GROWTH, CHEMICAL-COMPOSITION, AND NITRATE REDUCTASE-ACTIVITY OF RUMEX SPECIES IN RELATION TO FORM AND LEVEL OF N-SUPPLY

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH4+, NO3-, or both) and level of N supply (4 mM N, and zero-N following a period of 4 mM N). A distinct preference for either NH4+ or NO3- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO3- and free NH4+. Especially the accumulation of free NH4+ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH4+ and NO3-. Compared to mixed N supply, RGR of NO3- and NH4+-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH4+ supply probably resulted from direct or indirect toxicity effects associated with high NH4+ and/or low Ca2+ contents of tissues. NRA in NO3- and NH4NO3 plants was very similar with maxima in the leaves of ca 40 mumol NO2- g-1 DW h-1. 'Basal' NRA levels in shoot tissues of NH4+ plants appeared relatively high with maxima in the leaves of ca 20 mumol NO2- g-1 DW h-1. Carboxylate to organic N ratios, (C-A)/N(org), on a whole plant basis varied from 0.2 in NH4+ plants to 0.9 in NO3- plants. After withdrawal of N, all accumulated NO3- and NH4+ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50-80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH4+ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N(org) ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N(org) ratio and NRA of plants in their natural habitats are briefly discussed.