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 32^oC.  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 105^oC to a  constant weight while ash content was obtained by the incineration of 2.0g samples placed in a muffle furnace maintained at 600^oC 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 40^oC – 60^oC 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 (g_π) was determined on an osmometer.  Leaf water potential  (g_w) and g_π   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 g_w and the second for g_π.  Leaf discs were equilibrated for 2 hours in a sample chamber before g_w determinations.  Leaf  g_π 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 g_π determined after 30 minutes of equilibration in a sample chamber.  Leaf pressure potential (g_p) was calculated as the difference between g_w and g_π.   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 (g_w) of – 0.32mPa, an osmotic potential of – 2.60mPa, a pressure potential (g_p) of 1.23mPa, and a relative water  content of 95%.   Table 1 shows variations in g_w, g_π  and g_p during the five-week period of  the experiment.  The g_w, g_π  and g_p 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 g_w, 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 Y_w and Y_π of water stressed plants.  These decreases are normal responses when plants are subjected to water stress, (Hanson, 1982).  The decrease in Y_π 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)%

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.

REFERENCES

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