Transpiration, potassium uptake and flow in tobacco as affected by nitrogen forms and nutrient levels

Abstract

Background and aims: Ammonium can result in toxicity symptoms in many plants when it is supplied as the sole source of N. In this work, influences of different nitrogen forms at two levels (2 and 15 mm N) on growth, water relations and uptake and flow of potassium were studied in plants of Nicotiana tabacum 'K 326'.

Methods: Xylem sap from different leaves was collected from 106-d-old tobacco plants cultured in quartz sand by application of pressure to the root system. Whole-shoot transpiration for each of the treatments was measured on a daily basis by weight determination.

Key results: Total replacement of NO(3)(-)N by NH(4)(+)-N caused a substantial decrease in dry weight gain, even when plants grew under nutrient deficiency. Increasing nutrient concentration resulted in a greater net dry weight gain when nitrogen was supplied as NO(3)(-) or NH(4)NO(3), but resulted in little change when nitrogen was supplied as NH(4)(+). NH(4)(+)-N as the sole N-source also caused reduction in transpiration rate, changes in plant WUE (which depended on the nutrient levels) and a decrease in potassium uptake. However, the amount of xylem-transported potassium in the plants fed with NH(4)(+) was not reduced: it was 457 % or 596 % of the potassium currently taken up at low or high nutrient level, respectively, indicating a massive export from leaves and cycling of potassium in the phloem.

Conclusions: Ammonium reduces leaf stomatal conductance of tobacco plants. The flow and partitioning of potassium in tobacco plants can be changed, depending on the nitrogen forms and nutrient levels.

Fig. 1.

Time-course of whole-shoot transpiration of tobacco plants as dependent on N-forms and nutrient. Bars denote s.e. of the mean, n = 5.

Fig. 2.

Carbon isotope fractionation (δ¹³C, ‰) of leaf tissues of - and NO₃-fed plants at both low and high nutrient levels during a 10 d study period. L and H represent the low and high nutrient level, respectively. Bars denote s.e. of the mean, n = 5.

Fig. 3.

Flow profiles for uptake, transport and utilization of K⁺ in tobacco plants supplied with a low nutrient concentration (2·5 mm K and 2 mm N) over a 10 d experimental period, starting 106 d after sowing. The values of K⁺ deposition and the statistical significance are given in Table 3. The width of arrows and the height of histograms are drawn in proportion to the net flows and deposition of K⁺. The numbers indicate the values of uptake, transport and utilization (mmol K⁺ per plant over the 10 d study period).

Fig. 4.

Flow profiles for uptake, transport and utilization of K⁺ in tobacco plants supplied with a high nutrient concentration (6 mm K and 15 mm N) over a 10 d experimental period, starting 106 d after sowing. The values of K⁺ deposition and the statistical significance are given in Table 3. The width of arrows and the height of histograms are drawn in proportion to the net flows and deposition of K⁺. The numbers indicate the values of uptake, transport and utilization (mmol K⁺ per plant over the 10 d study period).