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PHYSIOLOGICAL BASIS

Metals are predominantly accumulated in the roots causing an unbalanced shoot to root ratio of metal concentrations in most plants. However, in hyperaccumulators, the shoot to root ratio of metal concentrations are abnormally higher in the leaves and much lower in the roots. As this process occurs, metals are efficiently shuttled from the root to the shoot as an enhanced ability in order to protect the roots from metal toxicity.

Delving into tolerance: Throughout the research of hyperaccumulation, there is a conundrum with tolerance. There are several different understandings of tolerance associated with accumulation; however, there are a few similarities. Evidence has conveyed that the traits of tolerance and accumulation are separate to each other and are moderated by genetic and physiological mechanisms. Moreover, the physiological mechanisms, in relation to tolerance, are classified as exclusion: when the movement of metals at the interfaces of soil/root or root/shoot are blocked, or accumulation: when the uptake of metals that have been rendered as non-toxic are allowed into the aerial plant parts.

Characteristics from certain physiological elements:

There are certain characteristics that are specific to certain species. For example, when presented with a low supply of Zinc, T. caerulescens had higher Zinc concentrations accumulated compared to other non-accumulator plant species. Further evidence indicated that when T. caerulescens were grown on soil with an adequate amount of contamination, the species accumulated an amount of Zinc that was 24-60 times more than Raphanus sativus (radish) had accumulated. Additionally, the capacity to experimentally manipulate soil metal concentrations with soil amendments has allowed researchers to identify the maximum soil concentrations that hyperaccumulation species can tolerate and the minimum soil concentrations in order to reach hyperaccumulation. Furthermore, with these findings, two distinct categories of hyperaccumulation arose, active and passive hyperaccumulation. Active hyperaccumulation is attained by relatively low soil concentrations. Passive hyperaccumulation is induced by exceedingly high soil concentrations.