The combination of K + deficiency with other environmental stresses: What is the outcome?

Potassium (K+) is a macronutrient known for its high mobility and positive charge, which allows efficient and fast control of the electrical balance and osmotic potential in plant cells. Such features allow K+ to remarkably contribute to plant stress adaptation. Some agricultural lands are deficient in K+, imposing a stress that reduces crop yield and makes fertilization a common practice. However, individual stress conditions in the field are rare, and crops usually face a combination of different stresses. As plant response to a stress combination cannot always be deduced from individual stress action, it is necessary to gain insights into the specific mechanisms that connect K+ homeostasis with other stress effects to improve plant performance in the context of climate change. Surprisingly, plant responses to environmental stresses under a K+-limiting scenario are poorly understood. In the present review, we summarize current knowledge and find substantial gaps regarding specific outcomes of K+ deficiency in addition to other environmental stresses. In this regard, combined nutrient deficiencies of K+ and other macronutrients are covered in the first part of the review and interactions arising from K+ deficiency with salinity, drought and biotic factors in the second part. Information available so far suggests a prominent role of potassium and nitrate transport systems and their regulatory proteins in the response of plants to several stress combinations. Thus, such molecular pathways, which are located at the crossroad between K+ homeostasis and environmental stresses, could be considered biotechnological targets in future studies.