Toxicity Effect of Cr Stress on Seed Germination and Seedling Growth in Lactuca Sativa

The impact of Cr6+ on the growth of lactuca sativa in Greenhouse Cucumber was investigated. The seeds of lacuna sativa Italian bolting resistance lettuce were treated by different Cr6+ concentration to study the effects on its seed germination and seedling growth. The results showed that the seed germination rate, vigor index of seedlings decreased with increment of Cr6+ concentration to varying degrees, and vigor germination, vigor index, raw weight, root length significantly lower. The absorption of lettuce seedlings on different nutrient elements is impacted by the concentration of Cr6+.


Introduction
As one of the toxic materials that has a wide range of existence in the environment, chromium pollution degree becomes more and more grievous and now Cd is the main pollution element in the nature ［1］ .Cr 6+ is harmful to human which can spoil human's health .Because of its great toxicity, Cr 6+ is carcinogenesis to human [2].Plant's roots have a great effect of absorbing and enriching Cr 6+ ,so chromium pollution has endangered the growth of the crops and seriously affects crops production [3,4].In order to identify the degree that different Cr 6+ concentration threat lettuce, we depend on the method--ecologicaltoxic effect method [5].The more serious the degree of pollution, the heavier the harm to the crop and even make plants die [6]. A large number of studies show that Cd has a prevailing threaten effect to crops, plants in the nature and mainly perform in following aspects:Low concentration of Cr is helpful to their germination and growth; high concentration of Cr shows the threaten effect. Great different tolerance [7] are shown between different corps. Plants enrich the heavy metal during its growing and development. Average chromium content of normal plants is 0.2mg/kg dry weight but chromium excess will produce poisonous effect to plants [8].Wu Yixue's study [9]shows that with the rise of Chromium concentration, the threaten effect to lettuce is gradually strengthened and inhibits growth of lettuce, even causes plants death, especially has an obvious influence on basic seedling's index.The effect to radicle is greater than seedling.There is a wide range of researches on chromium harm but all those researches are focus on heavy metal's enriched effect to corps, threaten effect, physiological and biochemical effect、 tolerance of corps,heavy metal's fouling mechanism、remedy and prevention to environmental pollution [10]. This experiment uses the method of Hoagland solution which including Cr and sandy cultivation, cultivates lettuce in illumination box for simulating growth environment of lettuce, observes status of lettuce growth, measure lettuce seeds germination, growth state of roots、seeds and leaves. In order to provide scientific basis of lettuce production、 improving quality、 Chromium prediction,this experiment uses statistical analysis to study lettuce status of germination and growth with different concentration of Cr.

Material and Experimental Processing
Lettuce seeds that the experiment used is Italian very late bolt lettuce produced by Nanjing Sheng Fa Seedling limited Company.
Seed Disinfection:Soak lettuce seeds in water overnight for fully water absorption,use 30% sodium hypochlorite(30% sodium hypochlorite is prepared by volume proportion )to disinfect for 5min, use clear water to wash cleanlily(until no smell of sodium hypochlorite,about 10 times).
Germination Cultivation：Selecte the same size plumping seeds to put on the culture dishes that have 2 layers of gauze, each dish sets 15 seeds. Sock lettuce seeds into culture medium with filter paper wetted by chromium solution.Cultivate seeds in constant temperature light incubator that has 16 hour light and 8 hours dark. Each day supply distilled water to keep filter paper humid and each 2 days supply nutrient solution including chromium.Cultivate for 7 days then observe and count growth and germination status of lettuce seeds.

Testing Items and Method
According to previous research, count 3d germination energy, 7d germination rate.Germination test lasts 7 days, count sprouting number,main root length,seedling fresh weight in the end of the test.
Germination Stop cultivating seedlings until seedlings have 4-5 leaves and show obvious difference.Determinate fresh weight of seedlings ,put seedlings in envelope then let envelope in oven.Until temperature of oven is up to 105℃,begin timing. After deactivation of enzymes for 15min,adjust temperature of oven to 85℃.Remove the dry matter that were dried to constant weight and weight dry matter with electronic balance( electronic balance is accurate to 1/10000 gram).Measure indexes of root length of seedlings,chromium concentration of seedlings,nutrient absorption of seedlings,etc. From Table 1,chromium concentration doesn't has a significant influence on germination energy and germination rate of lettuce seeds but each one of them is decrease in treated sample and has a trend that decrease as the chromium concentration is increasing when its concentration is about 0-20mg/L.When chromium concentration is more than 40mg/L, germination energy and germination rate has a significant influence between chromium concentration that is about 0-20mg/L.When chromium concentration is more than 40mg/L，each treated sample doesn't have difference.

Different concentrations Cr 6+ forces tolactucasativa's germination index (GI) and vigor index (VI)
Table2 different concentrations Cr 6+ forces tolactucasativa's germination index and vigor index Tip: The different lowercase letters after numbers of the same column means that the significance of level of 5%, The same below.
From Table 2,Cr 6+ doesn't have a significant effect of lettuce seed's GI. The effect of VI shows a decrease trend as chromium content is decreasing. When chromium has a significant difference on vigor index when its concentration is up to 10mg/L.If chromium concentration is more than 40mg/L,it will show a significant difference on treated samples of CK,5,10,20mg/L.When chromium concentration is more than 40mg/L, there is no significant difference between each two of treated samples. From Table 3, different concentration of Cr 6+ shows a significant difference on cultivation of lettuce seeds. will inhibit the growth of root and show withered symptom.From Table 3,control group show significant difference concentration of Cr 6+ is more than 5mg/L.When concentration of Cr 6+ is more than, there is no significant difference on each treated samples. By the analysis of the data which comes from inhibition rate of root length, inhibition rate of root length is increasing as concentration of Cr 6+ is increasing, which manifests that growth of root and stem became slow, when concentration of Cr 6+ is more than 40mg/L,inhibition rate of root length is more than 90%,radicles can't form and seedlings aren't vertical.

Different concentrations Cr6+ forces to lactucasativa's root length and leaf length
Table4 different concentrations Cr 6+ forces to lactucasativa's root length and leaf length Tip: means that the plants is dead in the treated ,the data is not shown. From Table 4,the growth of lettuce seedlings has a significant difference with different concentration of Cr 6+ and growth tendency of root-leaf is decrease as concentration of Cr 6+ is increasing. With chromium concentration is increasing, growth of lettuce seedling's root-leaf is continuous reduction,leaves are even withered in high chromium concentration. When chromium concentration is 40、60mg/L,lettuce seedlings can't sprout and growth. Table 4 shows the data analysis is more accurate, when chromium concentration is more than 5mg/L each treated sample has significant difference between blank control sample and each treated sample has significant difference from themselves.
From Table 3 and Table 4,growth of lettuce seedling roots have significant difference with threaten of Cr 6+ ,which shows that length of root become shorter and growth of root become worse, even seedling roots can't form lateral root system.From those data, high concentration of Cr 6+ has a great threaten effect to the growth of lettuce seedling.  Table 5,fresh weight and dry weight of lettuce seedlings dramatically reduce as chromium concentration is increasing and fresh weight and dry weight of lettuce don't change a lot when chromium concentration is more than 10mg/L.

Effect of different concentrations Cr 6+ stress on absorbing NPK of lactuca sativa
Fig2 Effect of different concentrations Cr 6+ stress on absorbing NPK of lactuca sativa NPK of lettuce seedling comes from nutrient solution. From Fig 2 we can see the trend of total nitrogen、potassium and phosphorus concentration , nitrogen and potassium concentration in seedlings are decreasing as chromium concentration is increasing. When chromium concentration is in 0 to 10mg/L, their concentration decreases little. When chromium concentration is more than 10mg/L,it has a large amplitude of influence which explains that chromium concentration has a great inhibitory effect to the absorption of N、P. When chromium concentration is more than 10mg/L, it will have a strong toxicity and will effect the growth of seedlings.
Phosphorus concentration is increased at first then decreased and the change range is smaller than total nitrogen 、 potassium's. In the chromium concentration of 0-10mg/L, total phosphorus concentration is increased in a little range. When its concentration is up to 10mg/L, phosphorus concentration of lettuce seedlings decreases gradually which explains that 10mg/L of phosphorus concentration has a strong toxicity to lettuce seedlings.

Conclusion
In germination experiment,the significance of germination energy,germination rate,germination index ,vigor index become large as the chromium concentration is increasing and show an obviously decrease trend. With chromium concentration is increasing, Inhibition of root is enhanced even seeds can't form radicle and chromium concentration has a great significance on the growth of roots.
In the experiment of seedlings's cultivation, the growth of plants is worse and worse even can't survive. The threaten of chromium to seedling root is more serious than its to stem and leaf. It also inhibits the growth of root. With the increase of chromium concentration, the absorption and enrichment become gradually stronger; lettuce has a more concentration of chromium.
Because lowest concentration of Cr 6+ is 5mg/Land the data that the effect of low concentration of Cr 6+ to lettuce sprouting and growth of lettuce seedling ,so we need further research and observation.