Introduction

Selenium is an essential element for humans but qualified as non-essential for plants. However, the presence of selenium in the substrate or applied by foliar spraying, plants accumulate it in their tissues, working as primary source of this element in the diet (^{Broadley et al., 2006}). In most soils worldwide selenium concentration is low, from 0.01 to 2 mg kg^-1 with an average of 0.4 mg kg^-1, but can be found in soils called seleníferous with concentrations of up to 1200 mg kg^-1 of It (^{Fordyce, 2005}). It is considered that selenium is essential in humans for its role as cofactor of enzymes related with antioxidant metabolism (^{Rayman, 2008}). In plants selenium has a positive effect on the antioxidant capacity, acting more effectively as selenite than selenate (^{Cartes et al., 2005}). In Mexico studies on the relationship between selenium and nutritional quality of food in terms of its antioxidant capacity are few and therefore the aim of this study was to document the effect of selenium applications on lettuce, checking plant growth, mineral composition of leaves and antioxidant capacity of foliar cell extracts. This under the assumption that the application of this element will modify the cellular redox balance enhancing the antioxidant capacity of plant tissues.

Materials and methods

This research was carried out in the Universidad Autonoma Agraria Antonio Narro (UAAAN) in Saltillo, Coahuila, Mexico, under greenhouse conditions, with an average temperature and relative humidity of 17.8 °C and 68% respectively. Lettuce seeds (Lactuca sativa) variety Great Lakes were used, these were placed in germinating trays 200 cavities, using as substrate peat moss and perlite for germination. After 40 days after sowing, seedlings were transplanted in 20 L polyethylene pots using as substrate a mixture of peat moss and perlite with 70:30 V:V. Treatments consisted of foliar application of selenium in concentrations of 0, 5 and 10 mg L^-1 at 15, 30 and 45 days after transplantation. Sodium selenite (Na₂SeO₃, Sigma Aldrich, USA) was used as selenium source without mixing with adjutants.

Leaves from three plants at physiological maturity per treatment at 20 and 35 days after transplanting (DAT) were collected, these were ground, obtaining a fresh leaf extract where the redox potential (ORP) was determined ( mV) using a potentiometer HI98185-01 (HANNA, Inc., USA) using the technique described by ^{Benavides-Mendoza et al. (2002)}. Catalase activity (EC 1.11.1.6) was also quantified in leaves with physiological maturity at 35 days after transplanting (DAT) following ^{Ramos et al. (2010)} technique.

A UV-Vis Biomate 5 (Thermo Electron, USA) spectrophotometer at 590 nm was used and the results were expressed in mM consumption of H₂O₂ min^-1 total^-1 protein (mg/g). Fresh weight of aerial parts and root was determined at 50 DAT taking three full plants per treatment. These were carefully washed and root was separated from stem and leaves at crown height. Weights were measured with an Adventurer Pro (OHAUS, Inc., USA) scale. Then the same samples were placed in a Robertshawa dehydrator oven at 60 °C for 72 h to weight them on an analytical scale Pioneer (OHAUS, Inc., USA). The dried samples were determined mineral concentration in leaves.

For total nitrogen the micro Kjelhdal (^{AOAC, 1980a}) method was used, while for phosphorus used a spectrophotometric (^{AOAC, 1980b}) method, K, Ca, Mg, Na, Fe, Mn, Cu and Zn were measured through an atomic absorption spectrophotometer Varian AA-1275 following ^{Fick et al. (1976)} technique. To determine selenium first digesting of 0.5 g of dry sample in concentrated nitric acid until all the organic part using a spectrophotometer of plasma atomic emission IRIS ADVANTAGE (Thermo Jarrell, USA). The statistical design was randomized complete block with three replications. Data analysis consisted of ANOVA and Tukey tests and was carried out with SAS version 6.0 (^{SAS Institute, 2001}).

Results and discussion

Foliar application of selenium has a substantial effect on redox potential (ORP) (Figure 1). ORP values indicate the antioxidant capacity, i.e. the ability of the system under analysis to donate electrons compared to hydrogen electrode (^{Benavides et al., 2002}); as lower the value of ORP greater the capability to donate electrons and work as an antioxidant.

Figure 1 Average and standard deviation of oxide reduction potential (ORP mV) in fresh lettuce seedlings extract to which was applied selenium as sodium selenite. Bars with different literal difference indicated by Tukey (p≤ 0.05). 

This result is possibly related to an increased antioxidant enzyme activity, which is known to increase in presence of certain selenium concentrations (^{Freeman et al., 2010}). This seems to be confirmed by the results obtained with leaf catalase activity (Figure 2) showing positive and significant trend by applying selenium to plants. These results are similar to those obtained by ^{Lingan et al. (2005)} who reported a positive effect of selenium on catalase activity.

Figure 2 Mean and standard deviation of catalase activity in lettuce seedlings to which was applied selenium as sodium selenite. Bars with different literal difference indicated by Tukey (p≤ 0.05). 

It is likely that the effect of selenium on the antioxidant potential is not associated with a higher photosynthetic activity and perhaps this explains the lack of differences in fresh and dry weight of root and leaves on lettuce plants in this study (Table 1). ^{Becvort et al. (2012)} obtained similar results applying selenium to tomato plants, obtaining a significant increase in antioxidant status but without differences in fresh and dried weight of fruits and root between treatments.

Table 1 Effect of foliar application of selenium on the fresh and dry weights of lettuce plants. 

^¥Promedios seguidos de distintas literales son estadísticamente diferentes según Tukey (P≤0.05).

As for mineral content in lettuce plants (Table 2) found differences only in nitrogen and phosphorus content by applying selenium. ^{Nowak et al. (2002)} found an increase in nitrogen content with the application of selenite in wheat plants. The data obtained regarding P match with ^{Wu and Huang (2004)}, who found an increase of this element in clover plants when low amounts of selenite was applied to the substrate with 0, 10, 20 and 30 mg kg^-1. Regarding the concentration of selenium it was found greater amount in treatment of 10 mg kg^-1, ^{Kápolna et al. (2009)} increasing radical selenium concentration from 0.045 to 2.0 μg g^-1 dry weight by applying 100 mg L^-1 of selenium by foliar spraying. These data is consistent with ^{Smrkolj et al. (2006)} who indicate the need to perform repeated spraying if the objective is accumulation of selenium.

Table 2 Effect of foliar application of selenium on mineral concentration in lettuce leaves. 

TRT=Tratamiento. ^¥Promedios seguidos de distintas literales son estadísticamente diferentes según Tukey (P≤0.05). ^£Se consideró un valor de cero en la concentración de Se cuando no fue detectado.

Conclusions

The application of selenium as foliar induced a higher antioxidant status in lettuce plants, increasing the concentration of N and P in leaves without changing the concentration of other elements or biomass.

Foliar application of selenium may be used as a tool to improve the nutritional quality of lettuce in terms of antioxidants.