Seven heathland species, four herbaceous plants and three dwarf shrubs, were tested for their capacity to utilize NH4+ or NO3-. When cultured in solution at pH 4.0 with 2 mol m(-3) N, all species showed similar growth responses with respect to N source. Nitrate was assimilated almost equally well as ammonium, with relative growth rate generally averaging 5-8% lower for NO3- grown plants, albeit not always significantly, However, N source was significantly and consistently correlated with biomass partitioning, as NH4+-fed plants allocated more dry matter to shoots and less to roots when compared to NO3-- fed plants, The strong difference in biomass partitioning may relate to the relative surplus of carbon per unit plant N (or, alternatively, the relatively suboptimal rate of N assimilation per unit plant C) in NO3--fed plants. Inherently slow-growing dwarf shrubs accumulated virtually no free nitrate in their tissues and reduction of nitrate was strictly root-based, Faster-growing herbaceous plants, however, partitioned the assimilation of nitrate over both shoots and roots, thereby accumulating relatively high tissue NO3--levels. ion uptake rates depended clearly on the 'relative shoot demand', At similar shoot demands, especially in the herbaceous species, specific uptake rates for N and total inorganic (non-N) anions were higher in NH4+-fed plants, whereas the uptake rate for total (non-N) cations was higher in NO3--fed plants, Rate of P uptake was enhanced with increasing plant demand, but was independent of the N source, Net H+ extrusions ranged from 1.00 to 1.34 H+ per NH4+, and from -0.48 to -0.77 H+ per NO3- taken up. [KEYWORDS: Ammonium; biomass partitioning; heathland plants; low ph; nitrate; nitrate reductase activity; relative shoot demand; specific absorption rate Zea-mays l; chemical-composition; functional equilibrium; ricinus-communis; vascular plants; lolium-perenne; root-zone; growth; form; absorption]
Original languageEnglish
JournalJournal of Experimental Botany
Journal publication date1995

ID: 57798