Department of Biology, Rivers State College of Education,
P.M.B. 5047, Port Harcourt
*Corresponding author
Studying the water balance and proximate composition in
plants subjected to these stresses compared the influence of drought and flooding
on cowpea seedlings. In drought plants the leaf water potential, its components
and the proximate composition were markedly reduced by the end of the experimental
period. In contrast, flooded cowpea seedlings did not show marked reduction
in water potential and other parameters studied. From these results, it was
concluded that cowpea strives better in a flooded environment than in a water
stressed environment. @JASEM
Water is absolutely necessary for
all life forms. It is necessary for the functioning of protoplasm. Thus the
adequacy and inadequacy of water is a limiting factor in both land and water
environments.
Excess water in the soil
displaces air from non-capillary pore-spaces thereby producing oxygen
deficiency. Thus as root growth ceases, physiological drought may set in.
Oxygen deficiency in any type of soil results in decreased transportation and
translocation and the production of adventitious roots (Shifferaw, et al,
1992). The effects of flood include poor development of roots and shoots (Onuegbu,
1999), decreased canopy height and dry matter (Scott et al,
1990), delayed flowering; poor fruit set and low yields (Uzo, 1997); low
percentage germination and leaf expansion (Hamada et al, 1990);
poor development of roots and shoots (Onuegbu, 1999); reduced number of noodles
and reduced amount of acids of Krebs cycle (Onwuegbuta-Enyi, 1999).
The effect of water stress on plants
is an important problem which in recent times has become the object of growing
attention. Water stress has been found to decreased leaf water potential and
photosynthetic activities (Soldatini, et al; 1990); decreased in
sucrose phosphate synthetase activity (Svilhra and Rodrignez, 1993) and in cell
enlargement and wilting of the leaves (Christina and Pfeiffer, 1997).
This work was aimed at comparing
the influence of drought and flooding on the seedlings of cowpea by studying
their effects on water balance and proximate composition.
MATERIALS AND METHODS
Plant Materials and Stress Conditions;Cowpea
seeds were put on moist filter paper in petri-dishes
at room temperature of 32oC. The seedlings were transplanted after
three days into pots (7.5cm diameter, 1.41 vol.) containing garden soil and grown
under greenhouse conditions. After two weeks the control plants continued to
be watered to reach field capacity. Immersing the pots in water flooded a second
group of plants, maintaining the water level 1 cm above the
soil surface. A third group of plants were subjected to drought by withholding
water. These treatments were imposed for a period of five weeks.
Proximate Analysis: The methods of AOAC (1984) were used
to determine the moisture content, ash, crude
protein and crude fat. Moisture was obtained by heating 2.0g samples in an oven
at 105oC to a constant weight while ash content was obtained by the
incineration of 2.0g samples placed in a muffle furnace maintained at
600oC for 4 hours. Crude protein (%N x 6.25) was determined by Kjeldahl
method (Kjedahl 1983) of nitrogen determination using 1.0g samples while crude
fat was obtained by exhaustively extracting 3.0g samples by sohxlet method using
petroleum ether of boiling point range 40oC 60oC as the
extractant. Carbohydrate was obtained by the difference method (summing the
value of moisture, ash, crude protein and crude fat and substracting the sum
from 100). The calorific value (energy) content was calculated by multiplying
the mean values of the crude protein, crude fat and total carbohydrate by the
water factor of 4, 9, 4 respectively, taking the sum of the products and expressing
the result in kilocalories as reported by Onyeike and Ikuru
(1998).
Plant Water Status:
Osmotic potential (gπ) was determined on an osmometer.
Leaf water potential (gw) and gπ were
determined daily and ten seedlings were used for these determinations. The
youngest leaf of each seedling was punched in two places with a 7mm borer to
obtain two 7mm leaf discs. One leaf disc was analysed for gw and
the second for gπ. Leaf discs were equilibrated for 2
hours in a sample chamber before gw determinations. Leaf gπ
was determined with the freeze-rupture technique. Leaf discs in a filter
paper-lined petri-dish were placed on dry ice for 5 minutes, thawed for 10
minutes and crushed on a filter paper disc and gπ
determined after 30 minutes of equilibration in a sample chamber. Leaf
pressure potential (gp) was calculated as the difference
between gw and gπ. Transpiration ratio
(wt
: wt) was determined as the ratio of the average water used per plant to
average dry weight per plant. The relative water content (RWC) was calculated
from the fresh and dry weights according to Racker (1978). Transpiration rate
(wt: wt) was determined as the ratio of the average water use (g) per plant to
average dry weight (g) per plant.
RESULTS
AND DISCUSSIONS
All the growth parameters (except
leaf area ratio) plant height, shoot dry matter, leaf area, Relative growth
rate, Net assimilation rate and leaf area ratio studied were significantly
reduced by both flood and drought stress (Fig.1). The reduced growth and dry
weight accumulation is as a result of reduced cell expansion and photosynthesis
as reported by Soldatini et al, (1990). Leaf area ratios increased in both the
flooded and water stressed plants. In the flooded plants the increase was as
much as two-folds. This ability to adjust leaf area is considered a sensitive
adaptation to stress at a whole plant level (Patterson, 1989). The decrease
observed in the net assimilation rate is probably as a result of respiration
losses in flooded plants.
The cowpea seedlings grown under
well watered conditions showed a leaf water potential (gw) of
0.32mPa, an osmotic potential of 2.60mPa, a pressure potential (gp)
of 1.23mPa, and a relative water content of 95%. Table 1 shows variations in
gw, gπ and gp during the
five-week period of the experiment. The gw, gπ
and gp of the flooded cowpea plants in this experiment were
similar to those of the control plants. In addition, the relative water
content of the flooded plants remained at the same level as in the plants grown
under normal conditions.
Under the conditions described
above, flooding does not cause water stress in cowpea plants. These results
are consistent with reports which suggest that the leaf water potential is not
reduced in plants subjected to flooding (Wample and Thronton, (1984), Soldtini et
al, (1990), Onwugbuta-Enyi & George, (2000).
This lack of water stress in the
flooded cowpea seedlings shows that plants under this condition can maintain a
water balance as a result of reduced water absorption by damaged roots. There
was marked reduction in the transpiration rate of both flooded and water stressed
plants. This reduction helped the plants to maintain their gw, at
the same level as that of the control. The maintenance of water balance occurs
at the expense of all expansion and growth rate. This result is in agreement
with other findings (Hanson, 1982, Soldatini et al, 1990, Onwugbuta-Enyi
and George, 2000).
There was a
decrease in both Yw and Yπ of water stressed
plants. These decreases are normal responses when plants are subjected to
water stress, (Hanson, 1982). The decrease in Yπ is a
manifestation of active osmotic adjustment and indicate that an active osmotic
adjustment is involved in maintaining a positive turgor pressure in cowpea
seedlings subjected to drought. The results agree with those reported by
Richter (1988).
The results of
proximate composition of flooded and water stressed seedlings of cowpea are
presented in Table 2. Low moisture content of 12.50% is an indication of
physiological drought.
TABLE 2: Proximate Composition
of Cowpea Seedlings Under Flood and Drought Conditions.Proximate Composition
(PC)%
Constituents |
Control Plants
|
Flooded Plans
|
Water Stressed
Plants
|
Moisture Content
Ash Content
Crude Protein
Crude Fat
Total Carbohydrate
|
83.17 + 0.2
12.71 + 0.01
28.88 + 0.4
17.40 + 0.20
26.75 + 0.50
|
83.09 + 0.03
8.10 + 0.1
22.90 + 0.20
9.40 + 0.50
2.18 + 0.20
|
12.50 12.50 + 1.2
6.24 + 0.08
16.03 + 0.30
7.18 + 0.30
18.96 + 0.26
|
Value are the mean + SD
of ten seedlings.
Th ash content of both the
control and flooded seedlings were significantly different from that of the
water stressed plants. This shows that both stresses inhibit absorption of
mineral nutrients. This result is in agreement with the findings of Grable
(1989).
Protein, fat and carbohydrate
contents were found to be very low in the water stressed plants. This is
reflected in the stunted growth of water stressed plants. Indeed the water
stressed plants were 20% smaller than the control at the end of the treatment
time period. Data presented in this paper show that cowpea seedlings are more
flood tolerant than water stressed tolerant.
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