| With salt-tolerant and salt-sensitive castor bean varietis (Fenbi10, Zibi7, Zibi8, and Zibi5) as the tested materials, the effects of salinity (NaCl) stress on water uptake and germination of seeds were studied. The salt-tolerant variety Zibi5was slected for further experiments. The effects of foreign amendment of GA3were studied on caster bean germination, the growth of radicals and shoots, seedling growth and physiological parameters. The main results were as follows.1. Water uptake and germination of castor seeds as influenced by salinity levelsThis study was conducted to evaluate the effects of different salinity levels (0and100mM NaCl) on the water uptake processes and germination of the seeds of four cultivars of castor bean. The water uptake of the four cultivars was fitted well with logarithm curve model. The water uptake quantity was positively correlated with water uptake time. Water uptake quantity and rate of the seeds treated with100mM NaCl were lower than that of seeds without NaCl treatment, indicating that saline stress had negative effects on water uptake. The two time sections of18-24h and36-48h probably were the key window periods of seed water uptake under saline conditions. Seed germination was significantly reduced under the salinity level of100mM. The lowest salinity injury rate was found in Zibi5, followed by Fenbi10, Zibi8and Zibi7.2. Effects of GA3amendment on seed germination, root and shoot length of castor under salinity stress conditions A laboratory test was first done to investigate the effects of different seed soaking durations (0,6,12,18,24,36, and48h) on castor bean (Zibi5) seed germination under non-saline conditions. Based on this test, castor bean seeds were treated with foreign gibberellin (GA3) lasted for different soaking durations, and then the responses of seed germination, shoot length and root length to salt stress was studied. The results showed that the germination rate and germination energy of Zibi5was significantly increased by seed soaking, and the optimum soaking duration was24h under non-saline condition. At the50mM salinity level, the castor bean seeds that were soaked by250μM GA3for24h had the highest germination percentage. When the soaking time is less than12h, the germination of castor bean seeds that were soaked by250μM GA3for12h and soaked by50μM GA3for6h were higher and the difference between them was not significant. At the100mM salinity level, the germination of castor bean seeds that were soaked by250μM GA3for12h reached the maximum. GA3treatment promoted root and shoot growth under salt stress, the promoting effect of50,100, and250μM GA3were significant. However, with the increase of soaking time, the promoting effect decreased. When the soaking time was more than12h, the root length and shoot length became short significantly. The root length and shoot length of castor bean seeds treated by50μM GA3for6h reached maximum. The appropriate seed soaking time was24h under non-saline condition. At the50mM salinity level, the more appropriate seeds treatment was50μM GA3for6h because it could promote seed germination, root and shoot growth. We could treat the castor bean seeds with250μM GA3for24h if we just want to get higher germination. At the100mM salinity level, the more appropriate seeds treatment was250μM GA3for12h.3. Effects of salt stress and GA3treatment on castor seedling growth and physiological parametersThe castor seeds were firstly soaked with0,200,250, and300μM GA3for12h and then cultured with1/2Hoagland solution containing50and100mM NaCl in pots filled with sand. The results indicated that the low level salinity (50mM NaCl) had relatively small effects on plant height, leaf growth and dry weight, and could increase stem diameter and fresh weight. However, the high level salinity (100mM NaCl) had significantly negative effects on castor seedling growth. It reduced plant height, stem diameter, fresh weight, dry weight, and the length and width of cotyledons and true leaf. Seed soaked with250μM GA3significantly increased plant height, stem diameter, fresh weight, dry weight, and promoted the growth of leaves, while other GA3treatments had little effect on seedling growth, indicating that seed soaking with250μM GA3could promote seedling growth and alleviate salinity stress.4. Effects of salt stress and GA3treatment on castor seedling physiological parametersAt10,20, and30d after seeding, compared with the0mM salinity level, the POD and SOD activity of castor bean seedlings at50mM salinity level was increased significantly. However, the POD and SOD activity of castor bean seedlings at100mM salinity level was decreased significantly. At the50mM salinity level, compared with the0μM GA3treatment, the GA3treatments increased the activity of POD and SOD of castor bean seedlings, and the POD and SOD activity of castor bean seedlings treated with200and250μM GA3reached the maximum. At the100mM salinity level, compared with the0μM GA3treatment, the POD and SOD activity of castor bean seedlings treated by200and300μM GA3decreased, but the POD and SOD activity of castor bean seedlings treated by250μM GA3increased. The change of CAT activity was inversely related with the change of POD and SOD activity. With the increased salinity, the content of proline and soluble protein increased at10,20, and30d after seeding. At the50mM salinity level, with increased GA3level, the content of soluble protein increased first then decreased. At the salinity level of100mM NaCl,, soluble protein content of castor bean seedlings treated by200and300μM GA3decreased, but the soluble protein content of castor bean seedlings treated by250μM GA3increased as compared with the0μM GA3treatment. The change of proline content treated by GA3was similar to the change of soluble protein at10d after seeding. The change of proline content treated by GA3, however, was inversely related with the change of soluble protein at20and30d. This study suggested that SOD and POD probably played a key role in scavenging O2-1and H2O2at low level salinity, and CAT may play an important role in scavenging high level of H2O2.GA3alleviated salinity stress on castor seedling by regulating the activity of antioxidase and the content of proline and soluble protein. |
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