The response to low K is partly driven by Ca
A common drawback in manipulating nutrient composition is the potential change in other cations due to electro-neutrality, making overall mineral conditions not comparable (Cramer et al. , 1986). Here, we have used low and high Ca conditions that were compensated for by sodium (Na) for the chemical form of nitrate. However, low Ca conditions were not associated with a drastic augmentation of the Na content in tissues and thus an onset of salt stress: in fact, (i ) Na always remained a minor component in leaves; and (ii) in roots, Na was increased by about 20% only under low Ca. Therefore, the effects we found here under low Ca conditions were not caused by salinity.
Our results clearly demonstrate that amongst effects observed under low K conditions, the decline in photosynthesis was in reality partly due to Ca. In fact, in young leaves, low Ca conditions could restore some photosynthetic activity (while it was not so in old leaves) (Fig. 2). In addition, the decline in the content of many proteins involved in photosynthesis could be compensated for by low Ca conditions (Fig. S5b). Surprisingly, low Ca aggravated the increase in leaf dark respiration triggered by low K conditions (Fig. 2), in agreement with TCAP intermediates being higher under both low K and low Ca (Fig. 4), whereas some proteins of respiratory metabolism were more abundant under low K + high Ca, not low K + low Ca (such as aconitase, Fig. 6). This contradiction is perhaps explained by (i) changes in post-translational modifications, (ii) the action of effectors on enzymes and/or (iii) a lower ATP/O2 efficiency of mitochondrial metabolism. In particular, we note that low Ca conditions led to a decline in Mg leaf content, and in principle, this must have affected mitochondrial-cytosolic ADP/ATP exchange and interconversion (Bligny & Gout, 2017).
It is worth noting that unlike photosynthesis, low Ca conditions did not compensate for the alteration of N metabolism at low K. In fact, low Ca aggravated the decline in the content of root proteins involved in N metabolism (Fig. S5a) and increased the content in non-aminated precursors (2-oxoglutarate) (Fig. 5c). The reverse was true at high K, where low Ca upregulated the content in proteins involved in root N metabolism, including nitrate transporters. However, nitrate assimilation measured using 15N-nitrate was increased by low Ca under low K conditions, and decreased by low Ca at high K (Fig. 3). This surprising result likely reflects the fact that under K-deficient conditions, the general effect of low K impacts not only on root development and protein synthesis but also on root cation balance (increase in Ca2+ but low content in unicharged species) and this leads to a decline in nitrate capture by root cells. In fact, Ca2+ addition has been found to be detrimental to nitrate influx in root cells (Kafkafi et al. , 1992; Aslam et al. , 1995). Accordingly, here15N-nitrate absorption was found to be low. When low Ca conditions are used, Na+ can partly substitute for K+ (Fig. 1) and this restores nitrate absorption, thus downregulating the synthesis of high affinity nitrate transporters.
Under high K conditions, such a role for Na+ ions is unnecessary and essentially, nitrate absorption is accompanied by K+ absorption. In cotton grown at 4 mM [K+], increasing [Ca2+] from 2 to 10 mM has only a small effect on nitrate (and K) absorption (Leidiet al. , 1991). Also in cotton grown at 2.5 mM [K+] in the presence of NaCl, low [Ca2+] (<0.25 mM) inhibits nitrate absorption (Gorham & Bridges, 1995). Similarly, nitrate absorption by wheat seedling is inhibited by Ca deficiency (Minotti et al. , 1968). K and nitrate absorption requires sufficient Ca2+ to maintain transmembrane electrochemical gradient and therefore, low Ca has a negative impact on nitrate absorption. It is also interesting that low Ca conditions at high K inhibited nitrate translocation to shoots (Fig. 3), probably due to similar ion imbalance in xylem sap loading and electroneutrality.