Response to salinity of tomato seedlings with a split root system:: Nitrate uptake and reduction

Soil water content and salinity level are seldom uniform in the field, particularly with the use of micro-irrigation systems that may water only a portion of the root zone. A split-root experiment was designed to evaluate growth, nitrate and water uptake and nitrate reduction, when half of the root system of tomato seedlings was stressed with salinity. This study also determines if non-stressed portions of the root system compensated for the decrease in water uptake by stressed portions. Tomato (Lycopersicon esculentum Mill.) seedlings were grown in a growth chamber under controlled conditions with half-strength Hoagland solution. Three different treatments were applied to the split-root system: 1) some seedlings had both parts of their roots in control solution, 2) other seedlings had half of their root system treated with salinity, and 3) the rest had both parts treated with salinity. Saline treatment of the whole root system caused a decline in water uptake and nitrate uptake. Nitrate reductase activity in leaves decreased due to the lower NO3 uptake, but also due to excessive chloride (Cl) accumulation in the shoot. When half of the root system was treated with salinity, water uptake was reduced in the treated part, but a compensatory increase in water uptake through the untreated part was observed. Nitrate uptake decreased slightly in the non-treated part and more strongly in the treated one, therefore, lower levels of nitrate in the shoot were obtained, compared to control plants. This lower nitrate uptake, and probably translocation to the shoot, may be the reason for the decrease in leaf NRA under these conditions.