Potassium (K) deficiency is a rather common situation which impacts negatively on biomass, photosynthesis and N assimilation, making K fertilization often unavoidable. 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 per se. To sort this out, 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 unlike low-Ca, does not suppress but aggravates the impact of low-K conditions on catabolism, including the typical fall-over in pyruvate kinase. We conclude that (i) the effects of K deficiency on key metabolic processes 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 acclimation to low K via the regulation of the content in enzymes involved in carbon primary metabolism.