poorest treatment in terms of phenolics accumulation.

CONCLUSION

In this study, the BW treatment provided the most efficient mixed light spectrum to stimulate phenolics productivity in red lettuce without a significant detrimental effect on other key properties.

Effects of NaCl and CaCl2 as Eustress Factors on Growth, Yield, and Mineral Composition of Hydroponically Grown Valerianella locusta

Corn salad (Valerianella locusta) is a popular winter salad, cultivated as an ingredient for ready-to-eat salads. The application of mild salinity stress (eustress) can increase the flavor and reduce the nitrate content of certain crops but, at the same time, a wrong choice of the eustress type and dose can negatively affect the overall productivity. In this research, the effects of different isosmotic salt solutions, corresponding to two different electrical conductivity (EC) levels, were investigated on the yield and mineral composition of hydroponically grown Valerianella locusta "Elixir". Five nutrient solutions (NS) were compared, including a basic NS used as the control, and four saline NS were obtained by adding to the basic NS either NaCl or CaCl2 at two rates each, corresponding to two isosmotic salt levels at a low and high EC level. Corn salad proved moderately susceptible to long-term salinity stress, suffering growth losses at both low and high EC levels of saline solution, except from the low NaCl treatment. Hence, it appears that mild salinity stress induced by NaCl could be employed as an eustress solution and corn salad could be cultivated with low-quality irrigation water (20 mM NaCl) in hydroponic systems.

Productive, Morpho-Physiological, and Postharvest Performance of Six Basil Types Grown in a Floating Raft System: A Comparative Study

Basil (Ocimum sp.) is one of the world's most famous culinary fresh herbs, characterized by rapid growth that makes it particularly suitable for hydroponic cultivation. This study aimed to evaluate the adaptability of six types of basil to a closed-loop hydroponic system (floating raft system) and their post-harvest performance. Twenty-three days after transplantation, productivity, morpho-physiological performance, and mineral profile (by ion chromatography) were evaluated. At 3, 6, and 9 days after harvest, the loss of water from the from leaves stored at 10 °C in the dark was evaluated. Although the total fresh production of Thai, Mexican, and Genovese did not differ significantly, the latter provided a higher fresh leaf weight (16.52 g of plant^-1) despite a lower leaf number (30.06 n. of plant^-1). Nine days after harvest, Thai and Mexican showed the lowest water loss. Although Mexican Purple had the lowest net CO₂ assimilation, it accumulated the highest concentration of ascorbic acid (909.41 mg 100 g fw^-1).

Stress induced production of plant secondary metabolites in vegetables: Functional approach for designing next generation super foods

Plant secondary metabolites are vital for human health leading to the gain the access to natural products. The quality of crops is the result of the interaction of different biotic and abiotic factors. Abiotic stresses during plant growth may reduce the crop performance and quality of the produce. However, abiotic stresses can result in numerous physiological, biochemical, and molecular responses in plants, aiming to deal with these conditions. Abiotic stresses are also elicitors of the biosynthesis of plant secondary metabolites in plants which possess plant defense mechanisms as well as human health benefits such as anti-inflammatory, antioxidative properties etc. Plants either synthesize new compounds or alter the concentration of bioactive compounds. Due to increasing attention towards the production of bioactive compounds, the understanding of crop responses to abiotic stresses in relation to the biosynthesis of bioactive compounds is critical. Plants alter their metabolism at the genetic level in response to different abiotic stresses resulting the changes in secondary metabolite production. Transcriptional factors regulate genes responsible for secondary metabolite biosynthesis in several plants under stress conditions. Understanding the signaling pathways involved in the secondary metabolite biosynthesis has become easy with the use of molecular biology. Therefore, aim of writing the review is to focus on secondary metabolite production in vegetable crops, their health benefits and transcription regulation under various abiotic stresses.

Response of Cyanic and Acyanic Lettuce Cultivars to an Increased Proportion of Blue Light

Indoor crop cultivation systems such as vertical farms or plant factories necessitate artificial lighting. Light spectral quality can affect plant growth and metabolism and, consequently, the amount of biomass produced and the value of the produce. Conflicting results on the effects of the light spectrum in different plant species and cultivars make it critical to implement a singular lighting solution. In this study we investigated the response of cyanic and acyanic lettuce cultivars to an increased proportion of blue light. For that, we selected a green and a red leaf lettuce cultivar (i.e., 'Aquino', CVg, and 'Barlach', CVr, respectively). The response of both cultivars to long-term blue-enriched light application compared to a white spectrum was analyzed. Plants were grown for 30 days in a growth chamber with optimal environmental conditions (temperature: 20 °C, relative humidity: 60%, ambient CO₂, photon flux density (PFD) of 260 µmol m^-2 s^-1 over an 18 h photoperiod). At 15 days after sowing (DAS), white spectrum LEDs (WW) were compared to blue-enriched light (WB; λ_Peak = 423 nm) maintaining the same PFD of 260 µmol m^-2 s^-1. At 30 DAS, both lettuce cultivars adapted to the blue light variant, though the adaptive response was specific to the variety. The rosette weight, light use efficiency, and maximum operating efficiency of PSII photochemistry in the light, F_v/F_m', were comparable between the two light treatments. A significant light quality effect was detected on stomatal density and conductance (20% and 17% increase under WB, respectively, in CVg) and on the modified anthocyanin reflectance index (mARI) (40% increase under WB, in CVr). Net photosynthesis response was generally stronger in CVg compared to CVr; e.g., net photosynthetic rate, P_n, at 1000 µmol m^-2 s^-1 PPFD increased from WW to WB by 23% in CVg, compared to 18% in CVr. The results obtained suggest the occurrence of distinct physiological adaptive strategies in green and red pigmented lettuce cultivars to adapt to the higher proportion of blue light environment.

Nitrate Uptake and Use Efficiency: Pros and Cons of Chloride Interference in the Vegetable Crops

Over the past five decades, nitrogen (N) fertilization has been an essential tool for boosting crop productivity in agricultural systems. To avoid N pollution while preserving the crop yields and profit margins for farmers, the scientific community is searching for eco-sustainable strategies aimed at increasing plants' nitrogen use efficiency (NUE). The present article provides a refined definition of the NUE based on the two important physiological factors (N-uptake and N-utilization efficiency). The diverse molecular and physiological mechanisms underlying the processes of N assimilation, translocation, transport, accumulation, and reallocation are revisited and critically discussed. The review concludes by examining the N uptake and NUE in tandem with chloride stress and eustress, the latter being a new approach toward enhancing productivity and functional quality of the horticultural crops, particularly facilitated by soilless cultivation.

Differential Response to NaCl Osmotic Stress in Sequentially Harvested Hydroponic Red and Green Basil and the Role of Calcium

Basil (Ocimum basilicum L.) is a heterogeneous reservoir of bioactive compounds that provide recognized benefits to human health, rendering it a model aromatic herb. Notwithstanding the application of nutritional stress, such as sodium chloride (NaCl) salinity, which mainly affects the primary metabolism, it also triggers adaptive mechanisms that involve the production of bioactive secondary metabolites. Genotype selection and the exogenous application of calcium chloride (CaCl₂) help minimize salinity's suppressive effects on growth. In the present study, we hypothesize that the ratio of different salt types may induce differential responses in the function of preharvest factors in hydroponic basil culture. In this perspective, the stock nutrient solution (Control) was supplemented with 12.5 mm NaCl + 8.33 mm CaCl₂ (Moderate Mix), 25 mm NaCl (Moderate NaCl), 25 mm NaCl + 16.66 of CaCl₂ (High Mix), or 50 mM of NaCl (High NaCl) with the objective of evaluating the different impact of salinity on yield, sensory quality (color and aroma profile), and the accumulation of minerals and bioactive compounds in two successive harvests of green and red basil cultivars. Although more productive (+39.0% fresh weight) than the red one, the green cultivar exhibited higher susceptibility to salinity, especially under the High Mix and High NaCl treatments. The addition of CaCl₂ to the High Mix solution reduced the sodium by 70.4% and increased the total polyphenols by 21.5% compared to the equivalent isomolar solution (High NaCl). The crop performance in terms of fresh and dry yield improved for both cultivars at the second cut. Regardless of cultivar and salt treatment, successive harvests also increased the concentration of phenols and vitamin C (29.7 and 61.5%, respectively) while reducing (-6.9%) eucalyptol, the most abundant aromatic compound in both cultivars. Salinity, as well as the mechanical stress induced by cutting, improved the functional quality of basil. However, the productive responses to the conditions imposed in our work once again highlighted the importance of genetic background. Specifically, CaCl₂ in the Moderate Mix solution preserved fresh leaf weight in the most stress-sensitive green cultivar.

Phytochemical Responses to Salt Stress in Red and Green Baby Leaf Lettuce (Lactuca sativa L.) Varieties Grown in a Floating Hydroponic Module

Lettuce (Lactuca sativa L.) is one of the most popular leafy vegetables, appreciated globally as a low-calorie food with bioactive compounds. The application of a low dose of abiotic stress is considered a sustainable pre-harvest strategy to modify the nutraceutical value of horticultural products. In this work, we explored the response of two differently colored (red or green) baby leaf lettuce varieties to four NaCl concentrations in the nutrient solution (from 1 to 30 mM), using a full factorial design. We focused on leaf morphological parameters and possible phytochemical enhancement of the main polyphenols and anthocyanins, analyzed by LC-MS. The response to low-to-moderate salt stress exposure was affected mainly by salt concentration for leaf traits or by the cultivar for leaf color, with very limited factors’ interactions. Multivariate analysis indicated a predominant role of the genotypic factor in shaping differences in the two weeks growing cycle for baby leaf lettuce. Phytochemically, different dose–response models to sub-optimal saline conditions may be applied to the various compounds. A significant hormetic stimulation was present only for cyanidin-malonyl glucoside, the main anthocyanin present in the red cultivar.

Isosmotic Macrocation Variation Modulates Mineral Efficiency, Morpho-Physiological Traits, and Functional Properties in Hydroponically Grown Lettuce Varieties (Lactuca sativa L.)

The management of mineral elements in agriculture is important for their nutritional role for plants and dietary value for humans, sparking interest in strategies that can increase mineral use efficiency and accumulation in plant food. In this work, we evaluated the effects of the isosmotic variations of the concentration on three macrocations (K, Ca, and Mg) in lettuce (Lactuca sativa L.). Our aim was to improve the nutritional components of this valuable dietary source of minerals. Using a full factorial design, we analyzed mineral utilization efficiency (UtE), leaf morphology, gas exchange parameters, phenolic profiles (through ultra-high performance liquid chromatography coupled to a quadrupole-time-of-flight (UHPLC-QTOF) mass spectrometry), and enzymatic activities in two phytochemically diverse butterhead lettuce varieties (red or green). Plants were fed in hydroponics with three nutrient solutions (NSs) with different ratios of K, Ca, and Mg. The variation of these minerals in the edible product was associated with alterations of the morphology and physiology of the leaves, and of the quality and functional properties of lettuce, with a trade-off between total accumulation and mineral UtE. Moreover, in non-limiting conditions of nutrient availability, significant mineral interactions were also present. The flexibility of the plant response to the different ratios of macrocations, and the observed large intraspecific variation, were adequate to provide mineral-specific phytochemical profiles to the edible product. Specifically, the full-red lettuce provided more interesting results in regard to the compositional and functional attributes of the leaves.

Reduction of Nitrate Content in Baby-Leaf Lettuce and Cichorium endivia Through the Soilless Cultivation System, Electrical Conductivity and Management of Nutrient Solution

Soilless cultivation systems are efficient tools to control nitrates by managing nutrient solution (NS) salinity and nitrogen availability, however, these nitrate-lowering strategies require appropriate calibration based on species/genotype-specific responses interacting with climate and growing conditions. Three experiments were carried out on lettuce and Cichorium endivia grown in ebb-and-flow (EF) and floating (FL) systems at two levels of NS salinity (EC = 2.5 and 3.5 dS m^-1) (EC2.5, EC3.5, respectively) under autumn and early-spring (lettuce) and winter and late-spring conditions (C. endivia). Nitrogen deprivation (NS withdrawal a few days before the harvest) was tested at EC2.5, in the autumn and winter cycles. The EF-system caused an increase in salinity in the substrate where roots mainly develop so it mimicked the effect of the EC3.5 treatment. In the winter-grown lettuce, the EF-system or EC3.5 treatment was effective in reducing the nitrate level without effects on yield, with the EF baby-leaf showing an improved quality (color, dry matter, chlorophylls, carotenoid, vitamin C, phenol). In both seasons, the EF/EC3.5 treatment resulted in a decline in productivity, despite a further reduction in nitrate content and a rise in product quality occurring. This response was strictly linked to the increasing salt-stress loaded by the EC3.5/EF as highlighted by the concurrent Cl^- accumulation. In early-spring, the FL/EC3.5 combination may represent a trade-off between yield, nitrate content and product quality. In contrast, in winter-grown endive/escarole the EC3.5, EF and EC3.5/EF reduced the nitrate level with no effect on yield, product quality or Cl^- uptake, thus proving them to be more salt-tolerant than lettuce. High temperatures during the late-spring cycle promoted nitrate and Cl^- uptake, overcoming the nitrate-controlling effect of salinity charged by the EF system or EC3.5. The nitrate level decreased after 3 day-long (lettuce) or 6 day-long (C. endivia) NS withdrawal. In C. endivia and EF-grown lettuce, it provoked a decrease in yield, but a concurrent improvement in baby-leaf appearance and nutritional quality. More insights are needed to fine-tune the duration of the NS removal taking into account the soilless system used and species-specific characteristics.

Changes in the primary and secondary metabolome of male green asparagus (Asparagus officinalis L.) as modulated by sequential harvesting

Rising temperature and solar radiation drive the mobilization and depletion of crown-deposited metabolites harnessed for asparagus spear regeneration during the spring harvest season. We presently examined how successive same-season harvests impact the sensory, nutritive and bioactive composition of select green asparagus genotypes. Soluble carbohydrates were analyzed by HPLC-RI, organic acids and polyphenols by HPLC-DAD and metals by ion chromatography. Higher sugars and lower acids accentuated sweetness and lower polyphenols contributed to reduced astringency at the beginning of the harvest season. This trend was reversed as the season advanced and spear sensory quality was compromised by declining sugars and rising acids; however, functional quality improved as antioxidant capacity increased along with the concentrations of quercetin-3-O-rutinoside (rutin) and ascorbic acid. The compositional changes presently described were uniform across all genotypes examined and thus contribute toward our understanding of seasonal variation in the sensory and functional quality of this acclaimed health-promoting product.