Plant Growth-Promoting Rhizobacteria as Tools to Improve the Growth of Kohlrabi (Brassica oleracea var. gongylodes) Plants in an Aquaponics System.

The use of nitrogen as a fertilizer can be highly risky when used excessively, and it is therefore necessary to find novel techniques to reduce its use. Aquaponics reduces the use of synthetic fertilizers and water, and the leaching of nitrate into the environment. One way to avoid problems due to a reduction in nitrogen availability could be the use of plant growth promoting rhizobacteria (PGPR). This study examines the effect of PGPR on kohlrabi plants grown with a traditional nutrient solution (100S), in combination with "fish water" (50F/50D), or with a supplement of synthetic fertilizers (50F/50D + S). Two formulations were used: T1 (Azospirillum brasilense and Pantoea dispersa) and T2 (Azotobacter salinestris). Irrigation with 50F/50D caused a reduction in several of the measured parameters. The combined application of 50F/50D with T1 attenuated the negative effects. T2 did not present significant effects on the parameters measured. The results obtained with 50F/50D + S hardly showed differences with the 100S. Thus, by irrigating with 50F/50D + S, we were able to maintain the yields while reducing fertilizer use and water. The combined use of T1 and 50F/50D was also positive; however, it would be necessary to continue adjusting the amount of nitrate supplied to maintain production.

Plant growth-promoting bacteria as affected by N availability as a suitable strategy to enhance the nutritional composition of lamb's lettuce affected by global warming.

Heat and nutritional stresses have a significantly effect on the accumulation of bioactive and other compounds harmful to human health, like nitrates, in green leafy vegetables like lamb's lettuce. Plant growth-promoting bacteria (PGPB) have shown to confer beneficial biochemical changes to various crops under different stresses. The hypothesis proposed here is that the combination of optimal N level (2.5 Mm, 12 mM or 20 mM of N) with the inoculation of PGPB in plants exposed to heat shock (43 °C) may be a good strategy to obtain healthier lamb's lettuce with a higher yield. Results showed that a dose of 20 mM N can be considered as overfertilization. Moreover, the inoculation of plants fed with fertilizers with reduced N and under heat stress, resulted in higher productivity and content of sugars (60 %), amino acids (94 %), nitrogen (21 %), and total phenolic compounds (30 %), and a reduced content of nitrates (27 %).

Assessing optimal nitrate/ ammonium- ratios in baby-leaf lettuce to enhance the heat stress tolerance under elevated CO2 conditions.

In recent years, the interest on baby-leaf lettuce has grown steadily, because it is richer in bioactive compounds than other traditional vegetables. However, the quality of lettuce is being increasingly affected by climate change. It is very rare for a climatic effect to occur in isolation. Even then, a large body of work has only focused on the effect of isolated heat stress, fertilization, and elevated CO2, on morphological, physiological and biochemical parameters. Thus, very few works have focused on how the combination of several of these factors can affect these parameters. For first time, the present work studied the combined effect derived from the application of two different levels of CO2 (400 and 1000 ppm of CO2), four different NO3-/ NH4+ ratios (100/0 (T-I), 100/0 before the short-term heat stress and finally without NO3- (T-II), 80/20 (T-III) and 50/50 (T-IV)), and a short-term heat stress (25 and 43°C), on some physiological and quality parameters (dry biomass, photosynthetic parameters, pigments content, lipid peroxidation and total soluble proteins content) of baby-leaf lettuce cv Derbi. Additionally, a comparison of that combined effect of all these parameters between inner and outer leaves was also performed. The results obtained indicated that the interaction between the nutrient solution containing a 50/50 NO3-/ NH4+ ratio and a high concentration of CO2 (1000 ppm) improved the biomass, photosynthesis, intercellular/external CO2 concentration ratio (ci/ca), stomatal conductance (gs), evapotranspiration (E) and lipid peroxidation, and protein content in this baby-leaf lettuce. The results obtained in this work lead us to conclude that this existing interaction between the NO3-/ NH4+ ratio and the elevated CO2 concentration may be considered as a new strategy for making baby-leaf lettuce more resistant to heat stress, in other words, stronger against the ever more frequent heat waves.

Heat-shock and methyl-jasmonate: The cultivar-specific responses of pepper plants.

Frequency, intensity and duration heat-related events have profound implications for future food supply through effects on plant growth and development. This concern needs effective and urgent mitigation tools. However, the effectiveness of potential solutions may decrease according to the specific cultivar response rather consider at specie level. The metyl-jasmonates are essential cellular regulators which are involved in pivotal plant development processes and related to confer protection to heat shock. Thus, our aim was to study the response of three pepper cultivars, Agio (Hungarian type), Basque (Chilli type), and Loreto (Lamuyo type), subjected to heat shock (40°C/72 h) and foliarly-sprayed with methyl-jasmonate (MeJA; 100 µmol), and the effects on several physiological traits. Our results show that despite the important differential impact of heat shock caused on each cultivar, MeJA application did not affect gas exchange, chlorophyll A concentration or efficiency of the photosystem in these cultivars. However, P concentration was reduced when MeJA was applied to Basque chilli, and a significant effect on leaf carbohydrates concentration was observed for Agio and Loreto. Moreover, Agio was the only cultivar in which the amino-acid profile was affected by MeJA under heat shock. Under that condition, putrescine increased for all cultivars, whist the effect of MeJA was only observed for spermine and histamine for Agio and Loreto. Thus, the results indicated that the ameliorative impact of MeJA on this stressor was clearly influenced by cultivar, revealing specific traits. Thus, these results could be used as valuable tools for the characterization of this intraspecific tolerance to heat shock during the vegetative growth stage of pepper.

Enhancement of Bioactive Constituents in Fresh Cauliflower By-Products in Challenging Climate Conditions.

In order to mitigate the detrimental impact that climate change is having on plants, the study of new practices that allow for the reduction of such effects has become imperative. In addition, the revaluation of the promotion of healthy plant by-products has also markedly increased in importance in recent years. In this work, the modifications in biomass and some antioxidant compounds of cauliflower by-products treated with putrescine under extreme temperatures in two different CO2 scenarios (the control (400 ppm) and a high concentration of CO2 (1000 ppm)) were studied. Additionally, the compositions of inner and outer leaves were also compared. According to results found in this work, cauliflower grown under elevated CO2 and treated with putrescine (2.5 mM) prior to heat stress showed the highest biomass accumulation (20%) compared to the control. Moreover, in the outer leaves from cauliflower grown under elevated CO2 and treated with putrescine prior to high temperature exposure, the highest biosynthesis of sugars (20%) was recorded. Although cauliflower by-products turned out to be rich in polyamines (208.6 nmoles g-1 fresh weight (FW) and 124.3 nmoles g-1 FW for outer and inner leaves, respectively) and phenolic compounds (1070.2 mg gallic acid equivalents ( (GAE) 100 g-1 FW in outer leaves and 772.0 mg GAE 100 g-1 FW in inner leaves), it was the outer leaves that after applying the new strategy showed the greatest increase in polyamines (68%) and phenolic compounds (39%), obtaining here the highest increase in antioxidant activity (3%). Thus, they should no longer be regarded as mere by-products and should be used for pharmaceutical or nutraceutical purposes. The novel strategy presented in this work may allow us to take advantage of both the continued increase in CO2 and heat waves that are becoming more frequent.

Effects triggered by foliar selenium application on growth, enzyme activities, mineral nutrients and carbohydrates in lettuce under an aquaculture system.

Se has beneficial effects on plants, through the stimulation of plant productivity, the reduction of abiotic stresses, and the improvement in N metabolism. Therefore, we investigated the effect of the foliar application of different concentrations of Se (0, 4, 8 and 16 µmol L-1) on lettuce plants grown in an aquaponics system (fish water) compared with a control (conventional soilless fertigation). The NO3- concentration supplied by the fish water was 47% of the control solution. The results showed a reduction in the fresh weight of lettuce plants irrigated with the fish water mixture treatment, along with an increase in sugar concentration. However, the application of Se at 4 and 16 µmol L-1, prompted a relief of this stress, reducing both lipid peroxidation and the sugar content, and increasing the nitrate concentration. In addition, in the case of the highest concentration of Se (16 µmol L-1), the values of nitrate were comparable those control plants. We show the importance of sprayed Se in increasing the efficiency of N utilization, in addition to lessening environmental issues for aquaponics culture.

Unraveling the nutritional and bioactive constituents in baby-leaf lettuce for challenging climate conditions.

The isolated effects of heat stress, fertilization and elevated CO2 on the content of several health-promoting compounds in plants have been quite studied. However, few studies have focused on two of these three factors together. This work provides information on how two different levels of CO2, four different NO3-/NH4+ ratios in the nutrient solution, and a short-term heat stress affect the biomass and nutritional quality of baby-leaf lettuce cv Derbi. Furthermore, the nutritional quality of the inner and outer leaves was also studied and compared. Results indicated that the strategy used led to a bigger and healthier baby-leaf lettuces. So, this lettuces contained a higher content of sugars, minerals and phenolic compounds and showed an enhanced antioxidant activity. On the other hand, results exhibited that whilst in inner leaves the biosynthesis of antioxidant compounds were favored, in outer leaves was favored the biosynthesis of sugars and mineral content.

Nitrogen management under increased atmospheric CO2 concentration in cucumber (Cucumis sativus L.): ameliorating environmental impacts of fertilization.

In the last years, the atmospheric CO2 concentration has increased significantly, and this increase can cause changes in various physiological and biochemical processes of plants. However, the response of plants to elevated CO2 concentration (e[CO2]) will be different depending on the nitrogen form available and the plant species. Therefore, hydroponic trials on cucumber plants, with two CO2 concentrations (400 and 1000 ppm) and two nitrogen sources (NO3-/NH4+; 100/0 and 90/10), were conducted. Physiological parameters-such as gas exchange, GS, GOGAT and GDH activities, cation composition, soluble sugar and starch content- were measured. The results showed that when plants were grown with NH4+ and e[CO2], parameters such as photosynthesis rate (ACO2), instantaneous water use efficiency (WUEi), the content of NH4+, Ca2+ and Mg2+, and the concentration of starch, were higher than in control plants (irrigated with nitrate as sole nitrogen source and ambient CO2 concentration (a[CO2])). Furthermore, an improvement in N assimilation was observed when the GS/GOGAT pathway was enhanced under these conditions (NH4+ and e[CO2]). Thus, our results contribute to the reduction of the negative environmental impacts of the use of nitrogen fertilizers on this crop, both by reducing nitrogen leakage (eutrophication) and greenhouse gas emissions.

The Effect of Foliar Putrescine Application, Ammonium Exposure, and Heat Stress on Antioxidant Compounds in Cauliflower Waste.

This work has been focused on the study of how we can affect the short heat stress on the bioactive compounds content. Some recent investigations have observed that management of nitrogen fertilization can alleviate short-term heat effects on plants. Additionally, the short-term heat stress can be also ameliorated by using putrescine, a polyamine, due to its crucial role in the adaptation of plants to heat stress Therefore, different NO3-/NH4+ ratios and a foliar putrescine treatment have been used in order to increase tolerance to thermal stress in order to take advantage of the more frequent and intense heat waves and make this crop more sustainable. So, other objective of this work is to make the cauliflower waste more attractive for nutraceutical and pharmaceutical preparations. Thus, the effect of a thermal stress combined with a 50:50 NO3-/NH4+ ratio in the nutrient solution, and the foliar application of 2.5 mM putrescine increased in the content of various sugars (inositol, glucose, and fructose), total phenolic compounds and polyamines, as well as in the antioxidant activity. The greatest accumulation of these compounds was observed in young leaves. Our results show from a physiological and agronomic point of view, that the foliar application of putrescine and the 50:50 NO3-/NH4+ treatment managed to alleviate the negative effects of the abiotic stress suffered at high temperature, yielding plants with higher antioxidant compounds content.

Effects of Different Nitrogen Forms and Exogenous Application of Putrescine on Heat Stress of Cauliflower: Photosynthetic Gas Exchange, Mineral Concentration and Lipid Peroxidation.

This study examines the effect of the exogenous application of polyamine putrescine together with the application of different ratios of nitrate/ammonium (NO3 -/NH4 +), on the physiology of cauliflower subjected to heat stress. The 50:50 NO3 -/NH4 + ratio was the best ratio against heat stress. As a result of the joint application of these compounds, a higher photosynthetic rate, a higher accumulation of both photosynthesis-related compounds and pigments, total proteins, and a change in the status of nutrients were obtained. Particularly, the decrease in content of calcium, chloride and sulphate in plants under heat stress is ameliorated by the ammonium effect. Additionally, it is important to highlight that cauliflower waste contains a higher content of mineral nutrients than floret cauliflower. These effects were more marked in young leaves. Furthermore, a synergistic effect for coping with heat stress between the polyamine and the nutritional treatment was observed. For this, both the application of putrescine and the feeding of plants with a 50:50 NO3 -/NH4 + ratio before heat stress is proposed for the first time as an agricultural practice for increasing the thermotolerance of cauliflower cv Moonshine. On the other hand, due to the lower lipid peroxidation rate obtained in cauliflower leaves, these plants could be used for health purposes as ointments or other nutraceutical products, making the cultivation of this kind of cruciferous more sustainable.

Foliar application of putrescine before a short-term heat stress improves the quality of melon fruits (Cucumis melo L.).

BACKGROUND: Climate change has caused an increase in the frequency and intensity of heatwaves, worldwide, which subject plants to thermal stress for short periods; this can affect the quality of melon fruits, both negatively and positively. Since the application of putrescine has been shown to help increase tolerance of abiotic stresses, the objective of this work is to determine the effects of the foliar application of putrescine (1.5 and 5 mmol L-1 ) before a short heat stress (HS) on the quality of melon fruits. RESULTS: The results indicate that HS had a positive effect on the quality of melon fruits, since it increased the total sugars and polyamines contents and the antioxidant capacity, and reduced the presence of substances undesirable in foods such as nitrate. However, the fruit quality was further increased by the combination of HS and putrescine (5 mmol L-1 ). In this case, the melon fruits showed increases in their antioxidant capacity and contents of polyamines, amino acids and minerals beneficial to health. The nitrate concentration was even lower than in the control fruits. CONCLUSION: This novel study highlights the possibility of improving the nutritional quality of melon pulp by applying foliar putrescine in combination with a short period of high temperature. © 2020 Society of Chemical Industry.

Exogenous Salicylic Acid Modulates the Response to Combined Salinity-Temperature Stress in Pepper Plants (Capsicum annuum L. var. Tamarin).

Growers in the cultivated areas where the climate change threatens the agricultural productivity and livelihoods are aware that the current constraints for good quality water are being worsened by heatwaves. We studied the combination of salinity (60 mM NaCl) and heat shock stress (43 °C) in pepper plants (Capsicum annuum L. var. Tamarin) since this can affect physiological and biochemical processes distinctly when compared to separate effects. Moreover, the exogenous application of 0.5 mM salicylic acid (SA) was studied to determine its impacts and the SA-mediated processes that confer tolerance of the combined or stand-alone stresses. Plant growth, leaf Cl- and NO3- concentrations, carbohydrates, and polyamines were analyzed. Our results show that both salinity stress (SS) and heat stress (HS) reduced plant fresh weight, and SA only increased it for HS, with no effect for the combined stress (CS). While SA increased the concentration of Cl- for SS or CS, it had no effect on NO3-. The carbohydrates concentrations were, in general, increased by HS, and were decreased by CS, and for glucose and fructose, by SA. Additionally, when CS was imposed, SA significantly increased the spermine and spermidine concentrations. Thus, SA did not always alleviate the CS and the plant response to CS cannot be directly attributed to the full or partial sum of the individual responses to each stress.

Merging Heat Stress Tolerance and Health-Promoting Properties: The Effects of Exogenous Arginine in Cauliflower (Brassica oleracea var. botrytis L.).

In the last decades, cauliflower consumption has increased due to its observed beneficial effects on human health, especially on chronic diseases. Furthermore, the use of arginine has been shown to improve the heat stress tolerance of plants by increasing the polyamine content. Thus, we aimed to investigate the effects of the exogenous application of arginine on the physical and chemical quality parameters of cauliflower florets under heat stress. For this, we applied two concentrations of arginine (1 and 4 mM) to the leaves of cauliflower (Brassica oleracea var. botrytis L.) plants grown in three different temperatures (ambient, elevated, and extreme). Our data show that potassium and phosphate, as well as iron were the most abundant macro- and micronutrients, respectively. The combination of high temperature and exogenous application of arginine increased the antioxidant activity, total content of phenolic compounds, polyamines, and proteins. The data presented herein indicate that the combination of an adequate heat stress and the appropriate foliar arginine treatment may be a useful strategy that could be used to increase the number of valuable plant compounds in our diet.

Differential Effects of Aquaponic Production System on Melon (Cucumis melo L.) Fruit Quality.

We investigated the effect on melon fruits of "fish water" alone or in combination with a supplement of synthetic fertilizers in a nutrient solution or foliar application of Ca(NO3)2. These treatments were compared with a traditional soilless system with synthetic fertilizers and no reuse of the nutrient solution. The results show that the treatments with recirculation of fish water and with the foliar supplement yielded fruits of greater weight and size but with reduced lightness and lower concentrations of proteins, NO3-, K+, and total amino acids. The supply of synthetic nutrients to the roots or leaves caused a reduction in the sugar concentrations and the antioxidant activity of these fruits. The use of fish water (alone or with an amendment) increased spermine and putrescine with respect to the traditional soilless crop management. The results for these bioactive compounds in melons should be considered for maintenance of health with age.

The Form in Which Nitrogen Is Supplied Affects the Polyamines, Amino Acids, and Mineral Composition of Sweet Pepper Fruit under an Elevated CO2 Concentration.

We investigated the effect of supplying nitrogen, as NO3- or as NO3-/NH4+, on the composition of fruits of sweet pepper (Capsicum annuum L. cv. Melchor) plants grown with different CO2 concentrations ([CO2]): ambient or elevated (800 µmol mol-1). The results show that the application of NH4+ and high [CO2] affected the chroma related to the concentrations of chlorophylls. The concentrations of Ca, Cu, Mg, P, and Zn were significantly reduced in the fruits of plants nourished with NH4+, the loss of Fe being more dramatic at increased [CO2], which was also the case with the protein concentration. The concentration of total phenolics was increased by NH4+, being unaffected by [CO2]. Globally, the NH4+ was the main factor that affected fruit free amino acid concentrations. Polyamines were affected differently: putrescine was increased by elevated [CO2], while the response of cadaverine depended on the form of N supplied.