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.