| Salinity and high temperature are major abiotic stresses limiting sustainable crop production in the world. Germination, emergence and early seedling growth are critical for crop establishment, especially under abiotic stresses. Salt stress can usually increase the accumulation of toxic ions in higher plants and are associated with decreased photosynthetic rate and subsequent reduced growth. Sweet sorghum(Sorghum bicolor L. Moench) is a medium salt tolerant crop containing high content of sugar, mainly sucrose, fructose and glucose, in the juice of the stalks, from which ethanol can be easily produced for biofuel. For this reason, sweet sorghum has become a popular crop throughout the world. In many tropical and temperate countries where sugarcane cannot be grown, growing interest is focused on the potential of sweet sorghum to produce bioethanol feedstock. However, salinity and temperature stresses (especially salinity) are hampering the production of sweet sorghum in many parts of the world. The appropriate application of plant growth regulators is an effective way to enhance germination, seedling growth, antioxidant defense system, and yield of crops under salinity and temperature stresses. However, there is little knowledge available on the effects of salinity and temperature stress on sweet sorghum and the effects of exogenous application of gibberellic acid (GA3), kinetin, and salicylic acid on seedling growth and physiological parameters of sweet sorghum plants. The knowledge in this field is of critical importance not only for breeders to develop salt-tolerant varieties but also for agronomists to construct a high-yielding cultivating system for sweet sorghum production grown in salt and temperature affected areas.The study of the present thesis consisted of three controlled experiments that were conducted at College of Agriculture, Yangzhou University, Yangzhou, China (32.39°N,119.41°N) in the 2013 and 2014 cropping seasons of sweet sorghum. The first one was a laboratory experiment done to determine the effects of three salinity levels (0,100,200, and 300 mM NaCl) and three temperature levels (25,35, and 39℃) on germination and early seedling growth of two varieties (Yajin 13 and Yajin 71) of sweet sorghum. Seeds were soaked in water solutions containing one of the three hormones [288.7 μM gibberellic acid (GA3),232.3 μM kinetin, or their combination] and then germinated under controlled conditions. Water imbibition, germination, radicle and shoot length, and the electrical conductivity of seed leachate were measured. Seed water imbibition was increased with increased temperature and decreased with increased salinity level. With the rise in temperature, seed water imbibition was increased by 7.5% and 12.5% at 35℃ and 39℃, respectively, compared to 25℃ after 4 h of imbibition initiation. All exogenous hormones significantly increased water imbibition at 4 h and 24 h as compared with the control. Yajin 13 had 17.9%higher germination percentage with 16.3% higher germination rate index than Yajin 71. Salinity and temperature stresses clearly affected germination and decreased subsequent shoot and root growth. There was no significant difference in germination percentage between 25℃ and 35℃, while germination percentage was statistically decreased by 27.6% at 39℃ as compared with 25℃. For germination recovery of high temperature (45℃) or high salinity (350 mM NaCl), the seeds of both varieties failed to germinate and to recover germination. All hormone treatments were effective in decreasing electrical conductivity of seed leachates in both varieties. The application of GA3, kinetin, or their combination alleviated the negative effects of salinity and temperature stresses on all the measurements. Among the tested hormones, GA3 was better in alleviating salinity and temperature stress in both varieties in germination and other parameters.The second experiment was done to determine the effects of exogenous GA3 (288.7 μM), kinetin (232.3μM), and salicylic acid (362.0μM) on morphological and physiological parameters of sweet sorghum variety Yajin 13 (more salt tolerant as compared with Yajin 71) under different levels of salinity (0,100 and 200 mM NaCl) and temperature (25℃ and 37℃). The parameters determined included emergence percentage, emergence rate, shoot and root length, root number, leaf number, fresh and dry weight of shoot and root, the activity of peroxide dismutase (POD) and superoxide dismutase (SOD), and the content of soluble protein, proline, malondialdehyde (MDA), and chlorophyll a and b. Salinity and high temperature significantly reduced emergence percentage, emergence rate, shoot and root lengths, leaf number, shoot fresh and dry weight, and chlorophyll a and b content. Increasing salinity reduced emergence rate in all hormonal treatments. Root number was increased with increased salinity level. At the high salinity and high temperature levels, dry shoot weight was decreased by 50% as compared with 0 mM NaCl level at 37℃. The activity of SOD and MDA was increased with the increase in both salinity and temperature. At 37℃, POD activity was increased by 88.9% at 200 mM NaCl as compared with 0 mM NaCl. Salinity, temperature, and hormone alone did not affect the content of soluble protein. There were no significant differences in chlorophyll a content between 0 mM NaCl and 100 mM NaCl at both 25℃ and 37℃. Hormonal treatments positively affected most of the observations. At the salinity level of 200 mM NaCl, salicylic acid increased emergence percentage, emergence rate, the content of chlorophyll b and soluble protein by 82.0%,130%, 7.9%, and 1.9%, respectively, as compared to the control. At 37℃, salicylic acid increased emergence percentage, emergence rate and root number by 72.5%,108.5%, and 63.8%, respectively, and decreased MDA content by 17.6% relative to the control. All hormonal treatments increased POD activity at the salinity level of 200 mM NaCl, whereas kinetin increased the activity of POD by 33.8% at 200 mM NaCl than control of 0 mM NaCl salinity level. Proline content was increased with increasing salinity at all hormonal levels.The third experiment was a pot study conducted under field condition for two cropping seasons of 2013 and 2014 to assess whether exogenous application of GA3 (288.7 μM), kinetin (232.3uM), and salicylic acid (362μM) as a foliar spray could ameliorate the adverse effects of salt (0,2, and 4 g NaCl/kg dry soil, equivalent to 0.3,2.3, and 4.7 dS/m) on two varieties (Yajin 13 and Yajin 71) of sweet sorghum. At 45 d after transplanting, stem diameter, chlorophyll content (SPAD reading), membrane permeability, total leaf area/plant, photosynthetic rate, stomatal conductance, and transpiration rate were measured and repeated each 15 d. When plants reach soft dough stage, plant height, internode number, plant fresh and dry weight, stem fresh and dry weight per plant, root dry weight, total juice content per plant, juice percentage and Brix percentage were measured. The content of Na+, K+, Mg2+, Ca2+, and P were also determined. The salinity level of 4 g NaCl/kg dry soil caused significant reductions in plant height, root dry weight, stem fresh and dry weight, plant fresh and dry weight, leaf area, chlorophyll content, photosynthetic rate, stomatal conductance, transpiration rate, K content, and juice content. The medium salinity level of 2 g NaCl/kg dry soil decreased the content of Mg2+, P, and Ca2+and increased Brix percentage. Yajin 13 was taller in average than Yajin 71 by 6.3% in all salinity and hormonal treatments. At the salinity level of 4 g NaCl/kg dry soil, stem fresh weight was decreased by 40.4% as compared with the non-salinity level. The highest Brix percentage value of 17.24 was recorded at the salinity level of 2 g NaCl/kg dry soil as compared with the salinity levels of 0 and 4 g NaCl/kg dry soil. Juice content was decreased by 28.3% at 4 g NaCl/kg dry soil as compared with 2 g NaCl/kg dry soil. High salinity significantly increased Na+ and membrane permeability. Na+content was increased gradually from 0.4 to 0.6 and then to 2.4 mg/kg with salinity increasing from 0 to 2 and then to 4 g NaCl/kg dry soil, respectively. The content of K was decreased by increased salinity, but there was no significant difference between 2 and 4 g NaCl/kg dry soil. There was no significant difference between 0 and 4g NaCl/kg dry soil in the content of P, Ca, and Mg. The content of Na+and P in Yajin 71 was greater than that in Yajin 13 by 43.5% and 14.1% respectively, and no significant differences between the two varieties in the content of other ions were observed. Photosynthetic rate, stomatal conductance and transpiration rate were decreased gradually with salinity increased. The salinity level of 4 g NaCl/kg dry soil caused reduction in photosynthetic rate, stomata-conductance, and transpiration rate, respectively, by 24.5%,28.2%, and 25.4% as compared with the salinity level of 0 g NaCl/ kg dry soil. Salicylic acid (SA) was more effective on most parameters than GA3 and kinetin. Salicylic acid caused reduction in relative membrane permeability by 25.5% at the level of 4 g NaCl/kg dry soil in Yajin 13 and by 26.4% in Yajin 71 at the level of 2 g NaCl/kg dry soil as compared with the level of 0 g NaCl/kg dry soil. Na+ content was decreased by 26.2%,40.0% and 65.9% by GA3, kinetin, and salicylic acid, respectively.Salicylic acid at the level of 4 g NaCl/kg dry soil improved stem diameter and total leaf area and internode number over the other hormones. Salicylic acid was the best hormone for reducing relative membrane permeability and Na+content under salt stress. GA3 and kinetin were more effective on photosynthetic rate, stomatal conductance, and transpiration rate. GA3 and kinetin increased photosynthesis rate by 41.2% and 37.5%, respectively, as compared with control. Both kinetin and GA3 were more effective on plant fresh and dry weight at high salinity level.The application of phytohormones like GA3, kinetin and salicylic acid by seed priming or by spraying method, was successful for ameliorating the negative effects of salinity and high temperature on germination, seedling growth, plant growth, photosynthetic efficiency, antioxidant defense system, and stem yield of sweet sorghum varieties. The appropriate application of exogenous hormones as seed priming or foliar spray can serve as a feasible way to promote seed germination, emergence, seedling growth, and yield development. |
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