Chemical root pruning of kangaroo paw ( Anigozanthos flavidus) by selected heavy metal carbonates

Abstract

The six heavy metal carbonates of copper (Cu), cobalt (Co), lead (Pb), nickel (Ni), strontium (Sr) and zinc (Zn) were tested for their ability to induce root pruning in container plants. The monocotyledon Australian native species Anigozanthos flavidus (kangaroo paw) was used as test material because it normally develops a coarse root system, and is susceptible to constriction and root circling in containers. Of the heavy metals tested, Pb and Ni caused toxicity to plants and markedly decreased shoot and root growth. Therefore, these two metals were unsuitable as root pruning agents, and at all concentrations tested did not increase shoot/root ratios; perhaps the best indicator of root pruning. Sr and Zn were not toxic at concentrations tested, but were ineffective at root pruning plants in containers (except for Zn at the highest concentrations tested). Cu and Co were the best heavy metals for inducing root pruning, while at the same time minimally affecting the growth of shoots. Cu increased the shoot/root ratios of plants, but concentrations of 1.0–2.0 mol eq l −1 were required. Co, however, was more effective than Cu, and significantly increased the ratios of shoots/roots at lower concentrations of 0.5–1.0 mol eq l −1. This feature of Co in combination with its general lower toxicity in the environment suggests that Co is the better heavy metal to use as a chemical root pruning agent.