Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations, nitrate uptake, and translocation of recently absorbed nitrogen, was investigated in 5-week-old tomato (Lycopersicon esculentum Mill cv. Ailsa Craig) plants grown hydroponically in a complete nutrient solution. Plants were subjected to dry air (relative humidity 2—4%) for 6 h. The transpiration rate increased several-fold and the shoot water content decreased by almost 20%, whereas root water content was unaffected. No effect on in vitro nitrate reductase (NR) activity was detected when using an EDTA-containing assay buffer. Replacement of EDTA with Mg2+ revealed a significant decline in shoot NR activity, which suggests phosphorylation of the enzyme during the stress treatment. Plants were grown in a split-root system, in which one root half was fed 15N-nitrate during the treatment, in order to determine nitrate uptake and translocation of recently absorbed nitrogen in the plants. Uptake of nitrate was substantially inhibited, but the proportion of absorbed 15N that was translocated to the shoots was only slightly affected. In untreated plants, 71% of the 15N recovered in roots had been retranslocated from the shoots, whereas in plants subjected to stress the delivery of 15N from shoots to roots appeared to be completely inhibited. The data show that lowered humidity in air has significant effects on both uptake of nitrate as well as translocation of nitrogen within the plants. Some of these effects appear to be common with those observed in plants subjected to reduced water potentials in the root environment and point to the possibility of the shoot water relations being highly influential on nitrogen uptake and translocation.