Photosynthetic activity, pigment composition and antioxidative response of two mustard (Brassica juncea) cultivars differing in photosynthetic capacity subjected to cadmium stress
Introduction
Cadmium (Cd) can be accumulated to higher levels in the aerial organs (Pence et al., 2000), preferentially in the chloroplasts and disturbs the chloroplast function by inhibiting the activities of enzymes of chlorophyll biosynthesis and CO2 fixation (Böddi et al., 1995; Krupa and Baszynski, 1995; Siedlecka et al., 1997) or the aggregation of pigment protein complexes of the photosystems (Horvath et al., 1996). The Cd-induced formation of active oxygen species (AOS), superoxide anion (O2·−), hydroxyl (OH−) radicals and H2O2 result in the damage of chloroplast. The presence of H2O2 in the chloroplasts restricts Calvin-cycle enzymes reducing carbon assimilation (Takeda et al., 1995). It induces changes in the functions of membranes by initiating peroxidation of polyunsaturated fatty acids (De Vos et al., 1993), oxidative damage by formation of oxygen free radicals or by the reduction in the status of enzymatic and non-enzymatic antioxidants (Shaw, 1995; Somashekaraiah et al., 1992).
Plants appear to possess a wide array of defense strategies to protect the photosynthetic apparatus and cellular membranes from AOS (Foyer and Harbinson, 1994). Production of antioxidative enzymes is one part of the defense system that plants require to protect against stress. Superoxide dismutase (SOD; EC 1.15.1.1) constitutes the primary step of cellular defense. It dismutates O2·− to H2O2 and O2. Further, the accumulation of H2O2 is restricted through the action of catalase (CAT; EC 1.11.1.6) or by the ascorbate–glutathione cycle, where ascorbate peroxidase (APX; EC 1.11.1.11) reduces it to H2O. Finally, glutathione reductase (GR; EC 1.6.4.2) catalyzes the NADPH-dependent reduction of oxidized GSSG to the reduced GSH (Noctor et al., 2002).
Mustard (Brassica juncea L. Czern & Coss) is recognized as an accumulator of heavy metals. It is, therefore, postulated that the mustard cultivars with diverse photosynthetic capacity detoxifies the AOS and protects the chloroplast functions differently from oxidative damage. In the present investigation, two mustard cultivars, Varuna and RH-30 (Khan, 2004), were used to study carbonic anhydrase (CA), ribulose-1,5-bisphosphate carboxylase (Rubisco), net photosynthetic rate (PN), stomatal conductance (gS), transpiration rate (E) and contents of chlorophyll (Chl), pheophytin and relative amount of anthocyanin, associated changes in the contents of H2O2, thiobarbituric acid reactive substances (TBARS), electrolyte leakage and the capacities of antioxidative enzymes SOD, APX, CAT and GR under Cd stress.
Section snippets
Plant material and growth conditions
The seeds of mustard (B. juncea L. Czern & Coss) cultivars Varuna (high photosynthetic capacity) and RH-30 (low photosynthetic capacity) were surface sterilized with 0.5% NaOCl for 20 min, rinsed and soaked overnight in sterile water for 12 h at 4 °C for uniform germination. The seeds were transferred to 23-cm-diameter earthen pots filled with 5 kg of reconstituted soil (sand:clay:peat; 70:20:10, by dry weight) in the green house of the Botany Department, Aligarh Muslim University, Aligarh, India,
Results
The Cd concentration in roots and leaves was greater in RH-30 than Varuna at all Cd treatments (Fig. 1A and B).
Significant reductions were found in photosynthetic parameters with all Cd treatments in both the cultivars (Table 1). PN was 3.5%, 30.9% and 35.5% less in Varuna and 4.7%, 35.0% and 50.0% less in RH-30 with 25, 50 and 100 mg Cd kg−1 soil, respectively, compared to the control. gS and E were significantly enhanced at 50 and 100 mg Cd kg−1 soil in RH-30, but in Varuna the changes in gS and E
Discussion
Plant species and genotypes significantly differ in the uptake of Cd and its subsequent translocation from roots into shoots (Metwally et al., 2005; Salt et al., 1995). In our study, Varuna accumulated less Cd in both roots and leaves than RH-30 (Fig. 1A and B). The accumulation of Cd in roots and shoots depends on binding to extracellular matrix (Horst, 1995), complexing inside the cell (Cobbett et al., 1998) and on the transport efficiency (Marchiol et al., 1996). Further, the transport
References (53)
- et al.
Molecular mechanism for relaxation of and protection from light stress
- et al.
A reappraisal of the use of DMSO for the extraction and determination of chl a and b in lichens and higher plants
Environ Exp Bot
(1992) A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein–dye binding
Anal Biochem
(1976)- et al.
Cadmium and zinc induction of lipid peroxidation and effects on antioxidant enzyme activities in bean (Phaseolus vulgaris L.)
Plant Sci
(1997) - et al.
Effect of heavy metal ion excess on sunflower leaves: evidence for involvement of oxidative stress
Plant Sci
(1996) - et al.
Dehydration damage to cellular membranes and heat-shock proteins in maize hybrids from different climates
J Plant Physiol
(1996) - et al.
Changes in the antioxidant enzymes efficacy in two high yielding genotypes of mulberry (Morus alba L.) under NaCl salinity
Plant Sci
(2001) - et al.
Effect of cadmium on organization and photoreduction of protochlorophyllide in dark-grown leaves and etioplast inner membrane preparations of wheat
Photosynthetica
(1995) - et al.
Manganese superoxide dismutase can reduce cellular damage mediated by oxygen radicals in transgenic plants
EMBO J
(1991) - et al.
Effect of aluminium on lipid peroxidation, superoxide dismutase, catalase and peroxidase activities in root tips of soybean (Glycine max)
Physiol Plant
(1991)