Introduction:
Nitrogen (N) is an essential macronutrient, playing the principal role in building block for the fundamental biomolecules such as amino acids, proteins, nucleic acids and chlorophyll. In agricultural systems, plants access soil nitrogen in two inorganic forms: nitrate (NO3⁻​), and ammonium (NH4⁺​). The relative availability of these forms in soil and plant preference for them, significantly influence the plant growth, metabolism and crop yield. Sugar beet (Beta vulgaris L.) is recognized as a nitrate-preferring species, but is capable to utilize ammonium. High concentrations of NH4⁺ as the sole nitrogen source is toxic‌ for sugar beet and induce obvious stress. The ammonium toxicity is complicated and directly impacts cellular homeostasis such as cytosolic acidification and ionic imbalance. These alterations can modify the cellular metabolism and promotes structural changes. 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:
This study was conducted hydroponically using a completely randomized design with two treatments (nitrate and ammonium, 5 mEq L^-¹) and eight repetitions for each treatment. Seedlings were grown for 30 days in a in a growth chamber at 33 ± 1^°C/22 ± 1^°C (day/night) and 85% relative humidity, with a 16/8 h (light/dark) photoperiod. Nutrient solutions were changed every six days, and pH was adjusted to 6.0 every two days. After harvesting, the growth parameters and some biochemical features such as chlorophyll content, soluble and insoluble sugar concentrations, lignin content of hypocotyls and total protein were assayed by spectrophotometric methods. The extension of xylem tissue was measured in plants by preparing the microscopic sections. The means were compared using 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. Although ammonium is a direct source for N, its exclusive usage severely inhibits sugar beet growth. This stunting is likely due to the high energetic cost of ammonium detoxification and diverting photosynthates from growth pathway. 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. These changes reduce cell wall flexibility and cause the growth limitations. Significantly, the soluble and non-soluble sugars contents were lower, but the total protein was higher in shoot of plants received ammonium. The reduced levels of sugars indicate limitations in carbon allocation, which is consistent with earlier studies suggesting that ammonium nutrition can disturb carbohydrate metabolism. In contrast, the increase in total protein of shoots may reflect enhanced nitrogen assimilation under ammonium supply. The chlorophyll content of sugar beet plants did not affect by nitrogen source. 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. Lignin accumulation could explain the observed growth inhibition due to decreased wall flexibility. Microscopic studies revealed that ammonium had obviously caused xylem tissue development in hypocotyl region. 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:
According to results, this study demonstrates that utilizing ammonium as the sole nitrogen source in Beta vulgaris (sugar beet) plants, imposes numerous significant physiological and structural constraints. Reduced sugar accumulation, increased lignification and alterations in xylem development, cooperatively inhibit overall plant growth.