| Drought stress is one of the major abiotic stresses that decrease the crop yield worldwide.It is crucial to develop more water-saving crop production systems.Many studies have shown that application of exogenous silicon could alleviate the damage of plants caused by drought.Therefore,silicon application may be one of the important approaches to improve crop drought tolerance.However,the mechanism for silicon-mediated drought tolerance has not been fully elucidated.Tomato(Solanum lycopersicum L.)is an important vegetable in the world,and it is crucial to improve the drought tolerance to ensure its high and stable production.Many previous studies on the mechanism for silicon-mediated drought tolerance have been focused on silicon accumulators,such as rice and sorghum,while there are few studies in less silicon-accumulating plants,such as tomato.Moreover,little information is available on the possible regulatory roles of silicon on carbon and nitrogen metabolisms in plants.Studying the effect and mechanism of exogenous silicon on drought tolerance of tomato not only has important theoretical significance,but also provides a theoretical basic for silicon application in agriculture production.In this study,the effects of exogenous silicon(2 mM)on carbon and nitrogen metabolisms were investigated in tomato seedlings grown in culture solution under simulated drought stress by 10%(w/v)polyethylene glycol 6000(PEG-6000).The main results are as follows:1.Under the simulated drought stress,the net photosynthetic rate of tomato seedlings was significantly decreased,while it was significantly increased by addition of silicon.Meanwhile,the biomass loss induced by simulated drought stress was also significantly increased by silicon.These results indicate that exogenous silicon could ameliorate the photosynthesis and increase the biomass accumulation.2.Under the simulated drought stress,the soluble sugar content was significantly decreased in the leaves,but it did not change in the roots.Addition of silicon significantly increased the content of soluble sugar in the leaves.Under simulated drought stress,the starch content in roots was significantly decreased by added silicon,but it was not affected by added silicon in the leaves.Under simulated drought stress,silicon treatment increased the activities of sucrose catabolism-related enzymes(sucrose synthase and invertase)in the leaves and roots.Similar phenomenon was also observed on the change of sucrose synthesis-related enzymes(sucrose-phosphate synthase and sucrose synthase).These results suggest that,silicon addition could enhance sucrose synthesis and catabolism under simulated drought stress and increase the contents of sucrose and hexose,which might have improved the osmotic adjustment ability of plants and guaranteed energy supply of tomato plants under stress conditions.3.Under the simulated drought stress,the activity of nitrite reductase was significantly increased by silicon in the leaves.Glutamine synthetase and glutamate synthase cycle is the main pathway of plants for the conversion of NH4+ to available organic nitrogen.Silicon treatment significantly increased the activities of these two enzymes,which were decreased under simulated drought stress.The activity of glutamic-oxaloacetic transaminase was also notably increased by silicon under simulated drought stress.These results indicate that application of exogenous silicon could improve the nitrogen metabolism of tomato plants under stress.Under the simulated drought stress,silicon changed the levels of individual free amino acids in the leaves and roots,and it inhibited the increase in total amino acid content induced by stress.Under the simulated drought stress,the total nitrogen content was significantly decreased,and addition of silicon had little effect on it.The accumulation of the free amino acid under simulated drought stress may be attributed to stress-caused disorders of nitrogen metabolism,whereas added silicon improved the the nitrogen metabolism and inhibited the accumulation of free amino acid in tomato plants.In conclusion,exogenous silicon may improve the drought tolerance of tomato seedlings through mediating the carbon and nitrogen metabolism. |
