Kinetics and Specificity of Amino Acid Uptake by the DuckweedSpirodela polyrhiza(L.) Schleiden

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Abstract
Borstlap, A. G, Meenks, J. L. D., van Eck, W. F. and Bicker, J. T. E. 1986. Kinetics and specificity of amino acid uptake by the duckweed Spirodela polyrhiza (L.) Schleiden.—J. exp. Bot. 37: 1020–1035. Uptake of 14C-labelled amino acids by intact, axenically grown plants of Spirodela polyrhiza (L.) Schleiden was investigated. Experiments in which uptake was measured from the decrease in the amino acid concentration in the medium, indicated that saturable uptake conforms to the sum of two Michaelis-Menten terms, possibly corresponding with a high-affinity and a low-affinity system. Further experiments with L-leucine, L-glutamic acid, and L-lysine, in which uptake was measured by assaying the amount of 14 in the plants, showed the presence of a non-saturable component in addition to the dual saturable uptake. Uptake of L-glutamic acid precipitously declined between pH 4·0 and 6· and that of L-leucine between pH 4·0 and 8· whereas L-lysine uptake was optimal at pH 6·0. No evidence was found that the apparent high-affinity and low-affinity systems responded differently to changes in external pH or to the addition of CCCP. The non-saturable uptake component was not affected by changes in external pH or by adding CCCP, and might have been due to free space uptake. Mutual inhibition of uptake was found between acidic and neutral amino acids (L-leucine, L-methionine, L-glutamic acid) and between basic amino acids (L-lysine, L-ornithine). The basic amino acids had no effect on the uptake of L-leucine, L-methionine and L-glutamic acid, although the uptake of basic amino acids was inhibited by glutaminc acid and several neutral amino acids. It is suggested that the duckweed has a high-affinity transport system for neutral and acidic amino acids, and a distinct high-affinity system for basic amino acids. It is argued that the first system transports zwitterionic amino acids (z-system), and that the second system transports cationic amino acids(y+-system). The specificity of the low-affinity system is less certain, but there is some evidence that it is similar to that of their high-affinity counterparts.