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Many reports have shown that plant growth and yield is superior on mixtures of NO 3؊ and NH 4 ؉ compared with provision of either N source alone. Despite its clear practical importance, the nature of this N-source synergism at the cellular level is poorly understood. In the present study we have used the technique of compartmental analysis by efflux and the radiotracer 13 , 1997). Although NH 4 ϩ should be the preferred N source, since its metabolism requires less energy than that of NO 3 Ϫ (Bloom et al., 1992), only a few species actually perform well when NH 4 ϩ is provided as the only N source. Among the latter are boreal conifers (Kronzucker et al., 1997), ericaceous species (Pearson and Stewart, 1993), some vegetable crops (Santamaria and Elia, 1997), and rice (Wang et al., 1993; Kronzucker et al., 1998). Most agricultural species develop at times severe toxicity symptoms on NH 4ϩ (Cox and Reisenauer, 1973; Findenegg, 1987); thus, superior growth in these species is seen on NO 3 Ϫ (Rideout et al., 1994). However, when both N sources are provided simultaneously, growth and yield are often enhanced significantly compared with growth on either NH 4 ϩ or NO 3 Ϫ alone. The effect is particularly well documented in corn (Below and Gentry, 1987; Smiciklas and Below, 1992; Adriaanse and Human, 1993) and wheat (Cox and Reisenauer, 1973; Heberer and Below, 1989; Chen et al., 1998), but it has also been reported in several other species (Hagin et al., 1990; Cao and Tibbits, 1993; Gill and Reisenauer, 1993), including rice (Ta and Ohira, 1981; Ta et al., 1981). Yield increases of 40% to 70% have been observed in solution culture (Weissman, 1964; Cox and Reisenauer, 1973; Heberer and Below, 1989), although, commonly, somewhat smaller enhancements are obtained in soil culture and under field conditions (Hoeft, 1984; Hagin et al., 1990). Several hypotheses pertaining to the enhanced growth and yield response on mixed N medium have been advanced (Lewis et al., 1982; Findenegg, 1987; Gill and Reisenauer, 1993), but mechanistic examinations of these effects have been lacking. In the present study we have used compartmental analysis with the short-lived radiotracer 13 N to examine the reciprocal effects of NH 4 ϩ and NO 3 Ϫ on each other in root tissue of intact rice plants with respect to N-flux partitioning and storage capacity at the subcellular level. MATERIALS AND METHODS Plant Growth ConditionsRice (Oryza sativa L. cv IR72) seeds were surfacesterilized in 5% NaOCl for 10 min, rinsed with deionized water, and left to imbibe in aerated deionized water at 30°C in a water bath for 48 h. The partially germinated seeds were then placed onto plastic mesh mounted on Plexiglas discs (Atohaas Americas Inc., Philadelphia, PA) and the discs were transferred to 40-L hydroponic Plexiglas tanks located in walk-in, controlled-environment growth chambers. Growth chambers were maintained at 30°C Ϯ 2°C, 70% RH, and set to a 12-h/12-h photoperiod. A photon flux of approximately 500 mol m Ϫ2 s Ϫ1 , measured at plant level (with a ...

SUMMARYMeasurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice {Oryza sativa L.) roots are reported. Initially 11-d-oId plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a welldefined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH/. But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO., uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed.

Nitrogen limitation compromises the realization of yield potential in cereals more than any other single factor. In rice, the world's most important crop species, the assumption has long been that only ammonium-N is efficiently utilized. Consequently, nitrate utilization has been largely ignored, although fragmentary data have suggested that growth could be substantial on nitrate. Using the short-lived radiotracer "$N, we here provide direct comparisons of root transmembrane fluxes and cytoplasmic pool sizes for nitrate-and ammonium-N in a major variety of Indica rice (Oryza sativa), and show that nitrate acquisition is not only of high capacity and efficiency but is superior to that of ammonium. We believe our results have implications for rice breeding and molecular genetics as well as the design of water-management and fertilization regimes. Potential strategies to harness this hitherto unexplored N-utilization potential are proposed.

S Ll M M . A R YRice plants {Oryza sativa L., cv, [R34) were grown with their roots sandwiched between cylinders of an anaerobic low-Zn Molhsol. After periods of root-soil contact of up to 12 d (total plant age c. 28 d) the profiles of different Zn fractions, reduced and oxidized Fe, and pH in the soil near the root 'plane' were determined. The concentration of easily plant-extractable Zn in the soil (measured by extraction in 1 M KCl) was negligible, and so it was necessary for the plants to induce changes in the soil to solubilize Zn. After 6 d, there was a substantial accumulation of Zn associated with organic matter and amorphous ferric hydroxide within 4-5 mm of the root plane. Over tbe next 6 d. tbe accumulation continued but there was a substantial depletion of the accumulated fractions within 2 mm of the root plane. The zones of accumulation and depletion coincided with zones of Fe(III) accumulation and soil acidification in which the pH decreased from the bulk soil \alue of 7*3 by over 0-2 pH units; i.e. a two-fold increase in H' concentration. The acidification was the result of H"* released from the roots to balance excess intake of cations over anions, and H* generated in the oxidation of Fe(II) by root-released O^. At the high pH and CO^ pressure of tbe experimental soil (7-3 and c. 0-9 kPa, respectively), soil acidity diffusion is fast and consequently the pH drop at the root surface was small. The rate of Fe oxidation peaked before 6 d, but tbe acidification and Zn accumulation continued beyond 6 d unabated. It is concluded that Fe oxidation released Zn from highly insoluble fractions, and that this Zn was re-adsorbed on Fe(OH),, and on organic matter in forms that were acid-soluble and therefore accessible to tbe plants.