Adequate supply of potassium improves plant water‐use efficiency but not leaf water‐use efficiency of spring wheat

Enhancing crop water‐use efficiency (WUE) is a major research objective in water‐scarce agroecosystems. Potassium (K) enhances WUE and plays a crucial role in mitigating plant stress. Here, effects of K supply and PEG‐induced water deficit on WUE of spring wheat ( Triticum aestivum L. var. Sonett), grown in nutrient solution, were studied. Plants were treated with three levels of K supply (0.1, 1, 4 mM K + ) and two levels of PEG (0, 25%). WUE was determined at leaf level (WUE L ), at whole‐plant level (WUE P ), and via carbon isotope ratio (δ 13 C). Effects of assimilation and stomatal conductance on WUE L were evaluated and compared with effects of biomass production and whole‐plant transpiration (E P ) on WUE P . Adequate K supply enhanced WUE P up to 30% and by additional 20% under PEG stress, but had no effect on WUE L . E P was lower with adequate K supply, but this effect may be attributed to canopy microclimate. Shoot δ 13 C responded linearly to time‐integrated WUE L in adequately supplied plants, but not in K‐deficient plants, indicating negative effects of K deficiency on mesophyll CO 2 diffusion. It is concluded that leaf‐scale evaluations of WUE are not reliable in predicting whole‐plant WUE of crops such as spring wheat suffering K deficiency.