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 (WUEP), and via carbon isotope ratio (delta C-13). 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 WUEP. Adequate K supply enhanced WUEP up to 30% and by additional 20% under PEG stress, but had no effect on WUEL. E-P was lower with adequate K supply, but this effect may be attributed to canopy microclimate. Shoot delta C-13 responded linearly to time-integrated WUEL in adequately supplied plants, but not in K-deficient plants, indicating negative effects of K deficiency on mesophyll CO2 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.