Abstract

Introduction

Nitrogen (N) is an essential macronutrient, playing the primary role in building block for crucial biomolecules such as amino acids, proteins, nucleic acids and chlorophyll. In agricultural systems, plants primarily access soil nitrogen in two inorganic forms: the oxidized form, nitrate (NO3−), and the reduced form, ammonium (NH4+). The relative availability and plant preference for these forms significantly influence plant growth, metabolism and ultimately, crop yield and quality. Sugar beet (Beta vulgaris L.) is widely recognized as a nitrate-preferring species (Munekata et al., 2021). While sugar beets are capable to utilizing ammonium. High concentrations of NH4+ as the sole nitrogen source induce marked stress responses and leading to growth reduction and significant metabolic disturbances. The toxicity associated with ammonium is complicated and directly impacts cellular homeostasis such as cytosolic acidification, ionic imbalance, and excessive energy demand for ammonium assimilation (Balkos et al., 2010). These alterations can modify the cellular metabolism and promotes structural changes, such as increased lignification, which negatively affect sucrose accumulation and yield quality. The aim of this study was to investigate the effects of ammonium and nitrate on growth, xylem development and some biochemical features in sugar beet plant.

Materials and Methods

Plants were grown hydroponically in a greenhouse with two nitrogen forms (nitrate and ammonium, 5 mEq L-¹) and four replications in a completely randomized design. Seedlings were maintained for 30 days in a growth chamber at 33 ± 1 °C day/22 ± 1 °C night and 85% relative humidity, with a 16 h light/8 h dark photoperiod. Nutrient solutions were changed every six days, and pH was adjusted every two days. After harvesting, the growth parameters and some biochemical features were assayed. Chlorophyll content was spectrophotometrically quantified according to the method described by Moran (1982), soluble and insoluble sugar concentrations were determined using the colorimetric technique outlined by Dubois et al. (1956) and the lignin content of hypocotyls was assayed by Samar et al. (1984) method. Total protein was assayed via the method of Lowry et al. (1951), with bovine serum albumin as the standard. The extension of xylem tissue was measured in plants by preparing the microscopic sections. The means were compared using Sigma Stat (3.02) software and Tukey's test at a probability level of 5%.

Results and discussion

Ammonium application as the sole nitrogen source strongly inhibited the growth of sugar beet plants. The fresh and dry weights of plants, root and shoot heights and leaves numbers and area were significantly decreased by ammonium compared to nitrate. Although ammonium is a direct N source, its exclusive use severely inhibits sugar beet growth, consistent with findings in other plants. This stunting is likely due to the high energetic cost of ammonium detoxification and diverting photosynthates from growth pathway, that potentially exacerbated by rhizosphere pH changes. Mechanistically, ammonium stress induces elevated putrescine, altered phytohormone signaling (e.g., increased ethylene and suppressed auxin transport via AUX1/PIN2), and the induction of lignin biosynthesis, which reduces cell wall flexibility and causes the observed growth limitations. Significantly the soluble and non-soluble sugars contents were lower, but the total protein and chlorophyll contents were higher in ammonium fed plants compared to nitrate fed ones. The reduced levels of sugars indicate limitations in carbon allocation, which is consistent with earlier studies suggesting that ammonium nutrition can disturb carbohydrate metabolism (Roosta & Schjoerring, 2007). In contrast, the increases in total protein and chlorophyll contents may reflect enhanced nitrogen assimilation under ammonium supply. Moreover, ammonium significantly increased the lignin content by induction of peroxidase enzyme activity. It has been reported that ammonium can stimulate lignin biosynthesis by induction of relative enzymes (Yang et al., 2022). Lignin accumulation could explain the observed growth inhibition due to decreased wall flexibility. Additionally, microscopic studies were revealed that ammonium had obviously caused to development of xylem tissue in hypocotyl region. This phenomenon also has been described in earlier anatomical studies under ammonium stress in Populus and Arabidopsis (Kogel, 2020). Under ammonium nutrition, increased lignin biosynthesis diverts carbon flux from primary carbohydrate metabolism. As a consequence, carbon skeletons that would normally be used for sugar and starch biosynthesis are channeled into the phenylpropanoid pathway, leading to greater lignification and lower soluble sugar accumulation in plants.

Conclusion

This study comprehensively demonstrates that utilizing ammonium exclusively as the sole nitrogen source imposes numerous significant physiological and structural constraints on Beta vulgaris (sugar beet) plants. Key findings indicate that reduced net sugar accumulation, notably increased lignification within structural tissues and significant alterations in xylem development collectively inhibit overall plant growth. Even though, ammonium led to an obvious enhancement in both total cellular protein and chlorophyll contents, but these specific biochemical improvements ultimately proved insufficient to offset the pervasive detrimental effects impacting essential cell wall flexibility and the overall efficiency of primary carbon metabolism pathways.