Introduction
Potassium (K) is an essential nutrient (up to 5% of plant dry weight)
involved in crucial physiological processes such as electrochemical
homeostasis, stomatal aperture and enzyme catalysis (Wang & Wu, 2013;
Anschütz et al. , 2014). K deficiency inhibits plant growth and
primary production and therefore, intense efforts are being devoted to
improve K acquisition by plants (Shin, 2014; Rawat et al. , 2016).
K-deficient soils are relatively common in countries of the
intertropical region, such as Brazil, Equatorial Africa, China and
South-Eastern Asia (like Indonesia), and Australia
(www.fao.org) and thus important crops
like oil palm, sunflower, cotton, or rice are concerned. In addition,
K-deficient areas have variable geological substratum and soil total
base content (in particular exchangeable calcium), and as a result
background nutrient (ionic) conditions that accompany K deficiency are
highly variable. For example, in Sumatra (Indonesia) where oil palm is
cultivated, both Ca-poor (histosol) and Ca-rich (cambisol) soils can be
found, and such a difference in Ca has an important impact on K
fertilization decisions.
Symptoms of K deficiency in plants have been studied for more than fifty
years and include metabolic effects such as an inhibition of glycolysis
due to a decrease in pyruvate kinase activity, as well as accumulation
of typical metabolites such as putrescine (Jones, 1961; Jones, 1966;
Okamoto, 1966; Freeman, 1967; Okamoto, 1967; Okamoto, 1968; Besford &
Maw, 1976; Armengaud et al. , 2009; Hussain et al. , 2011;
Sung et al. , 2015). Metabolomics analyses of K deficiency inArabidopsis showed a reconfiguration of amino acid and organic
acid synthesis, consistent with a change in nitrogen assimilation (in
favour of neutral amino acids), and an inhibition of the most
K-sensitive enzyme, pyruvate kinase, thereby leading to altered
glycolytic metabolism (Armengaud et al. , 2009). Recently,
metabolic changes caused by low K availability have been described in
details in sunflower, and it has been shown that in addition to the
well-known build-up of putrescine, low K induced a strong increase in
respiratory CO2 efflux and modified the flux through the
C5-branched acid pathway; furthermore, the natural15N/14N isotope composition
(δ15N) in leaf compounds showed that there was a
change in nitrate circulation, with less nitrate influx to leaves under
low K (Cui et al. , 2019a).
Importantly, as pointed out recently (Cui et al. , 2020), K
deficiency is not associated with a general decrease, but actually leads
to a significant increase in cation load, which comes from the
considerable increase in Ca2+ (and occasionally in
Mg2+) in tissues. In fact, there is an antagonism
between K+ and Ca2+ (Dibb &
Thompson, 1985; Jakobsen, 1993; Daliparthy et al. , 1994), and the
K x Ca interference in fertilization has been documented for
nearly 50 years in many crops (such as poplar, sugarcane, sunflower,
rapeseed, tobacco, oil palm, soybean, castor bean or wheat). Also, if
K-deficiency occurs under low Ca conditions, typical K symptoms such as
putrescine accumulation are partly suppressed (Richards & Coleman,
1952; Coleman & Richards, 1956). Therefore, despite the role of
Ca2+ in signaling to remobilize vacuolar
K+ ions under low K conditions (Amtmann & Armengaud,
2007; Pandey et al. , 2007; Tang et al. , 2020), some of the
commonly observed K deficiency symptoms probably reflect the response to
a disequilibrium in external (soil) cation composition, in which
Ca2+ is over-represented. To our knowledge, this
question has never been tackled directly. In a recent meta-analysis of
papers dealing with the effect of nutrient antagonism on yield, the
antagonism between K and Ca appears to be understudied as compared to Kx Mg antagonism (Rietra et al. , 2017). Also, the detailed
metabolic responses to K availability when Ca is varied have not been
documented. In this context, putrescine is an interesting biomolecule,
since its role under K deficiency has been suggested to be to both
mitigate reactive oxygen species (ROS) production –and thus the
down-regulation of mitochondrial damage– and participate in cellular
Ca2+ homeostasis (Cui et al. , 2020).
To clarify these aspects, we cultivated sunflower plants under different
K availability conditions (low, 0.2 mM, and high, 4 mM), under low or
high Ca conditions, with or without addition of putrescine (experimental
design detailed in Fig. S1). We carried out physiological, ionomics,
metabolomics and proteomics analyses and also used15N-labelling to assess nitrate absorption. Our
objectives were to assess whether (i) some of the typical
metabolic symptoms of K deficiency could actually be suppressed by the
use of low Ca conditions, and (ii) putrescine addition alleviates
some metabolic effects of K deficiency or on the contrary, triggers a
low-K response. Our results show a considerable effect of Ca on
metabolism, low Ca conditions compensating for some of low-K symptoms.
Putrescine addition is found to improve N nutrition but exaggerates
glycolysis inhibition, suggesting that it plays a dual role under K
deficiency.