Selenium supply affects chlorophyll concentration and biomass production of maize (Zea mays L.)

Maize (Zea mays L.) is an important field crop; based on area and production, it ranks on the second place among the field crops in Kosovo. Chlorophyll is naturally present in plants as a photosynthetic pigment, giving their specific coloration (Withnall et al., 2003). It is one of the most important physiological parameters, which is closely related to plant photosynthesis and growth (Wang et al., 2008; Czyczyło-Mysza et al., 2013). The molecules possess a basic skeleton structure of porphyrin with a magnesium ion in the center and a long phytol group in the tail. The major chlorophylls in plants include chlorophyll a (Chl-a) and chlorophyll b (Chl-b) (Garousi et al., 2015a). Chlorophylls are essential tetrapyrroles responsible for harvesting solar energy, charge separation, and electron transport in photosynthesis. They mainly capture light in the antenna complex via photosystem II, with subsequent electron transport (Taiz and Zeiger, 2009). They differ slightly only in the composition of a side chain (in Chl-a, it is –CH₃, and in Chl-b, it is –CHO). Leaf chlorophyll content is highly correlated with the nutritional condition and as an indicator for survival and growth of plants (Gitelson, 2003).

Selenium (Se) exists in very small amounts in humans, animals, plants, and microorganisms. Although it has an importance as microelement in small amounts, toxicity occurs at high concentrations because of the replacement of sulfur with selenium in amino acids, resulting in incorrect folding of the proteins and, consequently, nonfunctional proteins and enzymes (Gul et al., 2017). Se is essential to many organisms, including some archaea, bacteria, protozoans, green algae, and nearly all animals. In plants, Se can be found in both inorganic and organic binding forms, including selenoamino acids and methylated compounds. Se is an example of an essential element becoming limiting in food commodities because of intensive plant production. Consequently, controlling the Se uptake and metabolism in plants will be important for biofortification of food and feedstuff. The availability of Se for plants depends on soil properties, including pH, salinity, and the content of CaCO₃ (Kabata and Pendias, 2001).

Sager (2002) reported that in Europe, Se occurrence in soils, crops, and groundwater is rather low, but it may be enriched from fertilization with organic amendments or selenium-containing mineral fertilizers. In soils, Se is a naturally occurring trace element that typically ranges from 0.01 to 2 mg kg⁻¹ (Hoewyk, 2013). For agriculturally used soils in eastern Austria, Se values of 0.2 mg kg⁻¹ have been reported (Aichberger and Hofer, 1989).

Most plants contain rather low foliar Se, around 25 μg kg⁻¹ and rarely exceeding 100 μg kg⁻¹. However, some plants (Astragalus spp, Stanleya albescens, and some woody asters) exhibit a great capability to accumulate Se, and they may concentrate Se to extremely high levels above 1,000 mg kg⁻¹ that may be toxic to humans and animals (Garousi et al., 2015b). Edelbauer and Eder (2001) reported from a longterm grassland experiment that Se application with either sewage sludge or mineral fertilizer increased the Se contents in the uppermost soil layer but did reduce the Se of the grass. Furthermore, foliar Se application increases stepwise the Se contents in plant components of oilseed rape in the following order: leaves > stems > roots > siliques ~ seeds) (Száková et al., 2017). Despite substantial literature on Se uptake by plants and crops such as wheat, little consideration has been given to maize (Z. mays L.), a low “Se indicator” plant but one of the world’s most widely grown cereals. To date, there have been few publications on Se uptake and assimilation in this plant (Longchamp et al., 2011). Exogenous Se at low concentration can reduce the intensity of peroxide influence on membrane lipids, affect the activity of redox enzymes, and thereby change the oxidation–reduction status of the cell, increasing stress tolerance (Vikhreva et al., 2002). A positive influence of Se on changes in the activity and permeability of the cellular membrane may be one of the impacts of Se on plants (Filek et al., 2008). Most species of crops, however, do not appear to require Se for their growth, and, in general, these plants have a low tolerance for this element (Terry et al., 2000). Plants subjected to higher Se stress exhibit different physiological changes, including stunted root growth, reduced biomass, chlorosis, reduced photosynthetic efficiency, and ultimately plant death. Spallholz and Hoffman (2002) suggested that uptake of high concentration of Se may cause symptoms of injury including stunting of growth, chlorosis, withering and drying of leaves, decreased protein synthesis, and premature death of plants. The form in which Se appears in soils and its availability for plants are determined by many physicochemical factors such as soil pH, oxidoreduction potential, contents of humus, clay minerals, Fe oxides and other elements, microbiological activity, and also the nature and character of the absorbing surfaces (Placzek and Patorczyk, 2014). The aim of this study was to investigate the response of maize seedlings to Se supply at different concentration in biomass accumulation, chlorophyll and carotenoids content.

The influence of Se on plants largely depends on its chemical form and its concentration in nutrient solution (Combs, 2001). A stimulating effect with a Se dose of 1.30 mg kg⁻¹ on plant growth has been observed, whereas with a Se concentration of 6.57 mg kg⁻¹, the maize growth (both shoots and roots) was impaired compared to the control and further decreased with higher Se concentrations (Table 1). The biomass decreases with Se doses of ≥6.57 mg kg⁻¹ was stronger for shoots than for roots. With the highest Se dose, the biomass of shoots was lower by 80.3% (hybrid) or 84.4% (LP) and of roots by 60.0% (hybrid) or 50.7% (LP), respectively, compared to the control. Also Hartikainen et al. (2000) have shown that Se effects on plants depend on the concentrations; with lower doses, Se stimulated the growth of ryegrass seedlings, whereas with higher doses, it acted as pro-oxidant, reducing yields and inducing metabolic disturbances. The shoot-to-root ratio was highest in the control for both genotypes and decreased with higher Se doses reaching the lowest values with the highest Se dose. Even with a dose of 1.30 mg kg⁻¹, which enhanced both shoot and root biomass, the shoot-to-root ratio was lower than that of the control. Contrary to that, aqueous above-ground biomass extracts of catch crops stronger impaired root than shoot growth of maize seedling and thus increased the shoot-to-root ratio (Chovancová et al., 2015).

The chlorophyll (Chl-a and Chl-b) contents were affected by Se application in both maize genotypes (Table 2). There was a clear trend of Chl-a reduction with increasing Se concentration. Se application of 26 mg kg⁻¹ reduced Chl-a by 67.8% (hybrid) or 54.6% (LP), respectively, compared to the control. Contrary to that, Chl-b increased with applications of Se up to 6.57 mg kg⁻¹ before the values decreased again with higher Se application rates. With 6.57 mg Se kg⁻¹, Chl-b was 15.1-fold (hybrid) or 1.6-fold (LP), respectively, higher compared to the control. Lowest Chl-b was observed in the control (hybrid) or with 26 mg Se kg⁻¹ (LP). The Chl-b contents in the hybrid were strongly affected by Se application compared to those in the LP. This might be because the LP has large genetic variation to adapt to environmental stress tolerance, as a result of natural selection. Total chlorophyll (a and b) was highest with 1.30 mg Se kg⁻¹, lowest with the highest Se dose, and second lowest with the control.

Excessive Se concentrations not only reduced the physiological activities (Nowak et al., 2004) but also reduced the chlorophyll content (Nawaz et al., 2013). Rani et al. (2005) reported that the critical Se concentration in plant tissues, above which the yield in maize decreased, was 77 μg g⁻¹ DW. Also Nashmin et al. (2015) showed an effect of Se on Chl-a and Chl-b contents with the highest Se addition causing an increase in the Chl-a and Chl-b contents at all growth stages compared to the control. Nawaz et al. (2016) observed that the exposure to drought stress in plant applied with Se significantly reduced leaf photosynthetic pigments such as Chl-a, Chl-b, and carotenoids contents by 75%, 60%, and 71% respectively, compared to the control. Garousi (2016) showed that Chl-a and Chl-b were not impaired after 3 weeks of Se exposure up to 3 mg l⁻¹ from Se^IV or Se^VI, although reductions in the efficiency of the PSII photochemistry (This parameter measures the proportion of light absorbed from photo system II( PSII ) that is used in photochemistry ) occurred in the Se^VI treatment (but not in the Se^IV treatment).

The carotenoid content showed a relatively wide range across the Se treatments (Table 2). For the hybrid, the highest content of carotenoids was observed in the control and the lowest with the highest Se application (where it was lower by about 90% compared with the control). The overall contents were lower in the LP than in the hybrid. In the LP, carotenoids increased with the first Se dose compared to the control and then strongly decreased with higher doses. With the highest Se dose, the carotenoids were lower by 99% compared to the control. Similarly, Manion et al. (2014) also reported from solution culture with watercress that with increasing Se doses, the carotenoid contents decreased linearly.

For both genotypes, Chl-a concentration was positively correlated with carotenoids, shoot biomass, and shoot-to-root ratio, but for the hybrid, it also correlated with root biomass. There was no correlation of Chl-a with total chlorophyll (a and b). Chl-b was among all parameters only positively correlated with total chlorophyll (a and b) (in both genotypes). Carotenoids were (next to Chl-a) positively correlated with root and shoot biomass and shoot-to-root ratio; for the hybrid, it also positively correlated with total chlorophyll (a and b), but for both genotypes, it does not correlated with Chl-b. Total chlorophyll (a and b) was (next to Chl-b) positively correlated with root and shoot biomass; for the LP, it was also positively correlated with the shoot-to-root ratio. The root biomass was (next to already mentioned parameters) positively correlated with the shoot biomass and also with the shoot-to-root ratio for the LP. Shoot biomass was positively correlated to shoot-to-root ratios for the hybrid and the LP (Table 3).

Our study provided some evidence that higher concentrations of Se in the plant growth medium may reduce the chlorophyll content including Chl-a and Chl-b. These green pigments play a key role in photosynthesis and their content in crop leaves is of great importance for nutritional state diagnosis and yield formation. A Se concentration of 1.30 mg kg⁻¹ even stimulated plant growth, whereas concentrations of 13 and 26 mg kg⁻¹ dramatically reduced plant biomass, also affecting the root system. Positive correlations were observed between Chl-a and carotenoids with shoot (for both genotypes) and root (for one genotypes) biomass; whereas no correlation was observed between Chl-b and biomass. We can conclude that low amounts of Se application are favorable for both plant growth and chlorophyll and carotenoids contents, whereas high amounts of Se application negatively affect both.