Altered influx/efflux relations of nitrate in roots due to nutrient stress. II. Effect of calcium limitations

Abstract

The effect of calcium (Ca²⁺), aluminum (Al³⁺), and high salinity stress on the influx‐efllux relations of nitrate (NO₃ ^‐) was investigated in barley and squash seedlings using continuous monitoring techniques and ¹³N labelling. After 24 h of Ca²⁺ deficiency NO₃ ^‐ influx was substantially reduced at the level of unsaturated NO₃ ^‐ uptake. With prolonged Ca²⁺ deficiency the repression of NO₃ ^‐influx drastically increased in squash involving irreversible impairment of the whole root function whereas the impairment only progressed more moderately in barley. Treatment with 160 μM Al³⁺ for 24 h also induced a repression of NO₃ ^‐ influx probably due to an interaction between Al³⁺ and Ca²⁺ ions. In both Ca²⁺ deficiency and Al³⁺ toxicity experiments, the NO₃ ^‐ efflux was only slightly affected. Suddenly imposed high salinity at a species‐specific level resulted in an instantaneous repression of NO₃ ^‐ influx accompanied by a heavy release of NO₃ ^‐ to the ambient nutrient solution. The results are discussed in the light of the key role which Ca²⁺ is suggested to play in membrane integrity and at the level of intracellular processes and in view of the relations to carrier‐mediated NO₃ ^‐ influx.