Agriculture Reference
In-Depth Information
fraction of PSII reaction centers that is opened, as indicated by the decrease of qP, is
reduced in plants cultivated with nutrient solutions added with 250 µM chromium.
From 170 µg g
−1
DW, the chromium content in leaves of
L. perenne
induced a small
decrease in Fv/Fm, and a decrease of qP, indicating that chromium decreased the
capacity of reoxidizing QA during actinic illumination, and increased the capacity
pressure on PSII. This suggests a decrease in the fraction of open PSII becoming un-
able to carry out stable charge separation (Krause and Weis
1991
). The decrease in
qP from 250 µM chromium in the treated plants indicates that the down-regulation
of PSII electron transport by increased NPQ was not sufficient to match the de-
creased demand for electron through ATP and NADPH consumption. Altered PSII
centers still act as efficient excitation energy traps, but convert the trapped energy to
heat (Genty et al.
1989
). Altered PSII centers can be detected by an increase in Fo,
as recorded in chromium-treated plants. The increase in NPQ of chromium-treated
plants may indicate a control mechanism in the thylakoid membrane that adjusts
thermal dissipation of excitation energy in excess of that required for carbon me-
tabolism. It has been shown in many studies that an increase in the thermal dissipa-
tion in the PSII antennae competes with the excitation energy transfer from the PSII
antennae to PSII reaction centers, thus resulting in a decrease in the efficiency of
excitation energy captured by open PSII reaction centers ðF 0 v = F 0 mÞ (Demmig-
Adams et al.
1996
). Bishnoi et al. (
1993
) observed that the effect of chromium is
rather more important on the PSI than on the PSII activity in isolated chloroplasts
of pea. An inhibition of the electron transport processes and a shunt of electrons
from the electron-donating side of PSI to Cr(VI) is a potent explanation for the
chromium-induced decrease of photosynthetic rate. The electrons produced by the
photochemical process were not necessarily used for carbon fixation as evidenced
by the low photosynthetic rate of the chromium-stressed plants. Due to the known
oxidative potential of Cr(VI), it is possible that alternative sinks for electrons could
have been enhanced by reduction of molecular oxygen (part of Mehler reaction)
which in part explains the oxidative stress brought about by Cr(VI) (Shanker et al.
2005
). The overall effect of chromium ions on photosynthesis and excitation energy
transfer could also be due to Cr(VI)-induced abnormalities in the chloroplast ultra-
structure like a slight lamellar system with widely spaced thylakoid and a few grana
(Rocchetta et al.
2006
). Chromium is known to inhibit photosynthesis and the pho-
tosystem II (PSII) is known as the main target for this negative action (Davies et al.
2002
). However, the relationship between chromium and the primary reactions of
photosynthesis is not well described.
4.5 Chromium Toxicity to Chlorophyll Biosynthesis of Plants
Decreases in total chlorophyll content have been well documented under chromium
stress (Panda and Choudhury
2005
). This decrease suggests that the chlorophyll
synthesizing system and chlorophyllase activity are affected by the exposure to high
chromium concentrations (Van Assche and Clijsters
1990
). Iron depletion or sub-
