Zinc toxicity in tea



Zinc plays a vital role in the synthesis of nucleic acids and
proteins and helps in the utlilisation of phosphorous and nitrogen.
Zinc deficiency is common in tea, as these plants are grown on
highly withered acidic soils. However application of zinc sulphate
beyond a limit would become toxic to tea plants as the
requirement of zinc is low. Hence, attempts were made to
study the distribution of zinc through tea plants and to fix
the critical limits for its deficiency as well as toxicity.
As a first step the distribution and accumulation of soil
applied zinc was studied in potted plants.

There were nine treatments including an untreated control,
where no zinc was applied. Zinc acetate was added to soils
so as to form 8 treatments having 10, 25, 50, 100, 500, 750,
1000 and 2000 ppm of zinc. One year old plants of UPASI-9
were grown in these pots and the trail was continued for
136 days. The experiment Fig.1. Zinc toxicity symtoms ; a)
initial stage b) final stage was conducted in two sets.
The plants grown in the soil containing 2000 ppm zinc died
within 15 days while the plants supplied with 1000 ppm zinc
died within 36 days after imposing treatments. The plants
raised under 750 ppm zinc showed toxicity symptoms like
leaf browning and scorching. The symptoms of browning were
noticed along the mid-rib starting from about 1.5 to 2.0 cm from
the petiole towards the leaf tip and expanded throughout the
leaf (fig.1), at later stages. The surviving plants were sacrificed
and separated into root, stem and leaf after 136 days of
experimentation and analysed for Zn, Mn and Mg and the
results are given in table1.

Zinc toxicity interfered with the uptake of Mn and Mg.
Addition of zinc increased the translocation of Mn to leaves,
while it suppressed the uptake of magnesium. Higher the
concentration of applied zinc, higher was its accumulation
in leaf, stem and root. The rate of soil applied zinc has got
positive and significant correlation with the zinc content of
leaf (r = 0.87, P = 0.01), stem (r = 0.87, P = 0.01) and
root (r = 0.75,P = 0.05). The relationship for applied zinc
with manganese content of leaf and stem was linear and
positive (r = 0.86, P = 0.01; r = 0.73, P = 0.05, respectively),
which confirms the synergism between zinc and manganese.

Based on this study it is concluded that zinc at high levels
can cause chlorosis and browning of leaf . The toxic levels
of zinc appear to be 660 ppm in root or/and 170 ppm in
stem or/and 160 ppm in leaf. It can even become lethal to
plants when the concentration of zinc is more than double
the prescribed limits.