Low nitrogen supply decreases water‐use efficiency of oriental tobacco

Most studies indicating positive effects of nitrogen (N) supply on biomass water‐use efficiency (WUE) used two contrasting levels of N supply and monitored some of the relevant traits being associated with WUE. In order to investigate N effects on WUE over a wider range of N‐supply levels, oriental tobacco was cultivated in pots under six (Exp. 1) and four (Exp. 2) N levels until flowering stage. Water‐use efficiency, intrinsic water‐use efficiency (A/g), carbon isotope discrimination (CID), average daily canopy transpiration rate (E C ; from 12 to 7 d before harvest), and biomass‐partitioning parameters were determined. Water‐use efficiency increased from the lowest to the highest N level and was positively correlated with biomass. Intrinsic water‐use efficiency was positively correlated with leaf N and chlorophyll concentration. Whole‐plant CID was significantly less negative under high compared to low N supply. Biomass‐partitioning parameters were only slightly affected by N supply except for leaf‐area ratio, which significantly increased with N supply. The low planting density of Exp. 2 resulted in an increase of WUE and substantially less negative whole‐plant CID compared to Exp. 1. In both experiments, E C was lower under high compared to low N supply and was negatively correlated with WUE. It is concluded that plants under high N supply realized a higher WUE via increases of A/g and a reduction of E C .