Effect of instantaneous light intensity after magnesium suppression in tomato and bell pepper cultivation

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Introduction
Tomato (Solanum lycopersicum L.) and bell pepper (Capsicum annuum L.) crops require special attention when it comes to their nutrition.Deficiency of certain elements can have a detrimental impact on the initial stages of plant development.Among these elements, magnesium (Mg 2+ ) is particularly important as it plays a crucial role in various developmental processes.It is an integral part of the chlorophyll molecule, facilitating carbon dioxide (CO 2 ) assimilation, and serves as a cofactor for enzymes.Additionally, it forms a structural component of the ribosome (Bakshi and Gilroy, 2022).
Furthermore, magnesium is essential for chlorophyll synthesis and acts as an activator of the carboxylation of ribulose 1,5 bisphosphate (RuBP) by forming a complex with the carbamate group and RuBP activase.When magnesium is deficient, plants can exhibit symptoms of chlorosis, characterized by yellowing of the leaves between the veins, and a decrease in yield due to reduced carbohydrate transport from source to sink tissues (Faiz et al., 2021;Lisboa et al., 2021).
In field cultivation, plants are exposed to various climatic variations, with light intensity and quality being particularly influential on instantaneous photosynthesis, germination, and flowering induction.To optimize growing conditions, controlled environments are necessary, allowing for precise control over light quality and intensity through artificial means.Light-emitting diodes (LEDs) have emerged as a valuable tool in this regard, offering the ability to regulate both light intensity and spectrum while significantly reducing energy consumption.The precise control of LED spectra can also promote the accumulation of important plant metabolites (Tarakanov et al., 2022).
Photosynthesis is a crucial process for plant growth, where light energy is converted into chemical energy.Plants capture CO 2 from the environment and release oxygen into the atmosphere during this process.Understanding the light saturation points in vegetables is essential to maximize production yields.Light detection in plants involves specialized molecules called photoreceptors, consisting of a protein linked to a pigment called chromophore, which absorbs photons.Morphologically, light restriction can impact plant growth from the early stages, causing etiolation and making seedlings more fragile during transport and planting (Kochetova et al., 2022).
Conversely, excess light can also damage plant structures, affecting photosynthesis and carbon dioxide uptake.Inadequate ventilation and exhaust can lead to a CO 2 deficiency in crops, hampering development and maturation.Therefore, maintaining a balanced light environment is crucial to minimize damage (Lazar et al., 2022).
Overall, light intensity and relative air humidity have both positive and negative effects on plant rooting and nutrient production.Research results have suggested that achieving a balance between light intensity and relative humidity is beneficial for both rooting and overall plant development (Ding et al., 2022).In light of the above, this study aimed to assess the effect of instantaneous light intensity following magnesium suppression on the cultivation of tomatoes and bell peppers.

Material and Methods
Two separate experiments were conducted in September 2022 using the Mariana tomato variety (Sakata®) and the Magali R pepper variety (Sakata®).These experiments took place at the Faculty of Agricultural and Technological Sciences, situated in the municipality of Dracena, São Paulo State, Brazil.The experimental design was completely randomized (CRD) in a 2x5 double factorial scheme.The first factor consisted of the presence or absence of magnesium in the nutrient solution, interacting with four levels of light intensity: 0 (control), 600, 1200, and 1800 μmol m -2 s -1 of photosynthetically active radiation (PAR).
After 30 days of cultivation in the nutrient solution, tomato and pepper plants were subjected to five levels of instantaneous light intensity, as described earlier, provided by light-emitting diode (LED) lamps.A portable gas exchange device, specifically the Infra-Red Gas Analyzer (IRGA) by ADC BioScientific Ltd (model LC-Pro), was employed to determine various parameters, including the rate of CO 2 assimilation (Aμmol CO 2 m -2 s -1 ), transpiration (Emmol H 2 O m -2 s -1 ), stomatal conductance (gsmol H 2 O m -2 s -1 ), and internal concentration of CO 2 in the substomatal chamber (ci -μmol mol -1 ), with a constant CO 2 level of 380 ppm.To calculate water use efficiency (WUE), the following formula was applied: Statistical analysis involved a series of steps.Firstly, normality tests using the Shapiro-Wilk test were conducted to ensure the data met the required assumptions.After confirming normality, an analysis of variance (ANOVA) was performed using the F test (p<0.05)for the nutrient factors.The means of these factors were compared using Tukey's test at a 5% level of significance, following the method of Banzatto and Kronka (2013).
For the light intensity factor, regression analysis was employed, evaluating linear, quadratic, and cubic models.The significance and coefficient of determination (R²) were considered in model selection.Additionally, a Pearson correlation analysis was conducted.Furthermore, a principal component analysis (PCA) was carried out, following the procedures outlined in Galindo et al., (2022), utilizing the statistical software R (R Core Team, 2015).

Results and Discussion
In the tomato crop, there was an interaction between the factors for the rate of CO 2 assimilation (A).When cultivated with all nutrients and at a maximum light intensity of 1,303.83μmol m -2 s -1 of photosynthetically active radiation (PAR), a positive linear response increased light intensity was observed (Table 1).It is important to note that the suppression of magnesium led to a reduction of approximately 59.27% in comparison to plants grown with all nutrients.
Similarly, the A of pepper plants showed a negative response to magnesium restriction, which was approximately 72% lower than in plants receiving all nutrients.Both nutritional conditions, with all nutrients and in the absence of magnesium, exhibited quadratic responses, with maximum points at 1,272.00 and 1,180.00μmol m -2 s -1 of PAR, respectively (Table 2).
Table 1.Instantaneous mean values and regressions of the CO 2 assimilation rate (A -μmol CO 2 m -2 s -1 ); transpiration (Emmol H 2 O m -2 s -1 ); stomatal conductance (gsmol H 2 O m -2 s -1 ), internal CO 2 concentration in the substomatal chamber (ci -μmol mol -1 ) and water use efficiency (WUE) of tomatoes grown with all nutrients and with the suppression of magnesium interacting with light intensities.

Nutrients (N)
A A significant negative Pearson correlation was observed between the A and the CO 2 concentration in the substomatal chamber (ci), indicating that as the ci increases, the rate of gas assimilation decreases.However, there was a positive correlation between the A and water use efficiency (WUE) (Figure 1).
When a plant is cultivated with magnesium restriction, the A is compromised due to the lower concentration of magnesium in the plant's internal tissues.This affects the growth of the aerial part of the plant (Pessoa et al., 2022).The critical range of magnesium in the dry weight of the leaf falls between 0.1% and 0.2% in various crops, including wheat, potato, rice, corn, sorghum, and barley.Although net CO 2 assimilation may be higher, it does not translate into increased plant biomass.This implies that the efficiency of converting carbon gas into sugars is compromised (Hauer-Jákli & Tränkner, 2019;Lazar et al., 2022).

Figure 1. Significant Pearson correlations between gas exchange parameters of tomatoes (T) and peppers (P) grown with all nutrients (All) and with magnesium suppression (-Mg)
Light intensity is also a factor that influences carbon dioxide assimilation.In vitro studies have shown that changes in carbon fixation physiology can alter leaf anatomy, resulting in smaller leaves with reduced leaf blade thickness (Calazans Júnior et al., 2022).These effects can be exacerbated by magnesium restriction (Lisboa et al., 2021).As a result, instantaneous and CO 2 -saturated photosynthesis, as well as the maximum rates of ribulose 1,5 bisphosphate (RuBP) and electron transport, are compromised when plants are grown with nutrient restrictions, such as potassium and magnesium deficiencies (Faiz et al., 2021).Additionally, chlorophyll fluorescence and electron transport decrease in both nutrient deficiencies, while dark respiration increases, indicating that plants require more energy to maintain their metabolic functions (Rogiers et al., 2020).
Studies by Faizan et al. (2022) suggested that foliar application of magnesium oxide nanoparticles (MgO-NPs) under conditions of arsenic stress in soybean plants can result in a 17% increase in height and a 15% increase in dry weight.This is attributed to the positive response of magnesium, which improved the net photosynthetic rate by 12.9%, stomatal conductance (g s ) by 13.4%, intercellular CO 2 concentration by 15.3%, and transpiration by 14.7%.Additionally, the efficiency of photosystem II (PSII) improved, leading to a more efficient response to oxidative stress with reduced hydrogen peroxide (H 2 O 2 ) and lipid peroxidation in leaves.These findings support the key role of magnesium in photosynthetic parameters and its ability to enhance resistance to oxidative stress.
Instantaneous transpiration (E) was significantly reduced in tomato plants when magnesium was restricted, resulting in a decrease of approximately 56.44%.The minimum point for transpiration occurred at 954.88 μmol m -2 s -1 of PAR.In the absence of magnesium, transpiration decreased by approximately 67.55% compared to plants grown with all nutrients.On the other hand, the transpiration rate of pepper plants grown with all nutrients exhibited a quadratic response, with a maximum point at 1,050 μmol m -2 s -1 of PAR (Table 2).Magnesium restriction affected gas exchange parameters in both vegetable species (Figure 2).
Lack of light also negatively affected gas exchange parameters, as shown in Figure 3.There was no significant difference in g s with varying light intensities in tomato plants when grown under magnesium restriction.In contrast, tomato plants with all nutrients exhibited a positive quadratic response, with a maximum light saturation point at 789.47 μmol m -2 s -1 of PAR, which was approximately 67.55% higher than under magnesium restriction (Table 1).In the case of pepper plants grown under magnesium restriction, there was a statistically significant difference in light intensity, and a quadratic response was observed with a maximum point at 666.66 μmol m -2 s -1 of PAR.The absence of magnesium resulted in a reduction of approximately 76.10% in g s (Table 2).
Both E and g s can be influenced by the nutrient concentrations within plant organs.In cucumbers, higher environmental CO 2 levels can promote root growth, but this increased root mass may not compensate for the reduced absorption rates of other nutrients such as nitrogen, potassium, calcium, and magnesium.Even when cultivated under low temperatures, the mass flow of nutrients can negatively affect the rate of transpiration (Li et al., 2023).Magnesium restriction led to higher ci in the tomato substomatal chamber, resulting in a 9.85% increase.The response was linear when magnesium was restricted, whereas, with all nutrients, a quadratic response was observed with a maximum light saturation point at 1,473.00 m -2 s -1 of PAR (Table 1).In the case of pepper plants, both with all nutrients and under magnesium restriction, a quadratic response was observed, with maximum light saturation points at 1,347.00 and 1,282.00m -2 s -1 of PAR, respectively (Table 2).
Furthermore, balancing the partial supply of nitrate (NO 3-) can have a significant positive impact on various physiological parameters.It can notably enhance the net photosynthetic rate, transpiration rate, g s , and ci in plants, particularly when they are facing water restrictions.Consequently, the provision of nitrogen can act as a mitigating factor, alleviating the adverse effects stemming from magnesium deficiency (Deng et al., 2023).Due to magnesium restriction, stomatal function may become compromised, leading to the closure of stomatal pores.This, in turn, results in a higher concentration of internal CO 2 .Such a scenario could impede efficient gas exchange and compromise the rate of CO 2 fixation, as evidenced in Tables 1 and  2.
Moreover, Modarelli et al. (2022) demonstrated that increasing light intensity in lettuce cultivation can elicit a more favorable response in liquid photosynthesis.This ensures a more consistent electron transport rate and, additionally, stimulates the synthesis of anthocyanins and carotenoids.These findings have positive implications for plant protection, especially when plants are exposed to highintensity light stress.Once again, tomatoes exhibited a quadratic response to WUE when grown with all nutrients and under magnesium restriction.The points of maximum light saturation were 1,229.81and 1727.27m -2 s -1 of PAR (Table 1).Similarly, pepper plants also demonstrated a quadratic response in terms of WUE.This was observed in plants grown with all nutrients and under magnesium restriction, with maximum light saturation points of 1,317.00 and 1,202.00m -2 s -1 of PAR, respectively, as indicated in Table 2.
The stomatal apparatus can undergo alterations due to nutritional factors and even variations in light intensity, resulting in changes in stomatal size and density.Consequently, both the size and quantity of stomata can exhibit rapid responses when exposed to sudden changes in light levels.This phenomenon implies that plants with faster stomatal opening also experience quicker induction of photosynthesis, which leads to a greater accumulation of biomass.
However, it may also result in lower specific WUE when subjected to varying light intensities (Xiong et al., 2022).The ability to manipulate stomatal regulation, including adjustments in stomatal size and density, can serve as a viable strategy to enhance crop yield.Additionally, it may help mitigate some of the current challenges associated with climate change in agricultural environments (Sinha et al., 2022).

Conclusions
Magnesium serves as a key limiting factor in the gas exchange responses of tomato and pepper crops.A light intensity of 1200 µmol m -2 s -1 of photosynthetically active radiation promotes the most favorable gas exchange outcomes in both tomato and pepper cultures.

Figure 2 .
Figure 2. Biplot graph of the principal component analysis (PCA) of the inference of contribution of the crop with all nutrients and with magnesium restriction in crop the tomato and bell pepper.

Figure 3 .
Figure 3. Biplot graph of principal component analysis (PCA) of the inference of contribution of instantaneous light intensities in tomato and pepper crops.
Instantaneous mean values and regressions of the CO 2 assimilation rate (A -μmol CO 2 m -2 s -1 ); transpiration (Emmol H 2 O m -2 s -1 ); stomatal conductance (gsmol H 2 O m -2 s -1 ), internal CO 2 concentration in the substomatal chamber (ci -μmol mol -1 ) and water use efficiency (WUE) of peppers grown with all nutrients and with the suppression of magnesium interacting with light intensities.
GA: General average; CV: Coefficient of variation; MSD: Minimum Significant Difference.** -significant at the 1% probability level (p <0.01); * -significant at the 5% probability level (0.01=<p<0.05).Means followed by the same letter do not differ statistically.The Tukey test was applied at the level of 5% probability of the event occurring; (-Mg): magnesium suppression Effect of instantaneous light intensity after magnesium suppression in tomato and bell pepper cultivation.Revista de Agricultura Neotropical, Cassilândia-MS, v. 11, n. 1, e8330, Jan./Mar., 2024.Tabela 2: GA: General average; CV: Coefficient of variation; MSD: Minimum Significant Difference.** -significant at the 1% probability level (p <0.01); * -significant at the 5% probability level (0.01=<p<0.05).Means followed by the same letter do not differ statistically.The Tukey test was applied at the level of 5% probability of the event occurring; (-Mg): magnesium suppression.