Abstract
Evaluation of nitrogen (N) status by leaf color is a kind of classic nutritional diagnostic method. However, the color of leaves is influenced not only by N, but also by other nutrients such as potassium (K). Two-year field trials with a factorial combination of N and K were conducted to investigate the effects of different N and K rates on soil plant analysis development (SPAD) readings and leaf N, K, magnesium (Mg), and chlorophyll concentrations. Visual inspections in leaf greenness revealed darker green leaves with increasing N rates, while paler green leaves with increasing K rates. Data showed that SPAD readings, chlorophyll, N and Mg concentrations, and the chloroplast area increased significantly with raising N rates, while declined sharply with the increase in K rates due to the antagonistic relationships between K+ and NH4 + as well as Mg2+. It was also probable that the increase in K promoted the growth of leaves and diluted their N and Mg concentrations. The paler leaf appearance resulting from the application of K may overestimate the actual demand for N in the diagnosis of rice N status. The strong antagonistic relationships between K+, NH4 +, and Mg2+ should be considered in rice production and fertilization.
Highlights
Rice (Oryza Sativa L.) is one of the most important crops feeding the world's population (Chapagain and Hoekstra, 2011)
Compared to the N0 treatment, the application of 270 kg N ha−1 increased the soil plant analysis development (SPAD) readings by 15.3%, 18.1%, 17.4%, and 17.7% in 2016 and by 24.3%, 23.5%, 21.0, and 18.4% in 2017, respectively, at the four K rates
Many studies have investigated the relationships between leaf N concentrations and SPAD readings in the past 20 years, and it has been universally accepted that there were significant relationships between leaf N concentrations and SPAD readings as well as leaf chlorophyll concentrations (Castelli et al, 1996; Huang et al, 2008; Lavres and Monteiro, 2010; Wang et al, 2014)
Summary
Rice (Oryza Sativa L.) is one of the most important crops feeding the world's population (Chapagain and Hoekstra, 2011). With the increasing population and decreasing arable land, it is of great importance to ensure food safety by improving rice yield per unit area (Jiang et al, 2016). Nitrogen (N) and potassium (K) are two elements that are required in great quantities for rice growth and yield formation (Pettigrew, 2008; Zhu et al, 2008; Oosterhuis et al, 2014). Farmers usually apply excessive amounts of N to ensure high grain yields, which greatly depend on N inputs (Castillejo et al, 2010; Kaufman et al, 2013). The supply of N significantly improves grain yield, the low N use efficiency for the surplus application of N has been a major
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