|Belimov, A.A., Safronova, V.I., Tsyganov, V.E., Borisov, A.Y., Tikhonovich, I.A.,||
All-Russia Research Institute
|Soboleva, V.N., Kvokova, N.A., Zaitseva, L.N.,
Poddubnykh, O.N., Dolgaya, L.N.
and Sukhanov, P.A.
State Center of Agrochemical Service
Legume crops such as garden pea (Pisum sativum L.) are very sensitive to the toxicity caused by cadmium (Cd) and other heavy metals (HMs) (3, 4, 5). It was also shown that different cultivars of the same plant species can vary significantly in the resistance to and accumulation of HMs (1, 2, 6, 7). The aim of the present work was to study the variability in P. sativum for their resistance to Cd and uptake of HMs from contaminated soil.
A total of 99 wild accessions and primitive cultivars of P. sativum obtained from the World Collection of the Vavilov Institute of Plant Industry (St.Petersburg) were screened for the resistance to Cd in pot experiments using quartz sand culture. The plants were inoculated with Rhizobium leguminosarum bv. viceae CIAM1026 in the amount of 108 cells per seed and cultivated for 30 or 50 days. Cadmium was added to the sand as CdCl2 at the 18th day after sowing in the following concentrations (mg Cd/kg): 13 (lethal), 7 (highly toxic) and 5 (moderately toxic). The response of pea plants to 13 mg Cd/kg was estimated by time of plant death (determined by visual observation). Otherwise the response was determined as a ratio between dry biomass (roots and/or shoots) of Cd-treated versus untreated plants. Accumulation of HMs by pea varieties was studied when the plants were grown until the maturity stage in pots filled with 5 kg derno-podzolic soil supplemented with HMs in the amount of (mg/kg): Cd - 5, Cu - 50, Zn - 30, Ni - 20, Cr - 20, Pb - 100 and Sr - 50. The concentration of HMs in plant shoots was measured using a plasma atomic absorption spectrophotometer.
At the lethal concentration (13 mg Cd/kg) several Cd-sensitive varieties died as soon as five days after treatment, whereas the most resistant varieties lived as long as 30 days after treatment. In the presence of 7 mg Cd/kg the biomass of the four most sensitive varieties was decreased by 35-40%, whereas the growth of the six most resistant varieties was inhibited only by 10-20%. On the basis of these two experiments, twenty five additional varieties were screened at cadmium concentration of 5 mg/kg. The five most sensitive varieties (188, 3273, 8456, 4788, 1250) showed a decrease in biomass of approximately 50% as compared with untreated controls. The 12 the most resistant varieties (1658, 1693, 3445, 4789, 4650, 2593, 8274, 7131, 3429, 2174, 3034, 7128) showed a decrease in biomass of less than 20%. The content of Cd in shoots of the plants growing in 5 mg Cd/kg sand varied from 33 to 134 mg/kg dry weight, and correlated (r = +0.47; n = 20; P = 0.036) with the resistance to Cd.
The pea varieties examined differed in their phenotypic symptoms of Cd-toxicity such as stem curves, decrease in turgor, chlorosis, various kinds of necrotic spots and leave dryness without chlorosis. In addition, the development of symbiotic nodules on the roots was inhibited by Cd, with nodule formation generally decreasing as symptions of Cd toxicity increased. We observed that the decrease in nodule formation under Cd stress appeared less pronounced in three pea accessions (1658, 2593 and 8274). In contrast, nodulation was strongly inhibited in accessions 4788, 188, 8456 and 8638.
When the plants were cultivated in soil contaminated with HMs, pea lines varied in the content of Cd, Cu, Ni and Sr by a factor of 3, Zn by a factor of 4, Cr by a factor of 7, and Pb by a factor of 9. Lines displaying the greatest concentration of Cd in shoots were 1658, 1930, 3034, 3312, 6063 and 8274, and those displaying the lowest level of Cd in shoots were 1982, 7131, 8083, 6883, 1027, 2175, 2593, 7700, 4788 and 1250. Distribution curves of HM content in shoots for the pea lines examined exhibited high values for skewness (As) and kurtosis (Ex) coefficients and significantly differed from Gauss distribution curves. Cluster analysis of the data for HMs content showed that the lines formed three homogenous clusters: those having low content of all HMs, those showing a high content of all HMs and those showing a low content of Cr but a high content of the other HMs. There was a general positive correlation between contents of different HMs in plants, with the notable exception that Cr content was not correlated with the content of other metals. The resistance of pea varieties to Cd, as measured by a decrease in plant biomass at toxic Cd concentrations, was not correlated with their capability to uptake Cd from contaminated soil at nontoxic concentrations. These results suggest the existence of different genetic and physiological mechanisms for resistance to and uptake of HMs metals by pea plants. Certain of the selected pea varieties can be used as a model systems in more detailed study of the impact of HMs on legume plants and interactions between legumes and symbiotic microorganisms under HMs stress.
Acknowledgment: We are very grateful to Prof. A. Martensson and Dr. B. Forsberg for element analysis of the plants. This work was supported within the framework of grant 971112 under INCO COPERNICUS.
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