Comparison of lead tolerance in Allium cepa with other plant species

Our previous studies demonstrated increased lead tolerance of Allium cepa adventitious root tip cells during treatment with lead. We then compared the degree of tolerance to lead of A. cepa and 22 groups of plants (species, varieties, populations). We showed that initially, constitutional tolerance of A. cepa to lead was moderate, then high, similar to inducible tolerance of calamine waste heap plants. It is noteworthy that this high tolerance was characteristic for only A. cepa plants developing from bulbs. Plants developing from seeds demonstrated a lower tolerance to lead (by 7 to 45%). The high constitutional tolerance of A. cepa growing from bulbs was related to the possibility of transporting large amounts of lead (40%) from the roots to the bulb. These results point to the role played by the storage organ (bulb) in raising the tolerance of A. cepa to lead, which is demonstrated for the first time in this study, and which is interpreted as an element of constitutional tolerance to lead. It was found that the constitutional tolerance of crop plants ranged from 78 to 2%, depending on the species, variety, development stage, and duration of treatment with lead. The inducible tolerance of plants from the waste heap ranged from 130 to 10%. In these plants, the increased tolerance resulting from the adaptation to growing on a polluted waste heap averaged 18.5%. The following order of decreasing tolerance to lead was found: I. Species with the highest tolerance, i.e. with inducible tolerance, from the waste heap: Silene vulgaris>Biscutella laevigata>Leontodon hispidus. II. Species with high constitutional tolerance: B. laevigata natural population>A. cepa growing from bulbs>Hordeum vulgare>Zea mays. III. Species with intermediate, most common, constitutional tolerance: A. cepa growing from seeds>Triticum vulgare>Pisum sativum>Cucumis sativus>Lupinus luteus>Raphanus sativus>Secale cereale. IV. Species with low constitutional tolerance: Brassica napus>Soja hispida>Phaseolus vulgaris. The species, varieties and populations differed considerably among each other in terms of lead uptake. Although no relationship was found between the degree of tolerance to lead and the amount of lead in a plantʹs tissues, the pattern of lead distribution was similar in almost all of the plants.