Short-term responses of Brassica rapa plants to low root temperature: Effects on nitrate uptake and its translocation to the shoot

Brassica rapa L. plants were grown hydroponically for 5 or 6 weeks at 20°C and then half batches of plants were transferred to tanks in which the root temperature was lowered decrementally over 1 h to 7°C. Changes in nitrate uptake rate (NUR) and nitrate transfer from roots were studied in relation to transpiration and root pressure xylem exudation flow rates over a 48- or 72-h period. The response of plants following the root temperature decrease was biphasic. During phase 1, NUR and water and solute flow rates through the root decreased sharply. Coping mechanisms came into operation during phase 2, and tended to offset the effects of low temperature. The 3-h cold-treated roots exhibited a very low NUR but 48-h cold-treated roots partly recovered their ability to absorb nitrate. Transpiration rate decreased more slowly (during 24 h) than both root xylem exudation and parameters of root conductivity (during 6 h). Beyond these respective times, transpiration rate was balanced while root xylem exudation clearly increased, but without returning to the level of control plants. Nitrate transfer to the root xylem was strongly and rapidly affected by low root temperature, but the subsequent readjustment was such that no or little difference compared with the control was apparent after 48 h. Water and solute flows were strongly decreased when nitrate was replaced by chloride in the culture solution during exudation sampling. The major role of nitrate in root hydraulic conductivity and root xylem exudation is discussed.