Effects of low temperature on nitrate uptake, and xylem and phloem flows of nitrogen, inSecale cerealeL. andBrassica napusL.

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Abstract
SUMMARY: The effect of low temperature on nitrate uptake and subsequent N translocation and cycling was investigated inSecale cerealeL. andBrassica napusL. transferred to 7 °C for 9 d or kept at a warm temperature (20 °C). Some plants were grown with a split root system and15NO3−, labelled to measure NO3−uptake, root‐to‐shoot and shoot‐to‐root translocation of N from NO3−, taken up. Other plants with single‐root system were subjected to15N pulse‐chase labelling to quantify endogenous N remobilization.Lowering growth temperature from 20 to 7 °C reduced nitrate uptake more strongly in rye (−59%) than in winter rape (−28%). A very large proportion of the nitrate taken up was further translocated to shoots in both species. However, lowering the temperature decreased, xylem N translocation by about 60 and 30% inS. cerealeandB. napus, respectively. Most root N in the latter species came directly from root absorption, while in rye roots the proportions ascribe able to direct root allocation and phloem flow were well balanced. Cold treatment did not significantly modify the imbalance between the two origins. Mobilization of endogenous N from roots to shoot, estimated by pulse‐chase labelling, was limited inB. napusand significant inS. cerealedespite a depressive effect of low temperature. In general, low temperature led to an increase in root N concentration in both species. It is Suggested that low temperature may directly affect the nitrate uptake system, and also that N accumulation in the roots (resulting from greater inhibition of N xylem flow than of NO3−uptake) might increase the inhibition of uptake by higher cellular NO3, and/or amino acid content. The usefulness (for spring growth) of root N accumulation triggered by low temperature is discussed.