| Plants encounter various environmental stresses conditions in life circle. It has been estimated that about one-third of the world's potentially viable land suffers from an inadequate supply of water. Drought stress causes various physiological and biochemical effects on plant. As adaptive response,plants can effectively adjust their physiology and metabolism to reduce stress injury. As secondary metabolites in organisms, Polyamines(PAs)are ubiquitous aliphatic cations of low molecular weight, mainly including putrescine(Put),spermidine(Spd),spermine(Spm),have long been recognized to be linked with growth, differentiation, senescence of plants and defense reaction of plants to various environmental stresses.In recent years, there are many studies on the relationship between PAs and plant tolerance to various stresses, such as hypoxia stress, salt and so on. Under many kinds of stress conditions,polyamine content in plants changes rapidly.However,the mechanism is still not well established in physiological regulation role of polyamine in its adaptation to drought stress in the vegtables.In this paper, therefore, two different drought-tolerant tomato cultivars seedlings were investigated under drought stress induced by 7.5% polyethylene glycol (PEG-6000) in a nutrient solution culture system. The relationship between endogenous PA levels and drought stress as well as the mechanisms for the protection of vegtable plants were systemically studied.The results are as follow:1. The relationship between endogenous hormonal ABA, JA and free polyamines metabolism under drought stressTime-course of changes in JA and ABA concentrations were monitored during 48 h in leaves of tomato seedlings exposed to 10% PEG-induced drought treatment. Concentrations of endogenous ABA, JA, free polyamines and the activities of PAO in roots and leaves of both genotypes seedlings were generally higher when compared with their respective controls under PEG-stressed. It was found significantly positive correlation between endogenous polyamine contents and PAO activity (R=0.708**) in roots and leaves of seedling. The time course demonstrated that naturally increasing of ABA concentrations of roots occurred prior to that of leaves. It was concluded that higher concentrations of endogenous Spm and Spd in roots and leaves stimulated rapidly accumulation of endogenous ABA and JA simultaneously with increasing PAO activities under drought stress.2. The effects of different concentrations of exogenous Spermidine on drought-resistance in tomato seedlings under drought stressThe most suitable concentrations of Spd was 0 .1 m mol/L, respectively, when two different drought-tolerant tomato cultivars seedlings was treated with different concentrations of exogenous Spd (0, 0 .01, 0.1, 1 m mol/L). Excessive amounts(1 mmol/L) of Spd was toxic to tomato seedling. The most suitable concentrations of exogenous Spd could mitigate the injuries caused by PEG stress by promoting the activities of some antioxidant enzymes and decreasing them embrane lipid peroxidation and keeping low ROS levels under drought stress in tomato seedlings.3. The effects of Exogenous Spermidine on photosynthetic gas exchange in tomato seedlings under drought stressDrought stress caused a significant decrease in the net photosynthetic rate(Pn), transpiration rate(Tr), stomatal conductance(GS)and intercellular CO2 concentration(Ci) of tomato seedlings. The intracellular CO2 concentration(Ci) was exhibited higher significantly than the control during the later period of drought stress,which suggested that the decrease in Pn in tomato seedlings leaves mainly resulted from the stomatal limitation during the earlier period of stress,however,non-stomatal limitation during the later period.Compared with the drought treatment, exogenous Spd increased Pn, Gs and Tr in two cultivars tomato seedling leaves,and decreased Ci during the later period under drought stress.These results indicated that exogenous Spd could improve photosynthesis of tomato seedlings by preventing stoma closure and stimulating CO2 uptake. The mitigative effects of exogenous Spd in drought-sensitive cv. HuangGuan were greater than those in high drought-resistant cv. MaoFen802. In addition, exogenous Spd could raise the water use efficiency, the water use efficiency of MaoFen802 were greater than those in drought-sensitive cv. HuangGuan under the same condition.4. The effects of exogenous spermidine and biosynthesis inhibitors on reactive oxygen levels and antioxidative systems of tomato seedlings under drought stressTwo different drought-tolerant tomato cultivars seedlings were investigated under drought stress induced by 7.5% polyethylene glycol (PEG-6000) in a nutrient solution culture system. The results showed that exogenous spermidine (Spd)(0.1 mmol·L-1 ) treatment decreased O2-·production rate,MDA and H2O2 contents and raised SOD,POD,CAT,APX and GR activities in tomato seedlings, and had no effect on MDHAR and DHAR activities. Spd treatment in'Huangguan'seedlings with lower drought-resistance was more effective than that in'Maofen No. 802'seedlings with stronger drought-resistance. It indicates that exogenous Spd treatment could mitigate the injuries caused by PEG stress by promoting the activities of some antioxidant enzymes and the contents of antioxidant substances, decrease them embrane lipid peroxidation and keeping low ROS levels under PEG stress in tomato seedlings. In summary, exogenous PAs enhanced the antioxidative ability of plant, protected the biomembranes of cell, alleviated the damage of PEG stress to the plants, and enhanced the adaptability of tomato seedlings to drought stress.The spermidine biosynthesis inhibitors (D-Arg and MGBG)decreased SOD, POD activities and raised O2-·production rate,MDA and H2O2 contents in two different drought-tolerant tomato cultivars seedlings, which indicated D-Arg and MGBG affected activities of some antioxidant enzymes and embrane lipid peroxidation and metabolism of ROS in tomato seedlings. The inhibiting effect of D-Arg could be reversed partly by the combination with exogenous Put which indicated that ADC pathway was important for Put biosynthesis in PEG-stressed tomato seedlings,and the combination of exogenous Spd or Spm could reverse the effects of MGBG on tomato seedlings.The effects of the spermidine biosynthesis inhibitors in high drought-resistant cv. MaoFen802 were greater than those in drought-sensitive cv. HuangGuan.5. The effects of exogenous spermidine and biosynthesis inhibitors on metabolism of nonstructural carbonhydrate and involved activity of enzymes of tomato seedlingsUnder drought treatment,nonstructural carbonhydrate of two tomato cultivars seedlings increased, including glucose, fructose, sucrose, the content of starch decreased. During drought stress of 7 days, the acyivities of sucrose synthase (SS) of leaves climbed up firstly, and then declined, whereas the acyivities of sucrose phosphate synthase (SPS) increased gradually. Compared with the drought treatment, exogenous foliar spraying Spd increased the contents of sucrose,glucose and fructose in leaves,and decreased the content of starch in leaves,In addition, exogenous spermidine increased activities of sucrose synthase and sucrose phosphate synthase. These two enzymes cooperated in order to accelerate the synthesis of sucrose, so that the sucrose and hexose accumulated which lowered the osmotic potential of cells, the osmotic potential decreased subsequently, avoiding the severe dehydration of cells, maintaining turgor pressure of cells, and the ability of seedlings resisting drought stress was improved. It indicated that regulation of carbohydrate metabolism with exogenous Spd was an important mechanism of alleviating the harm of drought stress to tomato seedlings.The spermidine biosynthesis inhibitors (D-Arg and MGBG) decreased the contents of starch, glucose, fructose and sucrose in two different drought-tolerant tomato cultivars seedlings, which indicated D-Arg and MGBG affected the metabolism of nonstructural carbonhydrates in tomato seedlings. The inhibiting effect of D-Arg could be reversed partly by the combination with exogenous Put which indicated that ADC pathway was important for Put biosynthesis in PEG-stressed tomato seedlings,and the combination of exogenous Spd or Spm could reverse the effects of MGBG on tomato seedlings.The effects of the spermidine biosynthesis inhibitors in high drought-resistant cv. MaoFen802 were greater than those in drought-sensitive cv. HuangGuan. |
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