Physiological and metabolic factors determining nitrogen use efficiency of tomato seedlings grown with elevated dissolved inorganic carbon and different nitrogen sources

[摘要] ENGLISH ABSTRACT:The aim of this study was to determine (l) the influence of elevated dissolved inorganic carbon(DIC) on the nitrogen use efficiencies (NUE) of tomato seedlings grown with different nitrogensources, (2) how changes in the regulation and activities of nitrate reductase (NR),phosphoenolpyruvate carboxylase (PEPc), carbonic anhydrase (CA) and subsequent changes inmetabolites would account for observed changes in NUE, and (3) to what extent elevated DICcontributed to the carbon budget of plants grown with different nitrogen sources. Lycopersiconesculentum cv. Fl44 seedlings were grown in hydroponic culture (pH 5.8) with 2 mM of eitherN03- or NH4 + and the solutions were aerated with either 0 ppm or 5000 ppm CO2 concentrations.The similar NUEs of NH/-fed plants grown with either root-zone CO2 concentration werelargely due to their similar RGRs and N uptake rates. Elevated root-zone DIC had an initialstimulatory effect on N~ + uptake rates, but it seems as if this effect of DIC physiologicalprocesses was cancelled out by the toxic effect of unassimilated NH/. The NUE for N03--fedplants supplied with 5000 ppm root-zone CO2 was higher relative to 0 ppm root-zone CO2 and itwas possibly due to the higher relative growth rates for similar N uptake rates of 5000 ppmcompared to 0 ppm root-zone CO2. Nitrate-fed plants grown with 5000 ppm compared to 0 ppmroot-zone CO2 had higher in vivo NR and in vitro NR and PEPc activities. These increases inenzymes activities possibly lead to increases in organic acid synthesis, which could have beenused for biomass accumulation. This would account for the increased relative growth rates ofN03--fed plants grown with 5000 ppm compared to 0 ppm root-zone CO2. The increasing rootzoneCO2 concentrations resulted in the Ó15N values of NH/-plants becoming more positiveindicating an absence of enzymatic discrimination. This may have been due to the inhibitoryeffect of DIC on Nll,+ uptake, causing plants to utilise both internal isotopes equally. The Ól3Cstudies showed that PEPc contributed equally to both N03-- and NH/-fed plants over the longterm. From this it can be concluded that the lower NUE of NH/-compared to N03--fed plantsgrown with 5000 ppm root-zone C02 was due to increased N uptake and exudation of organiccompounds into the nutrient solution. Experiments with 813C also showed that at increasing rootzoneCO2 concentrations, PEPc made a bigger contribution to the carbon budget via theanaplerotic reaction.

[发布机构] Stellenbosch University