Uptake and Transport

In bacterial systems, several families of nickel permeases and ATP-dependent nickel carriers have been characterized. No equivalent mechanism has yet been identified in animals or plants (17). In plant systems, most studies have been conducted at unrealistically high soil-nickel concentrations and as such may be relevant for nickel toxicity, but are not relevant for nickel uptake under normal conditions. Cataldo et al. (56) using 63Ni indicated that a high-affinity Ni²⁺ carrier functioned at 0.075 or 0.25μM Ni²⁺ with a K_m of 0.5μM which approaches the nickel concentration in uncontaminated soils (48). Either Cu²⁺ or Zn²⁺ competitively inhibits Ni²⁺ uptake suggesting that all the three elements share a common uptake system (57). Uptake at higher nickel-supply levels (0.5 to 30μM) was energy dependent and had a K_m of 12μM indicative of an active, low-affinity transport system.

No evidence suggests that associations with arbuscular mycorrhizal fungus increase nickel accumulation by plants (58,59).

Nickel, unlike many other divalent cations, is readily re-translocated within the plant likely as a complex with organic acids and amino acids (60). Nickel rapidly re-translocates from leaves to young tissues in the phloem, particularly during reproductive growth. Indeed, up to 70% of nickel in the shoots was transported to the seed of soybean (61). Nickel is associated primarily with organic acids and amino acids in the phloem. Above pH 6.5, histidine is the most significant chelator, whereas at pH <5, citrate is the most significant one (5).