Drought Stress Effect during Different Growth Stages on Yield, Osmolites and Photosynthetic Pigments Accumulation of Grain Sorghum Genotypes (Sorghum bicolor L.)

IntroductionOsmotic adjustment in plants can be achieved by the accumulation of compatible solution or metabolites. These compounds are known as compatible metabolites that accumulate naturally in tolerant plants due to non-interference in the normal metabolic response of plants to adapt or supplement. Proline, soluble sugars and other metabolites accumulation that are involved in osmotic adjustment have been reported for various plants. Different studies show that water absorption in sorghum plant, is due to osmotic adjustment and appropriate and fairly extensive root system. Moreover, there are some differences from genotype to genotype regarding the osmolites accumulation under drought stress conditions. Thus, the aim of this study was to investigate the effects of drought in the vegetative and reproductive growth stages on yield, its components and biochemical traits in grain sorghum genotypes.
Materials and MethodsIn order to evaluate the effect of water stress on grain yield and its components and some biochemical traits in grain sorghum genotypes (Sorghum bicolor L.), a field experiment as a split plot design was carried out with 3 replications in 2014 at the research farm of the southern Khorasan Agriculture and natural resources research and education center. Water stress treatments including normal irrigation (control), irrigation cut off in vegetative growth stage (emergence of terminal leaf as rolled) and irrigation cut off in generative growth stage (50% of plants in start of flowering) as the main plot and 10 genotypes of sorghum including KGS29, MGS2, Sepideh, KGFS27, MGS5, KGFS5, KGFS17, KGFS13 and KGFS30 were considered as sub plots. Each plot consists of 4 rows with a length of 6 m and row spacing of 60 cm, between plants on row was 10 cm. In addition, between each plot and the adjacent plot a row was considered to side effect reduction. To determine the yield components of each plot, half a meter in length was harvested and the number of plants, the number of panicles, grain yield, 1000 grains weight and the number of seeds per panicle were determined. To determine the yield, after removal of 2 marginal lines and a half meter of the beginning and the end of each plot, plants were harvested from the surface of 3 m2. Biochemical parameters including chlorophyll a, chlorophyll b, carotenoids, proline and carbohydrates were measured on the flag leaf after flowering stage in each plot. Flag leaves immediately wrapped in aluminum foil and transferred into liquid nitrogen tanks after separating from the plant. The samples were transferred to a freezer at -20 ° C to be measured traits on them. Measurement of the biochemical characteristics, such as chlorophyll a content, chlorophyll b, total chlorophyll a and b and carotenoid content was done according to Arnon method. Measuring the concentration of soluble carbohydrates was performed using sulfuric acid method. Measurement of free proline was done by Bates method. Sugar percentage of stem (Brix) was read by a refractometer after cutting and placement of juice out of it.
Results and DiscussionResults showed that water stress had a significant effect on grain yield, 1000 grain weight, the numbers of seed per panicle and caused to decrement of them. The performances of different genotypes varied significantly for all traits, indicating high variability among them. In case of 1000 seed weight, the interaction between water stress and genotype did not show a significant difference, however, other traits which mentioned above showed a significant difference in this aspect. Regarding the biochemical characteristics, the impact of drought in the vegetative and reproductive growth stages was different, as drought reduced the content of chlorophyll and carotenoid and increased the content of soluble sugar and free proline and stalk sugar content (Brix). In term of grain yield, genotype KGFS13 with the average yield of 5060 Kg per hectare and then genotype KGFS17 had the highest yield. Comparison of interaction between genotype and stress about carbohydrate (sugar solution) concentration of leaves indicated that genotype KGFS30 in severe drought stress, had highest level and genotype KGFS27 in normal irrigation and genotype KGS33 in medium drought stress condition commonly had the lowest carbohydrate content of leaves respectively.
ConclusionsOverall results indicate that proline and soluble carbohydrates and stem sugar content increased under drought stress and photosynthetic pigments are reduced.