Discussion
Response mechanisms to K deficiency have been extensively studied and include changes in metabolism (such as putrescine build-up), gene expression and physiological parameters such as photosynthesis, chlorophyll synthesis and growth. Our study is broadly consistent with documented effects of low K conditions on metabolism (Ashley et al. , 2006; Armengaud et al. , 2009; Wang & Wu, 2013). Typically, there were (i) a general decline in photosynthesis due to lower stomatal conductance and a down-regulation of the biosynthesis of the photosynthetic machinery; (ii) an inhibition of nitrate absorption and assimilation; and (iii) changes in respiratory metabolism, with higher respiration rates and higher content in several catabolic enzymes (such as malic enzyme) despite an inhibition of glycolysis due to lower pyruvate kinase activity. The typical inhibition of pyruvate kinase by low K conditions comes from the fact that this enzyme uses K+ as a cofactor (Evans, 1963; Nowak & Mildvan, 1972) but also, as found here, a lower enzyme abundance as also found in tomato (Besford & Maw, 1976). Here, we have carried out a dissection of metabolic responses to K availability when it interacts with calcium and putrescine to better understand specific pathways affected by K itself, and effects of Ca/K balance or putrescine accumulation.