ENRICHED RHIZOSPHERE CO2 CONCENTRATIONS CAN AMELIORATE THE INFLUENCE OF SALINITY ON HYDROPONICALLY GROWN TOMATO PLANTS

Our previous work indicated that salinity caused a shift in the predom inant site of nitrate reduction and assimilation from the shoot to the root in tomato plants. In the present work we tested whether an enhan ced supply of dissolved inorganic carbon (DIC, CO2 + HCO3-) to the roo t solution could increase anaplerotic provision of carbon compounds fo r the increased nitrogen assimilation in the root of salinity-stressed Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were grown in hydroponic culture with 0 or 100 mM NaCl and aeration of the root solution with either ambient or CO2-enriched air (5 000 mu mol mol(-1) ). The salinity-treated plants accumulated more dry weight and higher total N when the roots were supplied with CO2-enriched aeration than w hen aerated with ambient air. Plants grown with salinity and enriched DIC also had higher rates of NO3- uptake and translocated more NO3- an d reduced N in the xylem sap than did equivalent plants grown with amb ient DIC. Incorporation of DIC was measured by supplying a 1-h pulse o f (HCO3-)-C-14 to the roots followed by extraction with 80% ethanol. E nriched DIC increased root incorporation of DIC 10-fold in both salini zed and non-salinized plants. In salinity-stressed plants, the product s of dissolved inorganic C-14 were preferentially diverted into amino acid synthesis to a greater extent than in non-salinized plants in whi ch label was accumulated in organic acids. It was concluded that enric hed DIC can increase the supply of N and anaplerotic carbon for amino acid synthesis in roots of salinized plants. Thus enriched DIC could r elieve the limitation of carbon supply for ammonium assimilation and t hus ameliorate the influence of salinity on NO3- uptake and assimilati on as well as on plant growth.