irradiation was providing by 12 white fluorescent lamps (Philips
TL40, Argentina). After 2 days, acclimated plants were rinsed
with running tap water and transferred to clean polycarbonate
boxes containing either tap water corresponding to the control
(Cr-untreated plants), or 2, 5, and 10 mg L−1 Cr concentrations
(Cr-treated plants) (treatment period). Cr was provided as potassium
dichromate (K2Cr2O7) and Cr solutions were prepared in
tap water. Tap water was used in order to get a similar aquatic
medium than the pond where Salvinia plants were collected. In
addition, there was no difference in plant growth using a distilled
water Cr solution [23]. Plants under Cr treatment were
not cultivated in Hoagland’s solution to avoid chelation and/or
cation competition between Hoagland salts and potassium dichromate.
Tap water physico–chemical characteristics according to
regulation of the SEPAPYS (Servicio Provincial de Agua Potable y
Saneamiento, Tucumán, Argentina) were: pH 7.3; EC (S cm−1)
200; DO (mg L−1) 3.0; TDS (mg L−1) 100; Turbidity (NTU) < 1;
As (g L−1) 0.20; Cu (g L−1) 2.2; Fe (g L−1) 12; Mn (g L−1)
0.2; NH4
+ (mg L−1) < 0.02; NO2
−(mg L−1) < 0.05; NO3
−(mg L−1) 9.0;
PO4
2− (mg L−1) < 0.2; HCO3
−(mg L−1) 50.0; SO4
2− (mg L−1) 10.0;
Cl−(mg L−1) 12.0; Ca2+ (mg L−1) 10.0; Mg2+ (mg L−1) 5.0; Na+
(mg L−1) 20.0; K+ (mg L−1) 5.0; Cr (g L−1) < 1.2; Ag+ (g L−1)
0.05; Pb (g L−1) 2.1; Hg (g L−1) < 0.01; Cd2+ (g L−1) 2.5;
CN−(g L−1) < 0.001; Zn2+ (g L−1) 3.5; Hardness (mg L−1) 212.
DO = dissolved oxygen; EC = electrical conductivity; TDS = total dissolved
solutes; NTU = nephelometric turbidity unit). During the
experiment, tap water was added to reach initial volume level at
definite intervals (2 days) to compensate water loss by transpiration
and evaporation. Because Cr remotion from water by aquatic
macrophytes is a fast process [3], the Cr solution was renewed at 3
days to avoid excessive changes in Cr concentration. The pH of Cr
solution and control tap water ranged between 6.8 and 7.0 during
the experimental period and it was daily measured using a
glass combination pH-sensitive electrode coupled to a digital pH
meter (Hanna Instrument, Germany). At this pH value, spontaneous
reduction of Cr(VI) to Cr(III) and/or Cr(III) precipitation were
not detected [35]. Plants were grown for 6 days under previously
described conditions. It is worth noting that we chose as experimental
period 6 days because preliminary experiments carried out
during 9 days showed slightly symptoms of chlorosis and necrosis
in Cr-treated plants. For chemical determinations and respiration
measurements plants were harvested, rinsed in distilled water and
separated in floating and submerged leaves. In order to minimize
any diurnal effect on the carbohydrate content all samples were
collected at noon.
Wet plant biomass (FW) was immediately determined after harvesting
whereas the dry plant biomass (DW) was determined by
drying weighed wet samples at 80 ◦C in a hot air oven for 4 days
and weighed again.