Sensitivity Of Different Vegetable Seedlings To Lead

Vegetables, being indispensable food in people's daily life, are also very important cash crops. With the improvement of people's living standard, demand of people for"pollution-free vegetables"becomes stronger and stronger. However, vegetable origin has been seriously polluted by heavy metals. Among these heavy metals, lead is one of the most pervasive elements. Therefore, more and more attentions have been paid to Pb pollution in the garden soils.In this study, seven representative vegetables which are widely planted were selected, i.e. leafy vegetables (Chinese cabbage and cabbage), fruit vegetables (cucumber, cowpea and tomato) and root vegetables (radish and lettuce). A water culture experiment was conducted to evaluate the influences of Pb on the growth and development of these vegetable seedlings, including seed germination rate, seedling emergence rate, root length, shoot height and biomass (fresh weight). Physiological and biochemical responses of these vegetables to Pb were also studied by determination of root activity, chlorophyll content and catalase (CAT) activity. Furthermore, characters of Pb uptake and accumulation in different types of vegetables grown in hydroponic solutions were studied by determination of the Pb contents in the roots and shoots. Thus, the sensitivity to Pb of various vegetable seedlings were inferred, which will afford some useful information to safety of vegetable products and adjustment of planting structure. The main results were summarized as follows:1. In the early stage of vegetable growth, the edges of leaves were turning yellow and the whole leaves became curled due to high Pb concentrations. However, as time going, leaves in different treatments had no significant differences between each other. Roots were the most sensitive organs under Pb pollution, as the stems and leaves were much less harmed than roots by Pb. Lower concentrations of Pb played a stimulative role in the root growth of cabbage, lettuce, cowpea and tomato as showing by the root length. There was a highly significant negative correlation between Pb concentrations and root length of Chinese cabbage, radish, cowpea, cabbage and lettuce (P<0.05), respevtively. The seed germination and seedling emergence of cucumber, cowpea and tomato were inhibited remarkably under the high Pb concentration treatments. The ascending sort of Pb toxicity to seedling growth and development of tested vegetables was: cowpea> cucumber> Chinese cabbage> tomato> cabbage> radish> lettuce.2. Root activity, chlorophyll content and catalase activity variated between vegetable species. Under the higher Pb concentration treatment, the root activities of cowpea and lettuce increased with the increasing of Pb concentration, and the contents were higher than that of control. While the root activities of the other five vegetables increased under low Pb concentration and decreased under higher Pb. Lower concentrations of Pb stimulated the chlorophyll synthesis and photosynthesis, but chlorophyll content started to decline with the increasing of Pb concentration, thus, ruduced the photosynthesis. The CAT activities in cucumber and Chinese cabbage increased under low Pb concentrations, but it decreased gradually in Chinese cabbage with the increasing of Pb concentration. The CAT contents in other six vegetables increased by different degrees under high concentrations of Pb.3. With the increasing of Pb concentration, Pb contents in roots, stems and leaves of seven vegetables gradually increased, while the highest contents were observed in roots. For all the vegetable seedlings, Pb contents in roots and shoots had highly positive correlation with the Pb concentrations (P<0.05), and the correlation coefficients were all above 0.90. The differences of Pb accumulation depended on vegetable types and Pb concentrations in solutions. Taking Pb treatment of 8.0 mg·L^-1 as an example, the ascending sort of Pb accumulation capacity in the roots of these tested vegetable seedlings was: lettuce> cucumber> cabbage> Chinese cabbage> tomato> radish> cowpea, and the ascending sort of Pb accumulation capacity in the shoots was: lettuce> cabbage> Chinese cabbage > tomato> radish> cucumber> cowpea.