Effect Of Sulfur Nutrition On Root Hydraulic Conductance Of Crops Under Heavy Metal Stress

Heavy metal pollution deteriorates soil environmental quality, and this is very harmful to the circle of ecosystem and the environment which human live in; furthermore, the pollution mightily affects plant root system and plant growth. Root is the organ which perceives soil pollution directly; heavy metal in soil could influence root's growth and function, and consequently impacts whole plant growth status. Coppor and zinc are required microelements of plant. However, when concentrations are excessive, they could poison plant as well. Mercury, copper and zinc are main heavy metals influencing soil quality. The ability of absorbing water by root system plays vital role in plant growth. Lp_r reflects this ability and is one of the most direct physiology indexes of root to respond soil water. Does heavy metal stress affect plant root Lp_r? Is there dose—effect relationship between heavy metal stress and Lp_r? It's a key problem of agrology and environmental science, but correlative reports are rarely. Sulfur has unreplaceable effects on physiological function of crop. Sulfur is the main ingredient of Cysteine, Cystine and Methionine, and these amino acids are main ingredient of protein. When Sulfur is inadequate, normal physiological activities, yield, adaptive ability to changing environment will be impacted. Under heavy metal stress, organical sulfur compounds in plant are important to resisting heavy metal stress. Then, does sulfur nutrition contribute to ameliorating water transport in plant and alleviating impacts of heavy metals on plant root system. This question needs further studies. In this research, corn and wheat were selected as experiment materials. By different heavy metal treatments, aquasolution and pot experiment, method in different sulfur levels, were carried out to determine the root hydraulic conductance (Lp_r) of the crops by the pressure chamber. The adjustive ability of sulfur on Lp_r was studied. The main results were as follows:1. Compared to CK, Hg²⁺, Zn²⁺ and Cu²⁺ obviously restrained maize and wheat root Lp_r. The order of impact extent is: Hg²⁺>Cu²⁺>Zn²⁺, and with heavy metal stress time rising, Lp_r fell more sharply. Hg²⁺, as protein inhibitor generally, restrains AQP expression, and this is the main reason of influencing plant absorbing water. With Hg²⁺ concentration rising, the inhibitive effect enhances. Under Cu²⁺ stress, though there is a sharp fall of Lp_r as well, the difference between 200μmol/L Cu and 100μmol/L Cu is not clear. This may be due to different restraining mechanism of Cu²⁺ and Hg²⁺.2. In aquasolution experiment, without heavy metal stress sulfur seems to provide nutrition to maize root system; when there is Cu²⁺ intimidating, sulfur reduced the inhibitive effect resultful, and this protective function enhanced with sulfur concentration increasing.3. Stress time is an important factor of reducing Lp_r by heavy metals. Under all sulfur concentrations, Lp_r of intimidated 30min was less than 50% of that of 10min in same sulfur concentration, showing that heavy metal intension affects water transport significantly. However, the effect is alleviated after strengthening sulfur nutrition, indicating sulfur makes sense in resisting heavy metal pollution.4. In pot experiment, under low sulfur treatment Zn²⁺ stimulated root dry weight, ratio of root/shoot rising, but Lp_r reduced. After strengthening sulfur nutrition, these impacts were not lightened clearly, showing that Zn²⁺ stress may change water transport mechanism of wheat root.5. In pot experiment, according to Third Class Farmland Environmental Quality Standard for Soils, 500mg/kg Zn and 400mg/kg Cu were added into the soil. Under low sulfur treatment, the Lp_r of wheat root system reduced remarkablely, the inhibit-ratio is: Cu²⁺ 34.61% and Zn²⁺ 36.89%. After adding sulfur nutrition on the proportion of N∶P2O5∶K2O∶S = 0.2∶0.15∶0.3∶0.015, the inhibit-ratio changed to: Cu²⁺ 12.29% and Zn²⁺ 37.71%. Lp_r falls down sharply under Cu²⁺ stress, but sulfur nutrition could reduce this impact significantly, putting up the important role sulfur nutrition plays on resisting heavy metal stress.