The use of industrial and municipal wastewater is a common practice in
many parts of the world, particularly in developing countries including
Ghana. Access to adequate water of acceptable quality for irrigation in
the urban/peri-urban communities of Ghana has been a major concern. About
80-90% of vegetables consumed by the people in the urban communities are
produced in the urban/peri-urban areas where water of acceptable quality
may not be accessible (Cornish et al., 1999). Where accessible,
the high cost of irrigation water makes its use prohibitive. Growers of
vegetables therefore use wastewater from drains that receive effluents
from all sources and other urban polluted water bodies. Therefore heavy
metal contamination of agricultural soils and produce from wastewater
irrigation is of serious concern as a result of human health implications.
Market survey of commonly consumed vegetables in Ghana showed higher
than the permissible values recommended by World Health Organization (1989).
Hence the objective of this study was to find out the extent that transpiration
influences Cd and Pb uptake by some selected vegetables irrigated with
wastewater containing high Cd and Pb concentrations.
Researchers from several countries have found linear relationships between
dry matter yield and cumulative transpiration of plants grown in containers
or between dry matter and cumulative water use (transpiration and soil
evaporation) as established in field experiments (Hanks and Rasmussen,
1982). Bierhuizen and Slatyer (1965) introduced an equation proposed to
be universally applicable for different climatic regions:
where, Δe is the saturation deficit of the air (Pa), k is a crop-specific
constant (Pa), T is cumulative transpiration (kg m-2, product
of column of liquid transpired and density of liquid) and Y is the plant
yield (kg m-2).
Ions are dumped into the root xylem by the vascular system and are swept
along in the transpiration stream at concentrations that depend on the
rate of water entry into the xylem. The amount of ions absorbed is proportional
to the rate that dry mass accumulates. Inorganic ions like those of salts
of heavy metals generally are absorbed most rapidly during most phases
of growth (Kramer and Boyer, 1995).
The occurrence of soil dehydration decreases the uptake of ions because
of the accompanied reduction of transpiration, decreasing the bulk flow
of soil solution to the root. Since water for shoot growth has to be extracted
from the same xylem as transpiration that sets up and increases tension
in the xylem thereby affecting the growth of surrounding tissues. The
increased xylem tension by transpiration creates a competition between
water for growth and transpiration (Kramer and Boyer, 1995). Water to
be used for transpiration evaporates close to the xylem vessels and thus
bypassing many of the cells outside of the xylem.
Plants grown on heavy metal containing substrates such as Cd show disturbed
water balance. Poschenrieder et al. (1989) reported that Cd-treated
plants showed lower leaf relative water content and higher stomatal resistance
than the controls of an experimental setup.
Transpiration rates are influenced by the local meteorological events
such as wind, radiation and air humidity therefore the relationship between
metal uptake and water use rate becomes location specific.
MATERIALS AND METHODS
Mini-experimental plots of 1.8x1.8 m were set up on an experimental field
at the Kwame Nkrumah University of Science and Technology (KNUST), Kumasi,
agricultural experimental land (usually with grass vegetation) from May
to September, 2005. Kumasi lies approximately on latitude 6° 411
N and longitude 1° 381 W. Rainfall in Kumasi is bimodal
with a mean annual total of about 1,302 mm. The minor dry season occurs
in August and the major dry season starts from mid-November and ends in
February. The main wet season is from March to July whilst the minor rainy
season extends from September to November. Temperatures are uniformly
high throughout the year. The lowest mean annual temperature of about
24.6°C is usually recorded in August and the highest mean monthly
temperature of about 28.8°C occurs in February. Morning relative humidity
is uniformly high throughout the year. The mean monthly figures range
between 84.4 and 95.6% at 06:00 h and 39.6 and 75.1% at 15:00 h. The annual
evapo-transpiration in Kumasi is about 1234 mm with monthly values ranging
from 107 to 144 mm in the major dry season and 71 to 118 mm in the rainy
season. The plots were used to grow cabbage, carrots and lettuce in three
replications. Cadmium and Pb solutions of 0, 0.05 and 0.1 mg L-1
and 0, 30 and 50 mg L-1, respectively, were formulated and
used to irrigate the crops. These values were used to establish the impact
on crop concentration of Cd and Pb when irrigation water concentrations
exceed the FAO permissible values of 0.05 mg L-1 of Cd and
20.0 mg L-1 of Pb. Cadmium and Pb nitrates salts (Cd(NO3)2
and Pb(NO3)2) were added to 200 L capacity containers
filled to the 200 L mark with treated water from the mains supply and
stirred thoroughly to obtain the predetermined Cd and Pb solution concentrations
and tested to confirm the concentrations. The crops were irrigated every
other day using 11 L per plot on each occasion. A plastic watering can
was used to avoid introduction of additional metal which would have been
the case if a galvanized container had been used.
The vegetables were sampled at maturity stages, washed with distilled
water, chopped into pieces on a washed and rinsed kitchen chopping board
to an average size of 2 cm2, sun-dried for about 6 h before
oven-drying at 80°C for about 20 h. The dried samples were milled
to < 1 mm sizes.
The plant samples were digested using EPA Method 3052 (USEPA, 1996).
Nine milliliter of HNO3 and 2 mL of HCl were added to 0.25
g of plant sample in a Teflon tube. The content of the Teflon tube after
assembling the unit was weighed and assembled in a rotor tightened using
a torque wrench and placed in a MRS-200 microwave digester. The microwave,
which had been temperature programmed, was then switched on. The samples
were left in the microwave after digestion until the temperature reduced
to about 30°C. Teflon heads were unscrewed and removed from the rotor.
Each Teflon tube was reweighed to check for any loss of the content of
the tube. The digested sample in a solution form was poured into a 15
mL centrifuge tube. One milliliter of the digested sample was diluted
in a ratio of 1:4 using deionized water in a 15 mL centrifuge tube before
analyzing it for Cd and Pb with an Agilent 7500 ICP-MS. A standard reference
material 1573a of tomato leaves certified by National Institute of Standards
and Technology (NIST) in the USA was also digested and analyzed for Cd
as a quality assurance control.
Local meteorological data (wind speed, temperature, radiation and relative
humidity) within the period of the experiment gathered at the weather
station of the Department of Mechanical Engineering, KNUST, were used
to calculate the cumulative transpiration rates (m as column of water
transpired) of the vegetables from the various experimental plots using
where, Δe (Pa) is the average saturation deficit of the atmosphere
during the main vegetation period; kp
(Pa) is a crop-specific
constant and ρw
) is the density of
water. The mean saturation deficit during the main vegetation period (Δe)
was obtained by averaging hourly values for the period between 0630 and
2030 h during the main vegetation period (Ehlers, 1989).
The hourly values of the saturation deficit Δeji (Pa)
of the atmosphere at the ith hour of the jth day were calculated as follows
where, Tji (°C) denotes the air temperature and rHji
(%) the relative humidity at the ith hour of the jth day of a year.
Data generated were analyzed statistically using the SAS software package.
RESULTS AND DISCUSSION
Average daily temperatures recorded during the period of the experiment
ranged between 24.7 and 33.1°C. The highest temperatures were recorded
in the month of August. For relative humidity the values ranged between
85.6 and 94.7% at 06:00 h. At 15:00 h the values ranged between 68.8 and
The transpiration rates of the vegetables were affected by Cd and Pb
concentrations of irrigation water (Table 1). Irrigation water with Cd
concentration of 0.05 mg L-1 reduced the transpiration rates
of cabbage and lettuce by 55.87 and 10.66%, respectively but increased
that for carrots by 268.4% compared to the controlled crops. At Cd irrigation
water concentration of 0.1 mg L-1, transpiration rates of cabbage
and lettuce reduced by 60.97 and 16.56%, respectively while transpiration
rates of carrots increased by 186.86%.
||Relationship between irrigation water Cd and Pb concentrations
and cumulative transpiration rates of the irrigated vegetables
For Pb treated 30 mg L-1 concentration of irrigation water,
the transpiration rates of cabbage, lettuce and carrots were reduced by
56.6, 11.14 and 35.88%, respectively. At 50 mg L-1 Pb concentration
of irrigation water the transpiration rates of cabbage, lettuce and carrots
reduced by 72.85, 41.66 and 24.56%, respectively, compared to the controlled
Some studies carried out worldwide have shown or established that heavy
metals do affect the cumulative transpiration rates of crops due to their
uptake. For example, a study by Veselov et al. (2003) showed that
Cd treatment of wheat seedlings led to an inhibition of growth rate, transpiration
and ion uptake. These results are in accordance with reports indicating
inhibition of water conductance in roots by toxic metals (Barcelo and
Poschenreider, 1990). The decrease in transpiration of Cd-treated plants
is likely to be due to stomatal closure. Cd-induced reduction in stomatal
conductivity is in accordance with the literature (Pearson and Kirkham,
1981). Bazzaz et al. (1974) attributed reduction in transpiration
in response to Cd to (a) increasing resistance to water flow in the stem
and (b) inhibition of stomatal opening and increasing stomatal resistance.
The inhibition of stomatal opening suggests that Cd has direct effects
on the ion and water movement in the guard cells. The effect of Pb-contaminated
solution on crop was found to be less significant, possibly due to the
lower absolute toxicity of Pb and the restricted transport to the shoot
Another effect of transpiration in soil is that it lowers the soil water
content which may affect the solution-phase concentration of heavy metal
and in consequence its plant uptake. Whether transpiration and root uptake
respectively increases or decreases the metal solution-phase concentration
depends on the uptake mechanism. For passive uptake mechanism, water removal
by plant roots has no effect on the metal solution-phase concentration.
For an active uptake, soil water is depleted and metal concentration decreases
(Ingwersen and Streck, 2005).
The study showed that the presence of Cd and Pb in irrigation water reduces
the transpiration rates of irrigated crops. Transpiration from carrots
may be increased by the presence of Cd in irrigation water and Pb may
reduce the transpiration rate of carrots as experienced in cabbage and
lettuce by both Cd and Pb.