Introduction

Tomato (Lycopersicon esculentum Mill.) is one of the important vegetables in Mexico, occupying the ninth position in production (SIAP, 2016), so that improper management in crop nutrition can decrease yields, causing physiological, morphological changes and biochemicals that affect negatively, which affect the photosynthetic rate, the root surface due to adsorption problems and therefore a low aerial growth of the plants. Nutrients play an important role in increasing productivity and quality in crops by modifying the environment of plants (^{Briat and Gaymard, 2015}).

Although iron, zinc, copper, molybdenum, boron, manganese and chlorine microelements are essential for the growth and development of plants they are used in smaller quantities than macroelements (^{Soetan et al., 2010}), acting as enzymatic catalysts promoting reactions Chemicals (^{Sathiyamurthy et al., 2017}). Fe, Cu and Zn are important in the production of biomass and quality of vegetables (^{Briat and Gaymard, 2015}), so a deficiency in any of them can change the structure of the chloroplast, decrease photosynthesis and modify the transport of electrons In PSI and PSII (^{Eberhard et al., 2008}), Zn is an enzymatic activator and produces changes in plant metabolism including impaired carbohydrate, protein, auxin synthesis and damage to membrane integrity (^{Kyrkby and Römheld, 2008}).

For its part, Cu plays an important role in photosynthesis, respiration, detoxification of superoxide radicals and lignification (^{Perea et al., 2010}), also participates in various physiological processes, such as carbohydrate distribution, reduction and N fixation, protein metabolism, among others (^{Lin and Jin, 2018}). Fe participates in physiological processes such as chlorophyll biosynthesis, respiration and redox reactions (^{Rui et al., 2016}). The objective of the present work was to determine the supplementary effect in different concentrations of the Fe, Cu and Zn microelements on the agronomic yield of the tomato crop.

Materials and methods

Results and discussion

For the quality of fruit in the variable’s firmness, pH and vitamin C no statistical differences were observed (p≤ 0.05); however, for vitamin C the treatment with low levels of Zn exceeded the control with 27.39%, in total soluble solids (SST) if there were statistical differences. High levels of Fe were 16.45% higher compared to the control, while Fe deficiency was 5.62%.

The results of this work were different from those obtained by ^{Valentinuzzi et al. (2015)} who, when subjecting the strawberry crop to a deficiency of Fe, the Brix degrees and firmness of the fruits were not altered, the high doses of Zn in SST exceeded the control 15%, these results coincide with those obtained by ^{López et al. (2018)} who, when applying 10 µmol L^-1 of this microelement, found a higher concentration of SST and attributed it to the fact that Zn is indispensable in the enzymatic activity in photosynthesis, carbohydrate metabolism and sugar transport to demand points.

The three microelements evaluated did not show significant statistical differences in firmness, pH and vitamin C, similar results were found by ^{Alvárez et al. (2003)} who demonstrated that in Fe deficiency there were no differences in the variables of vitamin C, acidity and SST in Prunus persica cultivate Babygol. The SST variable showed highly significant differences (p≤ 0.05), where treatments with 550 g of Fe showed the highest values of Brix degrees with 5.38 (Table 5).

The leaf area (AF) of the tomato crop in both doses of Fe were significantly affected, with a higher leaf area in the high doses, exceeding the control in 138%, presenting an average leaf area of 2 917 cm², this was due to the low concentration of Fe caused a decrease in chlorophyll content and other components of the chloroplasts which decreased the growth capacity of the crop, similar results were found by ^{Incesu et al. (2015)} in the cultivation of Tangerine; however, in the different doses of Zn and Cu they were statistically similar to the control (p≤ 0.05)

For doses of Zn, the control showed a greater foliar area followed by high concentrations, contrasting data were reported by ^{Haleema and Hussain (2018)} in tomato culture who showed that the higher the dose of Zn the greater was the leaf area. In Cu it was observed that the high doses exceeded the control 63% (Figure 1), these results contradict what was mentioned by ^{Yruela (2005)}, who mentions that plants with excess Cu regularly show a decrease in biomass.

Figure 1 Effect of different doses of Fe, Zn and Cu on the leaf area (AF) in tomato cultivation. The different literals on the standard error bar, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

The foliar area ratio (RAF) between the treatments was statistically different in the Fe and Zn doses, the high iron doses exceeded the control 112% (Figure 2), indicating that an excess of Fe generates a reduction in growth and this it will depend on the concentrations in the solution, on the other hand the high doses of Zn were higher 50%; however, other results indicate that an excess of Zn causes a decrease of dry matter in the plant (^{Casierra et al., 2010}), causing a reduction in biomass production (^{Rizwan et al., 2017}), while the doses of Cu were statistically similar, ^{Yruela (2005)}, points out that excess concentrations of Cu inhibits growth, causing alterations in physiological processes such as photosynthesis and respiration, generating a reduction in crop biomass.

Figure 2 Effect of different doses of Fe, Zn and Cu on the leaf area ratio (RAF) in tomato cultivation. The different literals on the standard error bars, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

For the specific leaf area (AFE), the high doses of Fe and Zn exceeded the control with 44% and 73%, respectively, while in the doses of Cu there were no significant differences (Figure 3), from high doses to means (control) for Fe and Zn there was a decrease in AFE. According to ^{Santos et al. (2010)} the decrease may be due to morphological changes that determine the formation of small and thick leaves, meanwhile ^{Lizarazo et al. (2013)} found that their treatments with Zn deficiency and excess were not significant differences, and they mention that in plants with Zn deficiency or excess their leaves are thin. On the other hand, ^{Ghasemian et al. (2010)} mention that with the application of 50 kg h^-1 Fe increased significantly the number of pods, pod weight and biomass production in the Glycine max crop.

Figure 3 Effect of different doses of Fe, Zn and Cu on the specific leaf area (AFE) in tomato cultivation. Different literals on the standard error bars, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

In the leaf weight ratio (RPF) all treatments were statistically similar (Figure 4), in this same variable he found that at the beginning of the crop cycle in Pisum sativum there were no differences; however, at the end of the cycle there was a difference where the doses of 2 mg L^-1 were higher.

Figure 4 Effect of different doses of Fe, Zn and Cu on the leaf weight ratio (RPF) in tomato cultivation. Different literals on the standard error bars, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

The variable leaf area index (IAF) treatment with 550 kg ha^-1 of Fe (high dose) was 101% higher than the control, so it can be shown that high doses of Fe took advantage of the radiation present. According to ^{Barraza et al. (2004)} the increase in the IAF is physiologically manifested in a higher rate of photo translocation assimilated towards the demand points. There were no statistical differences in the doses of Zn and Cu (Figure 5).

Figure 5 Effect of different doses of Fe, Zn and Cu on the leaf area index (IAF) in tomato cultivation. Different literals on the standard error bars, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

The crop growth rate (TCC) (Figure 6) that indicates the productive efficiency of the biomass per unit of surface area occupied (soil) over time (^{Santos et al., 2010}) in the present work were statistically similar, ^{Pingoliya et al. (2014)} mention that the application of 2 kg ha^-1 of ferrous sulfate increased the biomass, production and yield of Cicer arietinum.

Figure 6 Effect of different doses of Fe, Zn and Cu on the growth rate of the crop (TCC) in tomato cultivation. Different literals on the standard error bars, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

The yield with the Fe and Zn doses did not show statistical differences, the Fe in the 450 g doses has reported a higher yield with 28 t ha^-1 compared to the control (Figure 7), different results were presented in the Vigna radiata culture where doses of 5 kg ha^-1 of Fe contributed to maximum yield (^{Jamal et al., 2018}).

Figure 7 Effect of different doses of Fe, Zn and Cu on tomato crop yield. Different literals on the standard error bar, between levels of the same element, indicate significant difference according to the LSD test (p≤ 0.05). 

The doses of Cu there were statistical differences, where the high doses were superior to the control 36%, different values when applying Fe, Cu and Zn in L. esculentum observed that the best yields were found with the doses of Zn followed by Cu exceeding to the control 20% and 4% respectively, on the other hand ^{Patil et al. (2008)} when applying Fe, Cu and Zn at 100 ppm, they found that Fe provided the best yields with 18.25 t ha^-1, followed by Zn with 17.57 t ha^-1 and finally Cu with 15.58 t ha^-1 in L. esculentum. In this work, the lowest yields were found with the low doses of Zn.

In the Table 6 shows the values of the foliar area, RAF, AFE, RPF, IAF and performance variables. High doses of Fe and Cu showed the highest foliar areas with respect to the control in 137% and 63%, respectively, similar values were reported by ^{El-Fouly et al. (2001)} who demonstrated that at concentrations of 33.6 ppm of Fe (considered high doses) the leaf area was greater in Helianthus annus.

Low doses of Cu and high doses of Fierro increased RAF by 91%. Similar results were reported by ^{Vinit-Dunand et al. (2002)} who point out that the application of Cu at low concentrations provided a greater ratio of leaf area compared to high doses (10 mg g^-1 substrate) in Cucumis sativus.

In the specific leaf area, high doses of Zn and low doses of Cu exceeded the control with 73 and 49%, respectively, ^{Rouphael et al. (2008)}, when applying Cu, they found that at a higher concentration, less specific leaf area, with doses of 0.3, 47 and 94 µM, observed an AFE of 273.2, 256.8 and 238.3 cm² g^-1, respectively, in Cucumis sativus.

In the RPF variable, high doses of Fe exceeded the control 33%, while in the IAF the high doses of Fe and low doses of Cu exceeded the control by 101 and 32%, respectively, ^{Caliskan et al. (2008)} showed that the application of 0, 200 and 400 g ha^-1 of Fe at the beginning of flowering in Glycine max the IAF was not significant, however, the effects were notable in the full pod stage, and the values of IAF increased significantly with high doses of Fe (400 g ha^-1).

In performance it was superior with high doses of Cu, followed by low doses of Fe, which exceeded the control with 5 t, different results were found by ^{Kumar et al. (2009)} when applying different doses of Cu in Triticum aestivum, the highest yield was in the doses of 1.5 mg kg^-1 of soil surpassing the control by 62.9% while, that the high doses presented lower yield than the control.

Conclusions

High doses of the microelements increased the variables of tomato fruit quality, especially total soluble solids, firmness and pH, although without statistical differences, on the other hand, low doses of Zn increased vitamin C; however, the values of AF, RAF, AFE, IAF and performance decreased. High doses of Fe improved the AF, IAF and PRF variables. Doses of high Cu favored performance and TCC. The deficiency of the Fe, Cu and Zn microelements limited the leaf area, while high doses improved the IAF and yield.