| Cadmium (Cd) is a highly toxic heavy metal. Cadmium is introduced into contaminated soils through industrial emission, atmospheric deposition, sewage irrigation, application of sludge and fertilizer contaminated with Cd, which results in the severe Cd pollution of the fields throughout China. The vegetable soils contaminated with cadmium are also widespread in China, especially in suburban areas. Cadmium will affect crop growth, production and quality; in addition it will transfer to the edible part of crops via soil-plant system to endanger the health of human beings through food chain. The soil cadmium contamination control technologies mainly include engineering method, chemical remediation, phytoremediation and agronomic regulation. Engineering method is not economically feasible, and is likely to cause secondary pollution. Meanwhile, the remediation effect of chemical method is unstable, and the remediation period of phytoremediation is relatively long compared with other methods. Consequently, researchers are attaching more importance to agronomic regulation practices. It’s considerable to apply agronomic regulation measures to alleviate toxicity of Cd to crops as well as to inhibit its translocation to crop’s edible part in soils of light Cd contamination. Foliar spraying regulation belongs to agronomic measures whose principle is utilize the antagonism between exogenous chemicals and Cd2+to alleviate its damage to plants and to prevent Cd uptake by plant root as well as its transfer to edible part. Zinc (Zn) and iron (Fe) are both essential microelements for plant growth which play crucial roles in many physiological activities. Zinc, iron and cadmium have similar bio-geo-chemical characters for they are all divalent cations. The Zn/Cd and Fe/Cd interaction have always been the hotspots of research. Some reports showed that exogenous application of Zn or Fe could reduce Cd absorption and accumulation in crops, but the results were not consistent. There exists a great difference in Cd accumulation among cultivars except for the difference among crop species. In order to probe the interactions of Zn/Cd and Fe/Cd, and the effects of foliar application of Zn and Fe on Cd absorption and its transfer to edible part (fruits) in different varieties of tomato (Solatium lycopersicum Mill.), we used two tomato varieties (’4641’and’Yufen109’) planted widely in Chongqing, China, and pot culture experiments were carried out to investigate the effects of foliar application of different levels of Zn (0,50,100,200and400μmol·L-1) and Fe (0,50,100,200and400μmol·L-1) on biomass, photosynthetic characters, activities of antioxidant enzymes, chemical forms and accumulation of Cd, fruit quality, and nutrients absorption in tomato when exposed to Cd (10mg·kg-1). Moreover, we have separated several genes related to cadmium accumulation and tolerance in tomato with semi-quantitative RT-PCR (Reverse transcription PCR) technology to inquire premiminarily into the molecular mechanisms of cadmium accumulation differences among genotypes of tomato so as to provide theoretical basis for the regulation of Cd contamination of vegetable soils and for the reasonable distribution of vegetable production in Cd-contaminated soils. The main results are summarized as follows:1. Foliar spraying moderate Zn (≤200μumol·L-1) increased dry weights of fruit, roots, stem, leaves and plant in both tomato cultivars under Cd contamination; while excess Zn would decrease dry weight of tomato plant compared with moderate Zn application. Low Zn application (100μumol·L-1) enhanced the net photosynthetic rate (Pn), the stomatal conductance (Gs) of tomato leaves in both cultivars, and the transpirate rate (Tr) in’Yufen109’leaves, whereas the Pn, Gs and Tr of tomato leaves in both cultivars then decreased significantly with high Zn (400μumol·L-1). The activity of catalase (CAT) in roots of both varieties and the activity of peroxidase of’Yufen109’ increased firstly, and then followed a downward trend with increasing levels of Zn; while the activity of superoxide dismutase (SOD) in roots of both cultivars decreased at first, and then increased with increasing Zn levels, but reduced again with400μumol·L-1Zn. Cadmium was mainly existed in the forms of hydrochloric acid-extractable Cd (FHCl), residual Cd (FR), and sodium chloride-extractable Cd (FNaCl) in tomato fruit. Foliar spraying moderate Zn (≤200μumol·L-1) obviously reduced the contents of all chemical forms of Cd of tomato fruit in both cultivars compared to the control, whereas the contents of part of these forms presented an upward trend with high Zn (400μumol·L-1) treatment. Cadmium was mainly accumulated in leaves and stem of tomato with the contents of Cd in the order of leaves> roots> stem> fruits among different parts of tomato plant. Cd content of all parts of tomato plant and Cd accumulation of plant all decreased with foliar spraying Zn; and Cd content and accumulation of fruits were in the order of’4641’>’Yufen109’in the presence or absence of Zn. The contents of fruit free amino acid in both tomato cultivars and nitrate in’Yufen109’decreased with foliar Zn application, while that of reducing sugar in both cultivars and vitamin C (Vc) in’Yufen109’increased after Zn application; besides, the contents of nitrate in tomato fruit increased at first, and then reduced with increasing Zn levels. Zinc content of all plant parts increased significantly after Zn application; in addition, low Zn application increased the contents of nitrogen (N) and phosphorus (P) in roots and stem of two tomato cultivars, while lowered the contents of N and P in leaves and fruits of both cultivars as well as potassium (K) content in all plant parts; and the contents of copper (Cu), Fe, manganese (Mn), calcium (Ca), magnesium (Mg) were also affected inordinately with increasing levels of Zn. Cadmium was in high significant negative correlation with Zn in leaves, while no significant correlation between them was found in fruits, roots and stem. Besides, Zn was in high significant positive correlation with Cu and Fe in fruits, and in significant positive correlation with Cu in stem and leaves. 2. Foliar spraying moderate Fe (≤200μmol·L-1) increased dry weights of fruit, stem, leaves and plant in both tomato cultivars when exposed to Cd, while dry weights of all plant parts and tomato plant increased at first, and then decreased with increasing levels on Fe application. Pn, Gs and Tr of tomato leaves in both cultivars increased with moderate Fe (100μmol·L-1) application, and then reduced with excess Fe (400μmol·L-1); on the contrary, concentration of interculluar carbon dioxide (Ci) decreased firstly, and then increased with increasing application of Fe. As for the two tomato cultivars, both photosynthesis and transpiration of’4641’are stronger than that of ’Yufen109’. The activity of POD in roots of both cultivars and the activity of CAT in’4641’ increased at first, and then decreased with increasing levels of Fe; while the activity of SOD in roots of both cultivars and the activity of CAT in’Yufen109’decreased at first, and then increased with increasing levels of Fe. Cadmium was mainly existed in the fractions of hydrochloric acid-extractable Cd and residual Cd in fruits when subjected to Cd. Foliar application of different levels of Fe reduced the contents of all forms of Cd compared to the control. Alike Zn application, Cd was mostly accumulated in leaves and stem with the contents of Cd in the order of leaves> roots> stem> fruits with foliar spraying Fe. Cadmium content of all plant parts decreased with foliar Fe application compared to the control, while displayed an upward trend when excessive Fe was applied. Within the cultivars, it had more significant effect of foliar spraying of Fe on reducing plant Cd accumulation of’Yufen109’than that of’4641’. The contents of reducing sugar and nitrate in fruits of both cultivars increased after Fe application, while that of amino acid in’Yufen109’decreased compared to the control; besides, high Fe application (400μmol·L-1) facilitated the biological synthesis of Vc in fruit. Iron fertilization obviously increased Fe content of tomato shoots especially of leaves, and it’s more effective to enhance Fe accumulation of fruit in’4641’ than that in’Yufen109’. There existed difference in the effect of Fe/Cd interaction on plant N,.P and K absorption or transport. Furthermore, foliar spraying moderate Fe boosted the concentration of Cu, Zn, Mn, Ca and Mg in tomato leaves; and it also affected the contents of these elements in other plant parts, but not consistent. No significant correlation was found between Cd and Fe in all parts of tomato plant; and Fe was in high significant positive correlation with Cu and Zn in leaves and fruits.3. In this study, all of Cd accumulation or tolerance-related genes expressed in tomato leaves except for IRT (includes IRT1and IRT2). The expression quantities of MT2, OAS, HMA and CaM2in leaves of’4641’were higher than that of’Yufen109’, whereas the expression of NRAMP and PCS in leaves was in the order of’Yufen109’>’4641’; however, no significant difference was found between two cultivars in the expression of the other five Cd accumulation or tolerance-related genes except for MT2. |
PDF Full Text Request