12.6: Potassium- An overlooked limiting element?
Nitrogen \((\mathrm{N})\) is one of the most abundant elements on the earth and plays an important role in modern agriculture (Chen et al., 2018). N has an irreplaceable role in organ construction, material metabolism, fruit yield, and the quality formation of fruit trees (Warner et al., 2004; Liu et al., 2010; Bai et al., 2016). Sufficient \(\mathrm{N}\) application not only improves the photosynthetic efficiency of the leaves but also promotes flower bud differentiation (Li et al., 2012; Wei et al., 2016; Liu et al., 2018), enhances fruit setting rate, and increases yield (Raese et al., 2007). Due to the one-sided pursuit of high yield and large fruits by fruit farmers, the excessive application of \(\mathrm{N}\) fertilizer in apple orchards has become common in China. The applied \(\mathrm{N}\) fertilizer dose is are more above the demand of the tree (Ge et al., 2018). Furthermore, the over application of \(\mathrm{N}\) increases the input cost of farmers, decreases fruit yield and quality (Chen et al., 2017), and directly leads to a decreased \(\mathrm{N}\) utilization rate. Excessive \(\mathrm{N}\) also indirectly results in adverse ecological effects, including the increased \(\mathrm{N}\) deposition, the intensification of the greenhouse effect, soil acidification, and the eutrophication of water bodies (Liu et al., 2013; Zhu and Zhang, 2017). Therefore, it is a main issue for Chinese apple production to promote the efficient utilization of \(\mathrm{N}\) to improve the quality and yield of apple.
Potassium \((K)\) is the most abundant inorganic cation, and it is important for ensuring optimal plant growth (White and Karley, 2010). K is an activator of dozens of important enzymes, such as protein synthesis, sugar transport, \(\mathrm{N}\) and \(\mathrm{C}\) metabolism, and photosynthesis. It plays an important role in the formation of yield and quality improvement (Marschner, 2012; Oosterhuis et al., 2014). K is also very important for cell growth, which is an important process for the function and development of plants (Hepler et al., 2001). In terms of the growth-promoting mechanism of \(K\), it is generally agreed that \(K\) stimulates and controls ATPase in the plasma membrane to generate acid stimulation, which then triggers cell wall loosening and hydrolase activation (Oosterhuis et al., 2014), thus promoting cell growth. \(\mathrm{K}\) has strong mobility in plants and plays an important role in regulating cell osmotic pressure and balancing the cations and anions in the cytoplasm (Kaiser, 1982; Hu et al., 2016a). Through these processes, \(\mathrm{K}\) is involved in the regulation of stomatal opening and closing, cell elongation, and other important physiological processes. There are many studies on the effect of \(K\) level on plant growth. Jin et al. (2007) found that the highest yield and fruit quality were obtained in Red Fuji apple under treatment with \(600 \mathrm{~kg} / \mathrm{ha}\) K; Wang et al. (2017) determined that \(6 \mathrm{mMK}\) treatment promoted pear growth and improved photosynthetic efficiency; and Lu et al. (2001) also reported increased production with better fruit quality parameters in navel orange supplied under \(500 \mathrm{~kg} / \mathrm{ha} \mathrm{K}\). There is an interaction between \(\mathrm{K}\) and other nutrient ions. High \(\mathrm{K}\) concentrations in the soil solution inhibit Mg uptake and may induce Mg deficiency in plants (Tränkner et al., 2018). However, \(\mathrm{K}\) deficiency could promote the absorption of \(\mathrm{Na}^{+}\)and \(\mathrm{Ca}^{2+}\) in maize (Du et al., 2017), and could inhibit \(\mathrm{N}\) absorption in cotton and significantly reduce the content of \(\mathrm{NO}_3^{-}\)in the leaves (Hu et al., 2017). It is evident that \(K\) affects significantly the absorption and utilization of other nutrients by plants, and the appropriate \(K\) level differs in different crops.
Among the interactions between \(\mathrm{K}\) and other nutrients, the interaction with \(\mathrm{N}\) is the most important. Some studies evaluated the relationship between \(\mathrm{K}\) and \(\mathrm{N}\) metabolism. In contrast to the antagonistic relationship between \(\mathrm{K}^{+}\)and \(\mathrm{NH}_4{ }^{+}\)nutrition, the acquisition rates of \(\mathrm{K}^{+}\)and \(\mathrm{NO}_3{ }^{-}\)are often found to be positively correlated (Rufty et al., 1981; Coskun et al., 2016), and sufficient K supply can promote \(\mathrm{N}\) metabolism and enhance the synthesis of amino acids and proteins (Ruan et al., 1998; Ruiz and Romero, 2002). Hu et al. (2016b) found that K deficiency could reduce Nitrate reductase (NR), Glutamine synthetase (GS), and Glutamate synthase (GOGAT) activities and inhibit nitrate absorption in cotton, whereas Armengaud et al. (2009) found that K deficiency could up-regulate the activities of GS and Glu dehydrogenase (GDH) in Arabidopsis. Metabolism of \(\mathrm{N}\) affected by \(\mathrm{K}\) appears to vary in different types of plants. Meanwhile, the level of \(\mathrm{K}\) has a significant impact on \(\mathrm{C}\) metabolism, and also a strong interaction exists between \(\mathrm{C}\) metabolism and \(\mathrm{N}\) metabolism in the metabolic process and energy level (Hu et al., 2017).
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Excerpted from:
Xu, X., Du, X., Wang, F., Sha, J., Chen, Q., Tian, G., ... & Jiang, Y. (2020). Effects of potassium levels on plant growth, accumulation and distribution of carbon, and nitrate metabolism in apple dwarf rootstock seedlings. Frontiers in Plant Science , 11 , 904. Accessed December 2023 at https://www.frontiersin.org/articles/10.3389/fpls.2020.00904/full . CC-BY-4.0