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.