Oxidative stress and antioxidants in tomato (Solanum lycopersicum) plants subjected to boron toxicity

Abstract

Background and aims: Boron (B) toxicity triggers the formation of reactive oxygen species in plant tissues. However, there is still a lack of knowledge as to how B toxicity affects the plant antioxidant defence system. It has been suggested that ascorbate could be important against B stress, although existing information is limited in this respect. The objective of this study was to analyse how ascorbate and some other components of the antioxidant network respond to B toxicity.

Methods: Two tomato (Solanum lycopersicum) cultivars ('Kosaco' and 'Josefina') were subjected to 0.05 (control), 0.5 and 2 mm B. The following were studied in leaves: dry weight; relative leaf growth rate; total and free B; H(2)O(2); malondialdehyde; ascorbate; glutathione; sugars; total non-enzymatic antioxidant activity, and the activity of superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, ascorbate oxidase and l-galactose dehydrogenase.

Key results: The B-toxicity treatments diminished growth and boosted the amount of B, malondialdehyde and H(2)O(2) in the leaves of the two cultivars, these trends being more pronounced in 'Josefina' than in 'Kosaco'. B toxicity increased ascorbate concentration in both cultivars and increased glutathione only in 'Kosaco'. Activities of antioxidant- and ascorbate-metabolizing enzymes were also induced.

Conclusions: High B concentration in the culture medium provokes oxidative damage in tomato leaves and induces a general increase in antioxidant enzyme activity. In particular, B toxicity increased ascorbate pool size. It also increased the activity of l-galactose dehydrogenase, an enzyme involved in ascorbate biosynthesis, and the activity of enzymes of the Halliwell-Asada cycle. This work therefore provides a starting point towards a better understanding of the role of ascorbate in the plant response against B stress.

Fig. 1.

Effect of 0·05 mm B (control) and B toxicity (0·5 and 2 mm) on (A) MDA and (B) H₂O₂ concentration in leaves of two tomato cultivars: ‘Kosaco’ and ‘Josefina’. Bars represent means ± s.e. (n = 9); for each cultivar, bars followed by the same letters are not significantly different using Duncan's test (P = 0·05).

Fig. 2.

Effect of 0·05 mm B (control) and B toxicity (0·5 and 2 mm) on (A) SOD, (B) CAT and (C) APX activity in leaves of two tomato cultivars: ‘Kosaco’ and ‘Josefina’. Bars represent means ± s.e. (n = 9); for each cultivar, bars followed by the same letters are not significantly different using Duncan's test (P = 0·05).

Fig. 3.

Effect of 0·05 mm B (control) and B toxicity (0·5 and 2 mm) on (A) MDHAR, (B) DHAR and (C) GR activity in leaves of two tomato cultivars: ‘Kosaco’ and ‘Josefina’. Bars represent means ± s.e. (n = 9); for each cultivar, bars followed by the same letters are not significantly different using Duncan's test (P = 0·05).

Fig. 4.

Effect of 0·05 mm B (control) and B toxicity (0·5 and 2 mm) on l-GalDH activity in leaves of two tomato cultivars: ‘Kosaco’ and ‘Josefina’. Bars represent means ± s.e. (n = 9); for each cultivar, bars followed by the same letters are not significantly different using Duncan's test (P = 0·05).