Perspectives
Taken as a whole, our results show that amongst typical symptoms of K
deficiency, the alteration of photosynthesis can be in part compensated
for by low Ca nutrition, via the upregulation of proteins involved in
the photosynthetic machinery. Similarly, the inhibition by low K of
pyruvate kinase could be partly alleviated by low Ca conditions. By
contrast, low Ca could not compensate for changes in N assimilation,
likely due to complicated interactions with other ions
(Na+) and transmembrane potential. Low Ca
down-regulated the amount of putrescine in roots only. Putrescine itself
was able to alleviate some effects of low K such as the reconfiguration
of S metabolism, but exaggerated other effects (pyruvate kinase content)
suggesting a role of putrescine in signaling.
Other consequences of K and Ca availability have not been extensively
discussed here, such as the modulation of micronutrients: (i )
iron (Fe) content and iron-dependent enzymes (2-oxoglutarate
Fe2+-dependent, ferrochelatase, ferritin) are affected
by K and/or Ca level (Figs. 1, S5), and (ii ) magnesium is
affected by both K and Ca (magnesium content, Fig. 1; magnesium
chelatase, Fig. S5). Of course, a detailed analysis of micronutrient
absorption and allocation would require further analyses, including the
use of isotopic tracers (54Fe,25Mg). Similarly, more work would be required to
better understand the specific metabolic effects of putrescine, using
isotopically labelled putrescine (15N or13C) in particular to identify key reactions and
quantify putrescine catabolism to NH4+(via CAO) and CO2 (via the TCAP).
Our results also raise the question as to whether low Ca conditions or
putrescine addition are viable solutions to alleviate some effects on K
deficiency in crops. Although our results are limited to young sunflower
plants under controlled conditions, it seems that none of these two
possibilities is ideal. In fact, low Ca further increases respiration
and thus the overall impact on carbon mass balance might not be
beneficial. Putrescine addition is costly (≈ $1 g-1)
and does not suppress the strong effect of low K on respiratory efflux
(Fig. 2). Therefore, monitoring plant K status using new technologies
such as metabolomics (Cui et al. , 2021), and sustainable K
fertilization might be better options to ensure optimal K nutrition for
crops in the field.