Mechanisms of copper uptake and transport in plants : a thesis presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Soil Science at Massey University, New Zealand
The Cu concentration in plants varies considerably between species. This suggests different abilities to either absorb Cu from soils or translocate Cu from root to shoot. The main objective of the thesis was to provide a fuller understanding of the mechanisms of Cu uptake and transport in plants which may lead to the development the strategies to improve Cu uptake by pasture crops. Glasshouse experiments with the Cu hyperaccumulator Haumaniastrum katangense showed that Cu hyperaccumulation in shoots was not found. It was concluded that H. katangense plants tested in these experiments were Cu tolerant rather than having hyperaccumulator status. The mechanism of high tolerance to Cu could be due to the restriction of Cu transport from roots to shoots. Nutrient solution culture experiments with the Ni hyperaccumulator plants Alyssum bertolonii and Berkheya coddii showed that co-hyperaccumulation of Cu and Ni did not exist. Alyssum bertolonii was not a Cu-tolerant plant, whereas B. coddii exhibited a much greater degree of tolerance to this metal, and the tolerance of B. coddii to Cu was not at the expense of Ni uptake. It was concluded that B. coddii should be considered as a possible plant for phytoremediation of soils contaminated with both Cu and Ni and it is recommended that field trials be carried out to establish this potential. NFT nutrient solution culture experiments showed that a large proportion of total Cu uptake by chicory and tomato plants was retained by roots except when plants were grown in the basal nutrient solution (0.05 mg Cu L-1). Copper retention by roots, limited Cu translocation to xylem and shoots. Large differences between measured and predicted Cu accumulation by shoots of tomato and chicory suggested that some xylem-transported Cu is recirculated to roots via the phloem. A Cu speciation study showed that more than 99.7% of total Cu in tomato and chicory xylem sap was in a bound form. Increased Cu concentrations in the rooting media induced selective synthesis of certain amino acids which include NA, His, Asn and Gln, all of which have high stability constants with Cu. Nicotianamine and His have the highest binding constants for Cu and the concentrations of NA and His in chicory and tomato xylem saps can account for all the bound Cu carried in the sap. Copper recirculation within plants was demonstrated by an experiment with hydroponically grown tomato plants in a split-root system. Significant amounts of Cu were translocated from roots bathed in a solution of high Cu concentration to another half root system exposed to low Cu. Shoot Cu concentrations were positively correlated to plant water use (mL g-1 DM). A Cu recirculation model was suggested. Efforts have been made to develop the strategies to improve Cu uptake by pastures. The initial uptake of Cu from CuSO4-fertilised soil can be increased by 10-21 % by addition of His and casein. Casein was generally more effective at increasing plant Cu uptake than His and other amino acids. The Cu(OH)2-based fertiliser was less effective than the CuSO4-based fertilisers in supplying Cu to ryegrass grown in pots of Ashhurst stony silt loam and Wairoa pumice soil. In general, among the three CuSO4 fertilisers, Ca-caseinate-CuSO4 resulted in higher Cu uptake by ryegrass grown in both soils. The factors constraining Cu uptake by ryegrass plants from Cu-fertilised soils were elucidated. Linear relationships between ryegrass Cu uptake and total soil solution Cu concentration were soil type dependent, despite each soil having similar soil solution Cu concentrations. Between 98.5-99.5% of the soil solution Cu was in complexed forms. No relationship between the Cu2+concentration in soil solution (expressed as pCu2+) and Cu concentration in plants was found. Free Cu2+ concentrations in soil solution were sensitive to pH change. The extent of the increase in free Cu2+ concentration per unit decrease in pH was dependent on soil type. It is suggested that the rate of Cu uptake by plants is likely to be dependent on both the concentration of organically complexed Cu in the soil solution and the stability of this complex to pH change.