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Sorghum is a grain plant grown as food or feeds and its adaptability to various types of environment is broad (Ali et al, 2009). As part of an on-going search for the best genotype that can be grown, several stress factors are being studied by many plant physiologists and plant breeders. For this study, water stress was the main focus. According to Blum (1996), drought affects the plant in inter-related organizations such as photosynthesis and transpiration or yield and solute accumulation. This implies that the results obtained from a single parameter cannot be a good conclusion to decide which may be the best genotype. Hence, it is best that all levels of the plant organization are investigated.
The effect of drought on the four Sorghum genotypes was evaluated using different parameters namely, glucose content, fructose content, sucrose content, rate of photosynthesis, chlorophyll content, stomatal conductance, internal carbon content, transpiration rate and yield. The four Sorghum genotypes, Awan, Tray, Wray and Smith were subjected to mild and severe water stress and compared to a control plant to determine the effects of water stress.
The amount of solute present in the plant is a distinctive feature of stress adaptation (Blum, 1996). The tendency for plants to use the reserve solutes, which may be in the form of sugars, depends on the concurrent demand for growth (Blum, 1996). Table 7 presents the effect of the different water stress levels on the amount of accumulated sugars (glucose, fructose and sucrose) on the sorghum genotypes.
The glucose content of the Wray genotype had the highest reduction of 55.0% under mild water stress and the Smith genotype with 91.9% decrease under severe water stress. The Smith genotype, with 16.2% reduction in glucose content was second to the Wray genotype under mild water stress. The glucose content of the Tray and Awan genotypes increased by 4.7 and 75.9%, respectively, under mild water stress. Severe water stress caused a 28.0 and 12.8% decrease in glucose content for the Wray and Tray genotypes. The Awan genotype showed tolerance for both the mild and severe water stress for glucose content with a 75.9 and 44.4% increase, respectively.
Table 7. Effect of different water stress levels on amount of accumulated sugars (glucose, fructose and sucrose) expressed in %(w/v) of four sweet sorghum genotypes
The trending observed for the glucose content is similar to that of the fructose content for all the genotypes. Under mild water stress, Wray genotype had the greatest decrease by 53.1%, followed by the Smith genotype with 32.4% reduction. The Tray and Awan genotypes again increased by 4.7 and 80.4% respectively. Severe water stress showed that the Smith genotype had the highest at 91.9% reduction in fructose content. The Wray and Tray showed 27.6 and 14.1% reduction respectively. Under both water stress conditions, the Awan showed tolerance for fructose content with 80.4% increase at mild water stress and 45.1% increase at severe water stress.
The sucrose content of the four genotypes showed varying responses to the subjected water stress. The Smith and Tray genotypes showed reduction under mild water stress at 29.6 and 21.6% respectively, with the Smith having the highest decrease. The Wray and Awan genotypes increased by 77.2 and 31.7% respectively under mild water stress. The only genotype which showed reduction in sucrose content at 7.4% when the plants were subjected to severe water stress was the Smith genotype. The Tray, Awan and Wray genotypes increased their sucrose content by 13.6, 31.8 and 166.7% respectively. Under mild and severe water stress, the Wray genotype showed the greatest tolerance.
The different trend observed in the sucrose content is explained in Blumâ€™s study (2005). According to his research, the stress decreased the amount of water needed for photosynthesis during formation of the grains which led to the movement of the sugars stored in the stems to the grains of the plant.
In general, the four Sorghum genotypes showed varying responses to the conditions they were subjected to. Among the four, the Awan genotype was the most tolerant under both mild and severe water stress because there was no observed reduction in the glucose, fructose and sucrose content.
Chlorophyll content, expressed as SPAD value, photosynthesis and stomatal conductance are simultaneously affected when a plant is subjected to water stress. Table 8 shows the effect of water stress on the SPAD value, photosynthetic rate and stomatal conductance of the Sorghum genotypes.
Table 8. Effect of different water stress levels on SPAD value, Photosynthetic rate and Stomatal conductance of four sweet sorghum genotypes
All four Sorghum genotypes experienced a decrease in SPAD value, photosynthetic rate and stomatal conductance. The Tray genotype showed the highest reduction of the SPAD value at the mild and severe water stress conditions with 17.9 and 24.5% respectively. The Tray genotype also showed the highest reduction in stomatal conductance (32.0%) with the plant in a mild water stress condition. The Awan, Wray and Smith genotypes subjected to mild water stress showed 17.2, 9.7 and 10.6% decrease in SPAD value, and 4.7, 5.0 and 15.3% SPAD value reduction under severe water stress. The Wray genotype showed tolerance under mild water stress and the Awan genotype under severe water stress.
For photosynthetic rate of plants under mild water stress, the Smith genotype experienced the highest reduction (19.3%) and under severe water stress, the Wray genotype showed the highest at 54.4% decrease in photosynthesis. The photosynthetic rate of the Awan, Tray and Wray genotypes under mild water stress decreased by 18.2, 23.9 and 14.3% respectively. Even greater reduction in rate of photosynthesis also occurred in the genotypes subjected to severe water stress. The Awan genotype further decreased to 40.3%, the Tray genotype down to 33.0% and the Smith genotype with a 31.1% decrease as compared to the control. The tolerant genotype which showed lowest reduction in photosynthetic rate is Wray (mild water stress) and Smith (severe water stress).
Stomatal conductance of the plants also showed reductions as compared to the fully-irrigated Sorghum plants. The Tray genotype, with a 32.0% decrease had the highest reduction under mild water stress, followed by the Wray (24.9%), Smith (24.5%) and Awan (24.3%). Severe water stress caused the greatest reduction of stomatal conductance in the Wray genotype at 57.8%. Compared to the same genotype in the mild water stress, the genotypes under the severe water stress had even lower stomatal conductance. The Awan at 38.3, Tray at 41.8 and Smith genotype at 43.1% decrease in stomatal conductance. Awan is the tolerant genotype for this parameter under both mild and severe water stress.
The observations from Table 8 imply that the genotypes are in the state of adapting to the stress to prevent desiccation (Blum, 1996). The decrease in chlorophyll content and consequently, the decrease in rate of photosynthesis resulted to the decreased stomatal conductance in the leaves of the plants (Blum, 1996). It may be possible that the leaves are almost wilting due to the severe decrease in stomatal conductance (Blum, 1996).
Table 9 shows the effect of water stress on the internal carbon dioxide (CO2) concentration and transpiration rate of the Sorghum genotypes. The plants under mild water stress, all decreased their internal CO2 concentration, with the Wray genotype having the highest reduction at 38.1%, the Awan, Tray and Smith had a decrease of 21.9, 9.0 and 11.8% respectively. The Smith genotype at 43.0% had the highest reduction in the internal CO2 concentration when they were subjected to severe water stress. Tray genotype also experienced a 10.4% reduction, while the Awan and Wray genotypes had an increase in the internal CO2 concentration with 31.1 and 8.2%. The Tray genotype was tolerant at mild water stress and Awan at severe water stress compared to the other genotypes under the same condition.
Table 9. Effect of different water stress levels on Internal CO2 concentration and Transpiration rate of four sweet sorghum genotypes
Transpiration rate of all the genotypes decreased for both water stress conditions. The Tray genotype (35.1%) showed the highest reduction under mild stress and the Wray genotype (57.8%) under severe water stress. The mild water stress reduced the transpiration rate in Awan, Wray and Smith by 25.6, 17.8 and 23.7%. Their genotype counterpart experienced further decrease in the transpiration rate under severe ware stress: Awan with 35.9%, Tray at 40.4% and Smith with 28.9%. The Wray and Smith genotypes were tolerant for mild and severe water stress respectively. Reduction in transpiration rate is an effect of the closure of the stomata in plants as a means to adapt to soil water depletion (Blum, 1996).
The yield potential of a plant is affected by the over-all interaction of the photosynthesis, transpiration and solute accumulation (Blum, 2005). It is hypothetical that high-yielding genotypes will continue to perform well under any environmental condition (Blum, 2005). Table 10 presents the yield of the four sorghum genotypes under the two water stress conditions.
Table 10. Effect of different water stress levels on Fv/Fm ratio, Juice yield and Sugar yield of four sweet sorghum genotypes
Juice yield (mL plant-1)
Sugar yield (mL plant -1)
The Fv/Fm ratio, as a measure of yield, decreased for all the four genotypes. It can be seen that under mild water stress, the Smith genotype had the highest yield reduction at 4.8%, and consequently, it also had the highest reduction in juice yield at 31.3%. The Wray genotype had the second highest reduction in the Fv/Fm ratio, followed by Tray and Awan at 3.9, 1.8 and 1.3% respectively. The Awan genotype showed tolerance for this parameter under mild water stress. The Tray genotype experienced the highest decrease in yield by 7.0% under the severe water stress. The Awan genotype showed further decrease in Fv/Fm ratio at 1.9%, but the Wray (0.81%) and Smith (1.3%) genotypes increased their Fv/Fm ratio when compared to values obtained from the mild water stress conditions. The Smith genotype was tolerant at this severe water stress condition.
As previously mentioned, the Smith genotype (mild water stress) had the highest reduction in juice yield, followed by Wray and Awan at 27.7 and 7.4%. Apparently, the Tray genotype showed tolerance in the mild water stress with an increase in the juice yield for about 6.9% and had the lowest reduction in sugar yield at 2.5%. Under mild water stress, The Wray genotype showed the highest decrease in sugar yield at 20.0%, followed by Awan (18.8%), Smith (3.6%).
All the four genotypes further decreased in juice yield and sugar yield when they were subjected to severe water stress. The severe water stress condition resulted to the highest reduction of juice yield and sugar yield in the Smith genotype, at 56.1 and 64.3% respectively. The juice yield reduction of Tray and Wray were 35.2 and 37.4%. In this parameter, the Awan genotype showed tolerance with the lowest juice yield reduction at 21.1%. The sugar yield reduction was highest in the Smith genotype at 64.3%. The decrease was also seen with the Awan genotype at 43.8% followed by Tray with 42.9% and the Wray genotype which showed the lowest reduction or tolerance with 42.4%.
Based on the results obtained from all parameters measured for the effect of water stress on four Sorghum genotypes, there is no single genotype which can be tolerant to all the applied conditions. The capacity of the plants to adapt to water stress is affected by the various genes that contribute to the traits of each genotype (Ali et al, 2009). Therefore, selection of the best genotype to plant will also depend on the proper evaluation of the environment where it is intended to be cultivated.