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Effects OF Drought (Water Stress) On Growth And Photosynthetic Capacity Of Cotton (Gossypium hirsutum L.)
 

Contact Person: María Elena Fernández-Conde (bv2cocum@lucano.uco.es)

Introduction

    Drought is a serious problem that affects many regions of the world, decreasing the photosynthetic rate of crops and limiting the productivity world-wide. Therefore, water availability is an essential factor influencing agriculture.
    Growth and photosynthesis are two of the most important processes abolished, partially or completely, by water stress (Kramer and Boyer,1995), and both of them are major cause of decreased crop yield.
    Cotton crops need irrigated lands to reach high productivities. Therefore, it is very important to know how much and how often cotton plants should be irrigated to complete a successful development.
    The aim of this work was to study how cotton plants are affected by different levels of water stress. We particularly investigated when the first signs of stress appear, which are the most sensitive developmental stages (seeds, seedlings or young plants), and what organs (cotyledons or leaves) are the most affected by drought.
 


 

Materials & Methods

     Gossypium hirsutum L.cv. palma was grown in a controlled environment chamber at day/night temperature 28/21°C, relative humidity 70/80% and with a photoperiod of 16 h per day. The photosynthetic photon flux was 200 micromol m-2 s-1 at plant height.
    Plants were grown hydroponically in plastic pots containing a nutrient solution with nitrate as the sole nitrogen source (Hewitt, 1966). Water stress was applied by addition of polyethylene glycol (PEG) (MW 6000) at 0 (control), 30 and 60 g l-1 for seedling until 14 days and 0, 20 and 40 g l-1 for plants until 28 days after germination. We used PEG 6000 because is an inert polymer, with a non-ionic and virtually impermeable long chain (Couper and Eley,1984), which allows to maintain an uniform water potential during the whole experimental period.
    Dry weight was measured after drying out cotyledons or leaves at 80°C for 72 h.
    Chlorophyll a and b and carotenoids were extracted with acetone (100%) in a ratio 1:20 (w/v). Pigment concentrations were calculated by Lichtenthaler method (1987).
    Gas exchange measurements were determined at saturating light intensity, regulated temperature and relative humidity, using an Infra-red Gas Analyzer (IRGA).
 
 


 

RESULTS
 
 
 

Fig. 1: Germination index of cotton seeds 72 h after imbibition at different concentrations of PEG.
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Fig. 2: Cotton seedlings grown for 14 days with different concentrations (g l-1)of PEG.

 
 
 
 
 
 
 
 
 

 
 
PEG (g l-1)
Parameter
0
30
60
RGR (d-1)
0.074 ± 0.008
0.061 ± 0.009
0.035 ± 0.006
Fresh weight (mg)
985 ± 63
720 ± 48
574 ± 54
Dry weight (mg)
82.1 ± 3.5
73.8 ± 2.5
65.6 ± 5.0
 
Table 1: Growth parameters of cotton seedlings grown for 14 days with different concentrations of PEG.
 
 

  

Click to enlarge

 Fig.3: Pigment content of cotton cotyledons between 5 and 14 days after sowing. Seedlings were treated with different concentrations of PEG.
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 

 

Fig. 4: Cotton plants grown for 28 days with different concentrations of PEG.

 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
PEG (g l-1)
Parameter
0
20
40
Leaf area (cm2)
56.3 ± 6.6
49.4 ± 6.5
17.7 ± 3.6 
Fresh weight (g)
1.26 ± 0.09
0.86 ± 0.06
0.29 ± 0.01
Dry weight (mg)
130 ±  8
104 ± 5
65 ± 4
SLA (m2 Kg-1)
43.3 ± 2.5
43.2 ± 2.8
27.3 ± 1.7
RGR (d-1)
0.105 ± 0.008
0.092 ± 0.006
0.057 ± 0.005
LAR (cm2 g-1)
64.3 ± 2.9
57.6 ± 1.8
38.0 ±  0.9
NAR (g m-2 d-1)
16.3 ± 1.4
15.9 ± 1.7
14.9 ± 1.2
 
Table 2: Growth parameters of cotton plants grown for 28 days with different concentrations of PEG.

 
 

Click to enlarge

 

Fig.5. Net photosynthesis, stomatal conductance, transpiration and water use efficiency of cotton plants between 18 and 28 days after sowing. Plants were treated with different concentrations of PEG.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Click to enlarge

 

 
Fig. 6: Pigment content of the first cotton leaf between 18 and 28 days after sowing. Plants were treated with different concentrations of PEG.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Conclusion

1. Water stress, induced by PEG, causes a decrease in the germination index and in the morphological development of organs from young cotton plants.

2. Dry and fresh weights of cotyledons and fully expanded first leaf were smaller at increasing stress level, which suggests that water absorption and/or retention, as well as biomass gain, are affected by water stress.

3. Water stress diminished the relative growth rate of cotton plants. The net assimilation rate was not significantly decreased, being the major factor responsible for this decrease the leaf area rate.

4. Net photosynthesis, stomatal conductance and transpiration in stressed plants were smaller than in the control, but the photosynthetic rate was more affected than transpiration. Accordingly, water use efficiency (WUE) decreases in PEG-treated plants.

5. The pigment content was smaller in cotyledons and leaves under stress. This fact may explain in part the decrease in the photosynthetic process.

6. The earlier the water stress is applied, the less successful is the development of cotton plants.
 
 


 

References

1. Couper, A. and Eley, D. (1948). Surface tension of polyethylene glycol  solutions. J. Polymer. Sci. 3: 345-349.
2. Hewitt, E. J. (1966). Sand and water culture methods in the study of plant nutrition.
Commonwealth Bureau of Horticultural and Plantation Crops, East Malling Tech. Commun. 22.
3.Kramer, P.J. and Boyer, J.S. (1995). Water relations of plants and soils. Academic Press, San Diego.
4.Lichtenthaler, H.K. (1987). Chlorophylls and carotenoids: Pigments of phothosynthetic biomembranes. Methods Enzymol. 148: 350-382.
 
 

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