Magnitude and kinetics of metal rhizotoxicity in cowpea

摘要

Many difficulties exist in establishing the concentrations of metals in solution that are toxic to the growth of plant roots. To limit these difficulties, short-term solution culture experiments were conducted using the same technique on 3-d-old cowpea (Vigna unguiculata (L.) Walp.) cv. Caloona seedlings. These were grown for 48 h in solutions with ca. 1000 mu M Ca and 5 mu M B plus one of 24 metals, concentrations of which were determined after filtering (0.22 mu m). The decrease in root elongationrate (RER) varied markedly among the metals tested, with a 50 % reduction in RER (i.e. EC50) evident at 0.02 mu M Ag to 132 mM K. The rapidity with which RER was reduced varied also, as did the symptoms of rhizotoxicity. A range of metals caused rupturing of the rhizoderrnis and outer cortex in the roots elongation and transition zones within 2 - 24 h of exposure. These metals were all highly rhizotoxic, but not all highly rhizotoxic metals caused ruptures. We conclude that rhizotoxicity results from disruption of a range of underlying biochemical mechanisms. There were some common effects, but no metal could be considered an analogue for another in all respects.