The effect of shoot demand for nutrients on nutrient uptake and translocation in the xylem exudate was studied in maize growing in nutrient solution at uniform shoot zone (24/2°C, day/night), but different root zone temperatures (RZT: 12°C, 18°C, 24°C). The shoot base (apical shoot meristem and zone of leaf extension) was either kept within or lifted above the cooling zone. In plants with their shoot base above the cooling zone (RZT: 12°C and 18°C) shoot growth was significantly increased but not root growth. Therefore, at suboptimal RZT shoot fresh weight increment d−1 g−1 root fresh weight, which was taken as a parameter for the shoot demand for nutrients per unit root, varied strongly depending on the temperature of the shoot base (shoot base temperature, SBT).

In short-term studies (2 h and 1 d after onset of temperature treatment) rates of nutrient (nitrogen, N; potassium, K; phosphorus, P; calcium, Ca) uptake or translocation in the xylem exudate were markedly decreased at suboptimal RZT (12°C, 18°C), irrespective of the SBT. In long-term studies (3, 5, and 10 d after onset of temperature treatment) uptake and translocation of K, N, and Ca, but not P, increased in plants at suboptimal RZT when the shoot demand was high (shoot base above the cooling zone) but decreased when the shoot demand was low (shoot base within the cooling zone).

These results suggest, that the increase of translocation rates of N, K, and Ca after long-term exposure to suboptimal RZT was a consequence of a higher shoot demand per unit root fresh weight and not due to a direct temperature effect on the nutrient uptake system.

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