Studies On Cadmium Tolerance And Detoxification In Tall Fescue And Kentucky Bluegrass

The plant tolerance to Cadmium(Cd) is one of the important bases in phytoremediation. Some turfgrass species showed the high tolerance to soil Cd contamination and the great potential in phytoremediation. In the present study, we evaluated comprehensively on Cd tolerance and accumulation in tall fescue(Festuca arundinacea), Kentucky bluegrass(Poa pratensis), perennial ryegrass(Lolium perenne) and creeping bentgrass(Agrostis stolonifera), compared the capability of Cd accumulation of tall fescue, kentucky bluegrass and a Cd hyperaccumulator(Solanum nigrum L.). We selected tall fescue(Cd-tolerant) and kentucky bluegrass(Cd-sensitive), which were significantly different in Cd tolerance, to study the mechanisms of Cd hypertolerance and detoxification. The main results are as follows: 1.Cadmium tolerance and accumulation in four cool-season turfgrassesTall fescue, Kentucky bluegrass, perennial ryegrass and creeping bentgrass were exposed to Cd(0, 50, 100, 200, 400 mg Cd kg-1) for 60 days, and four cool-season turfgrasses showed high tolerance and accumulation capability on Cd contaminated soil. As indicated by the data of turf quality, relative growth rate, turf density, shoot and root biomass, leaf electrolyte leakage, leaf relative water content, leaf Fv/Fm, four cool-season turfgrasses showed significant difference to Cd tolerance. Tall fescue was most tolerant to Cd stress, followed by perennial ryegrass Kentucky bluegrass and creeping bentgrass. Relative growth rate and root biomass were most sensitive parameters in all measurements of this study. The Cd concentration and accumulation in both shoots and roots were positively correlated to the increases of Cd stress. Kentuchy bluegrass accumulated the highest Cd concentration in the shoot, followed by tall fescue, creeping bentgrass, perennial ryegrass. However, perennial ryegrass accumulated the highest Cd concentration in the root. Under 50~400 mg Cd kg-1 stress, Kentucky bluegrass showed the highest translocation factor and phytoextraction rate, whereas perennial ryegrass was the lowest. Kentucky bluegrass showed the higher Cd accumulation capacity in shoot than other three grass species. 2.Comparison of cadmium tolerance and accumulation in tall fescue, kentucky bluegrass and black nightshadeTall fescue(Cd-tolerant) and kentucky bluegrass(Cd-sensitive), which were significant different in Cd tolerance and accumulation, were selected to compare with black nightshade(Solanum nigrum L.) in their Cd tolerance and accumulation. The plants were exposed to Cd(0, 40, 80 mg Cd kg-1) for 63 days. Results showed that Cd inhibited the growth of all plants. Under 40 mg Cd kg-1 treatment, the shoot and root biomass of black nightshade were significantly declined, while the biomass of Kentucky bluegrass and tall fescue did not show significantly decline when compared with the control. The total Cd accumulations in Kentucky bluegrass, tall fescue and black nightshade was 418.6, 383.4 and 9.4 mg m-2 under 40 mg Cd kg-1, and 1114.9, 761.5 and 63.7 mg m-2 under 80 mg Cd kg-1, respectively. Under 40 and 80 mg Cd kg-1 treatments, the phytoextraction rate was 2.10% and 4.64% in Kentucky bluegrass, and 0.71% and 1.40% in tall fescue, respectively. Black nightshade had the lowest phytoextraction rate, only 0.12% and 0.43% under 40 and 80 mg Cd kg-1 treatments. The results showed that the Cd tolerance and accumulation by Kentucky bluegrass and tall fescue were much higher than black nightshade, suggesting that Kentucky bluegrass and tall fescue have potential phytoremediation capability in Cd contaminated soils. 3.Chemical forms of cadmium in the plants tissues between tall fescue and Kentucky bluegrassChemical forms of Cd in different plant tissues are directly connected with the mechanism of Cd absorption, translocation and accumulation, which are important to understand the mechanism of detoxification of the heavy metals. There was no significant difference observed in root Cd concentration between Kentucky bluegrass and tall fescue, but the shoot Cd concentrations in Kentucky bluegrass were significantly higher than that in tall fescue. No significant difference of Cd concentration in rhizodermis + cortex tissues was observed between tall fescue and Kentucky bluegrass. However, in stele, the Cd concentration in Kentucky bluegrass was higher than that in tall fescue, indicating that more Cd was translocated into stele of roots for further long-distance transport in Kentucky bluegrass. In leaves, both Cd concentrations in vascular bundle and epidermis + mesophyll tissues of Kentucky bluegrass were significantly higher than that of tall fescue, showing that Kentucky bluegrass transported more Cd into the shoot. By analyzing the Cd chemical forms in the plant tissues, we found that a large proportion of Cd accumulated in the leaves was in forms of undissolved Cd phosphates in both grass species, which could be involved in the Cd detoxification. The superior Cd tolerance of tall fescue to Kentucky bluegrass was associated with less Cd translocation into the stele of roots and less Cd transported to shoots. 4.Cd distribution characteristics in the tissues between tall fescue and Kentucky bluegrassCd distribution characteristics in the tissues of tall fescue and Kentucky bluegrass were observed by histochemistry and fluorescence methods. Cd was absorbed on the surfaces of rhizodermis of root and root hair before entering into root tissue. Histochemistry and fluorescence methods showed that more Cd was found in the stele of root in Kentucky bluegrassthan that in tall fescue, in which was agree with the results from the Cd tissue measurement. In leaves, more Cd was accumulated in over the tissues of vascular bundle, mechanical tissue of vein and stratum corneum layer of epidermis. The Cd fluorescence in the stratum corneum layer of tall fescue was stronger than that of Kentucky bluegrass, while the fluorescence in the vascular bundle was weaker than that of Kentucky bluegrass, indicating that tall fescue could involved in a Cd discharging mechanism, thereby alleviating Cd tolerance.5.Cd accumulation in different leaves between tall fescue and Kentucky bluegrassThe shoot leaves of tall fescue and Kentucky bluegrass were separate into four portions: the young growing leaves, the function leaves, the old degenerating leaves and the withered leaves. The results found that the young and function leaves accumulated the lowest Cd concentrations, while old and withered leaves accumulated the significantly higher Cd concentrations than young leaves and function leaves in both grass species. it’s the old leaves accumulated 3(tall fescue) and 11 times(Kentucky bluegrass) in Cd concentrations than young and function leaves, respectively. The Cd concentration in withered leaves was even higher. By Cd histochemistry observation, we found that Cd was mainly accumulated in stratum corneum layer and vascular bundle in both young and function leaves. However, lots of Cd was observed in the mesophyll cells in old leaves. The most of Cd translocated into the old and withered leaves, indicating that there could be a young leaf protection mechanism involved in tall fescue and Kentucky bluegrassunder the Cd stress. Chelants was reported to accelerate Cd translocation in the plants. In this study, NTA showed the positive effects in enhancing the Cd concentrations into shoots, especially to the old and withered leaves in tall fescue, but no such effects observed in Kentucky bluegrass. 6.Cd accumulations in response of the stress period between tall fescue and Kentucky bluegrassThe Cd concentrations in the shoots and roots were positively correlated to the stress period in both grass species. Under the prolonged stress period, Cd concentration increase in the shoots was higher than that in the roots, showing a gradually increase of the translocation factors in both tall fescue and Kentucky bluegrass. The Cd translocation factor in tall fescue was significant lower than Kentucky bluegrass. The translocation factor of tall fescue was reached above 1.0 after 98 days stress period, while in Kentucky bluegrass only need 28 days of stress period. In tall fescue, NTA increased the translocation factor before 56 days of stress period, while in Kentucky bluegrass the increases only occurred after 56 days of stress period.