Table 1.

Exposure to excess metals has consequences for the biosynthesis of AsA and the antioxidant properties expressed as (1) the amount of reduced and oxidized ascorbate, i.e., AsA and DHA and (2) the AsA-GSH cycle activities. The effects of excess Cd, Al, As, Pb, Cu, Zn, Ni and Mn are shown along with the experimental upset and the use of plant material.

	Plant	Condition	Biosynthesis/content of total AsA	Antioxidant		Ref.
				Ratios	AsA-GSH cycle
NON-ESSENTIAL ELEMENTS
Cadmium (Cd)	Arabidopsis thaliana	0, 5, 10 μM CdSO4 24 h	↑ Total AsA (roots)		↑ APX, GR (leaves)	[26]
	Arabidopsis thaliana	0, 5, 10 μM CdSO4 24 h	↑ Total AsA (leaves)	↑ Reduced AsA (leaves) ↓ DHA/AsA (leaves)		[45]
	Arabidopsis thaliana	10 μM CdSO4 24 h	↑ Total AsA (roots)	↑ Reduced AsA (roots) ↓ DHA/AsA (n.s.)	↑ APX, GR (leaves)	[17]
	Phaseolus vulgaris (leaves)	2 μM CdSO4 0, 24, 48, 72 h	↑ Total AsA	Long term (>48 h) ↑ DHA Long term (>48 h) ↑ DHA/AsA	↑ APX, GR	[46]
	Phaseolus vulgaris	0, 100 μM CdCl2 4 days	↑ mRNA levels of GMP, GME, GalDH, GalLDH			[43]
	Pinus sylvestris	0, 5, 50 μM CdSO4 6, 12, 24, 48, 96 h	Transient ↑ total AsA followed by ↓ (50 μM Cd)	Transient ↑of reduced AsA followed by ↓ (>24 h)	↑ MDHAR Ttransient ↓ APX followed by ↑ APX (>24 h)	[47]
	Ceratophyllum demersum	10 μM CdCl2 1 week	↓ Total AsA	↓ Reduced AsA DHA > reduced AsA ↑ DHA/AsA	↑ APX (2x) ↓ MDHAR, DHAR	[48]
	Brassica juncea (shoots)	0, 50, 200 μM Cd(NO3)2 7 days	↓ Total AsA ( <controls) (200 μM Cd)	↑ Reduced AsA (50 μM Cd) ↓ Reduced AsA ( <controls) (200 μM Cd)		[49]
Cadmium (Cd)	Brassica juncea (roots)	0, 50, 200 μM Cd(NO3)2 7 days	↑ Total AsA (50 μM Cd) ↓ Total AsA ( >controls) (200 μM Cd)	↑ Reduced AsA (50 μM Cd) ↓ Reduced AsA ( >controls) (200 μM Cd)		[49]
	Bechmeria nivea Gaud	0, 1, 3, 7 mg/L Cd 1, 2, 3, 7, 10 days		↑ Reduced AsA followed by ↓ at high Cd concentration ↑ DHA ↑ DHA/AsA	↑ APX, GR	[50]
	Shorea robusta	1 mg/L, 5mg/L, 10 mg/L CdCl2 4 months		↑ AsA		[51]
Aluminum (Al)	Oryza Sativa	0, 80, 160 μM Al2(SO4)3 0, 5, 10, 15, 20 days	↓ Total AsA	↓ Reduced AsA ↑ DHA/AsA	↑ APX, MDHAR, DHAR, GR	[52]
	Nicotiana tabacum SR-1 wildtype	0, 400 μM AlCl3 24 h	↑ Total AsA	↑ DHA ↑ DHA/AsA	↑ APX	[53]
	Nicotiana tabacum overexpressing Arabidopsis thaliana cytosolic DHAR (DHAR-OX)	0, 400 μM AlCl3 24 h	↑ Total AsA	↑ Reduced AsA ↓ DHA ↓ DHA/AsA	↑ APX	[53]
Arsenic (As)	Shorea robusta	1 mg/L, 5mg/L, 10 mg/L As2O3 4 months		↑ AsA		[51]
Lead (Pb)	Shorea robusta	1 mg/L, 5mg/L, 10 mg/L Pb(C2H3O2)2·3H2O 4 months		↑ AsA		[51]
ESSENTIAL MICRONUTRIENTS
Copper (Cu)	Arabidopsis thaliana	2, 5 μM CuSO4 24 h	↓ Total AsA (roots) ↑ Total AsA (leaves)	↑ DHA/AsA (roots)	↑ APX (roots) ↑ GR (leaves)	[26]
	Arabidopsis thaliana	10 μM CuSO4 24 h	↓ Total AsA (roots) ↑ Total AsA (leaves)	↓ Reduced AsA, DHA (roots) ↑ Reduced AsA (leaves)	↓ APX (roots)	[17]
	Arabidopsis thaliana	0, 5, 25, 50, 100 μM CuSO4 1, 3, 7 days		Short term ↑ reduced AsA (1, 3 days) Short term ↑ DHA (3 days)	Short term ↑ DHAR Long term ↑ GR, MDHAR	[54]
	Phaseolus vulgaris (roots)	15 μM CuSO4 1, 5, 24, 48, 72, 96, 120, 168 h	↑ Total AsA	↑ Reduced AsA ↑ DHA ↑ DHA/AsA	Short term ↑ DHAR Long term slightly ↓ DHAR ↑ GR Long term ↑ MDHAR, APX	[55]
	Phaseolus vulgaris (leaves)	50 μM CuSO4 0, 24, 48, 72, 96, 120, 144, 168 h	↑ Total AsA	↑ Reduced AsA Short term ↓ DHA followed by transient ↑ DHA Long term ↓ DHA ↓ DHA/AsA	↑ MDHAR, GR Long term ↑ APX, DHAR	[56]
	Oryza Sativa (MSE-9)	0, 10, 50, 100 μM CuSO4 1, 5 days	↑ Total AsA		↑ APX Long term ↑ GR (>50 μM Cu)	[57]
	Morus alba L. cv. Kanva-2	0.0, 0.1, 1.0, 100 μM CuSO4 25, 50 days	↑ Total AsA	↑ DHA (Cu-deficient,-excess) ↑ DHA/AsA (Cu-deficient,-excess)	↑ APX, GR	[58]
	Hordeum vulgare L. cv. Obzor	15, 150, 1500 μM CuSO4 5 days	↑ Total AsA	↓ % reduced AsA		[59]
Znic (Zn)	Phaseolus vulgaris (roots)	50 μM ZnSO4 1, 5, 24, 48, 72, 96, 120, 144, 168 h	0–120 h ↓ Total AsA >120 h ↑ Total AsA	1–96 h ↓ reduced AsA >96 h ↑ reduced AsA >72 h ↑ DHA ↑ DHA/AsA	1–96 h ↑ APX	[60]
	Phaseolus vulgaris (leaves)	50 μM ZnSO4 1, 5, 24, 48, 72, 96, 120, 144, 168 h	↑ Total AsA	↑ Reduced AsA 48 h, >96 h ↑ DHA >96 h ↑ DHA/AsA	↑ DHAR Long term ↑ MDHAR, APX, GR	[60]
	Cajanus cajan LRG30 (long duration type)	2.5, 5.0, 7.5 mM ZnSO4·7H2O germinate for 6 days		↓ AsA	↑ GR	[61]
	Cajanus cajan ICPL87 (short duration type)	2.5, 5.0, 7.5 mM ZnSO4·7H2O germinate for 6 days		↓ AsA	↑ GR	[61]
	Brassica juncea	0.007, 0.05, 5.0, 10 mM ZnSO4 10 days	↑ Total AsA		↑ MDHAR, DHAR, APX, GR	[62]
Nickel (Ni)	Oryza sativa	200, 400 μM NiSO4 0, 5, 10, 15, 20 days		↑ Reduced AsA ↑ DHA ↑ DHA/AsA	↑ MDHAR, APX, GR	[63]
	Cajanus cajan LRG30 (long duration type)	0.5, 1.0, 1.5 mM NiSO4·6H2O germinate for 6 days		↓ AsA	↑ GR	[61]
	Cajanus cajan ICPL87 (short duration type)	0.5, 1.0, 1.5 mM NiSO4·6H2O germinate for 6 days		↓ AsA	↑ GR	[61]
Manganese (Mn)	Vigna unguiculata cv. TVu 91	0.2, 50, 100 μM MnSO4 6 days		↓ Reduced AsA ↓ AsA/total AsA		[64]
	Vigna unguiculata cv. TVu1987	0.2, 50, 100 μM MnSO4 6 days			↑ MDHAR, GR	[64]
	Hordeum vulgare L. cv. Obzor	183, 1830, 18300 μM MnCl2 5 days	Transient ↑ total AsA followed by ↓		↓ APX	[59]
	Oryza sativa L. cv. Pant-12	3, 6 mM MnCl2 0, 10, 20 days		↓ Reduced AsA ↓ DHA ↑ DHA/AsA	↑ MDHAR, DHAR, GR, APX	[65]

Abbrevations: AsA, ascorbate; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; MDHAR, monodehydroascorbate reductase; APx, ascorbate peroxidase; GR, glutathione reductase, DHAR-OX, DHAR overexpressing, n.s., not significant.