Effects of K deficiency have been investigated for several decades and recently, progress has been made in identifying metabolomics signatures thereby offering potential to monitor the K status of crops in the field. However, effects of low K conditions could also be due to the antagonism with other nutrients like calcium (Ca) and the well-known biomarker of K deficiency, putrescine, could be a response to Ca/K imbalance rather than K deficiency. We carried out experiments in sunflower grown at either low or high K, at high or low Ca, with or without putrescine added to the nutrient solution. Using metabolomics and proteomics analysis, we show that a significant part of the low-K response such as lower photosynthesis and N assimilation, is due to calcium and can be suppressed by low Ca conditions. Putrescine addition tends to restore photosynthesis and N assimilation but but aggravates the impact of low-K conditions on catabolism. We conclude that (i) effects of K deficiency can be partly alleviated by the use of low Ca and not only by K fertilization, and (ii) in addition to its role as a metabolite, putrescine participates in the regulation of the content in enzymes involved in carbon primary metabolism.