Flexibility of nitrogen metabolism in the tropical C3crassulacean acid metabolism tree species Clusia minor

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. The C3-crassulacean acid metabolism tropical hemiepiphytic tree, Clusia minor L., is highly flexible in terms of ecological sites occupied, life forms, and photosynthetic and metabolic pathways. We studied nitrogen uptake patterns in two glasshouse ¹⁵N-labelling experiments using hydroponically-grown plants and excised roots of pot-grown C. minor, and investigated leaf and root nitrate reductase activity (NRA) in a field study in Venezuela. The results of both ¹⁵N-uptake experiments indicate that C. minor utilized all offered nitrogen sources, but clearly preferred to take up NH4⁺ over glycine (GLY) and NO3^-. The uptake pattern of NH4⁺ and NO3^- was identical in intact plants and excised roots, and NH4⁺ was taken up to a much larger extent in both experiments. Gas chromatography-mass spectrometry (GC-MS) analysis of amino acids in excised roots revealed that glutamine (45 atom-%), glutamate (23 atom-%) and alanine (34 atom-%) accounted for most of the labelled soluble amino acids after 2-h labelling with ¹⁵NH4⁺. High amounts of ¹⁵N in GLY and serine confirmed that GLY was taken up as an intact molecule and metabolized in the excised roots. With ¹⁵NO3^- labelling, only a small amount of ¹⁵N was found in the amino acid fraction, indicating a low NO3^- assimilation rate by nitrate reductase. This was confirmed by low NRA of leaves and roots in C. minor plants in Venezuela. It appears, therefore, that a high degree of plasticity is not reflected in nitrogen uptake and metabolism. Although C. minor exhibits clear preferences for NH4⁺ uptake, it can utilize other sources of nitrogen, and the nitrogen uptake pattern represents an adaptation to the natural environments in which C. minor can grow.