INFLUENCES OF DIFFERENT NITROGEN-SOURCES ON NITROGEN-USE AND WATER-USE EFFICIENCY, AND CARBON-ISOTOPE DISCRIMINATION, IN C-3 TRITICUM-AESTIVUM L. AND C-4 ZEA-MAYS L. PLANTS

The impacts of various nitrogen sources, i.e. NO3- NH4+ or NH4NO3 in c ombination with gaseous NH3, on nitrogen-, carbon- and water-use effic iency and C-13 discrimination (delta(13)C) by plants of the C-3 specie s Triticum aestivum L. (wheat) and the C-4 species Zea mays L. (maize) were studied. Triticum aestivum and Z. mays were hydroponically grown with 2 mol m(-3) of N supplied as NO3-, NH4+ or NH4NO3 for 21 and 18 d, respectively and thereafter exposed to gaseous NH3 at 320 mu g m(-3 ) or to ambient air for 7 d. In T. aestivum and Z. mays over a 7-d gro wth period, nitrogen-use efficiency (NUE) values were influenced by N- sources in the decreasing order NH4NO3-N > NO3-N > NH4+-N and NO3--N > NH4NO3-N > NH4+-N, respectively. Fumigation with NH3 decreased the NU E values of plants grown with any of the N-forms. During 28- and 7-d g rowth periods, N-sources affected water-use efficiency (WUE) values in the decreasing order of NH4+-N > NO3--N approximate to NH4NO3-N in no n-fumigated T. aestivum, while fumigation with NH3 increased the WUE o f NO3- -grown plants. There were insignificant effects of N-sources on WUE values of Z. mays over 25-and 7-d growth periods. Furthermore, de lta(13)C values in plant tissues (leaves, stubble and roots) were high er (less negative) in NH4+-grown plants of T. aestivum and Z. mays tha n in those supplied with NH4NO3 or NO3-. Regardless of the N-form supp lied to the roots of the plant species, exposure to NH3 caused more-po sitive delta(13)C values in the plant tissues. These results indicate that the variations in N-source were associated with small but signifi cant variations in delta(13)C values in plants of T. aestivum and Z. m ays. These differences in delta(13)C values are in the direction expec ted from differences in WUE values over long or short growth periods a nd with differences in the extent of non-Rubisco (ribulose-1,5-bisphos phate carboxylase-oxygenase, EC 4.1.1.39) carboxylate contribution to net C acquisition, as a function of N-source.