Optimal NH₄⁺/NO₃⁻ ratios enhance the shade tolerance of soybean seedlings under low light conditions

Abstract

In the maize–soybean intercropping system, shade is the major chronic restraint that affects normal growth of soybean. Different spatial patterns of this system affect the microclimate of soybean through shading from maize plants. However, the negative impacts of shading stress can be mitigated by providing optimal ratios of different fertilizers. Therefore, to test this hypothesis, soybean plants were grown under different light conditions (normal light or shade) to evaluate the response to varying NH4+/NO3− ratios. Seeds of soybean (Glycine max L. cv. Nan‐99‐6) were grown in nutrient solution with a total concentration of 5 mM N using different NH4+/NO3− ratios (T0 = 0:0, T1 = 0:100, T2 = 25:75, T3 = 50:50 and T4 = 75:25) for 40 days in a greenhouse at PPFD 320.95 μmol m−2 s−1 (low light) or 967.53 μmol m−2 s−1 (normal light). Under low light, growth and photosynthesis of soybean seedlings significantly decreased as compared to normal light conditions. However, the optimal ratios of NH4+/ NO3− improved growth and photosynthesis of soybean seedlings under both light conditions. Our results indicated that soybean seedlings supplied with optimal NH4+/NO3− ratios (25:75 and 50:50) have maximum biomass yield, chlorophyll pigments, leaf gas exchange, photochemical activity and root growth as compared to low and high NH4+/NO3− ratios (T1 and T4). High ratios of NH4+/NO3− (T4) resulted in reduced plant growth due to nutrient accumulation in plant tissues; therefore, we suggest that optimal ratios of NH4+/NO3− (T2 and T3) can enhance the shade tolerance of soybean seedlings.

The impact of different growth media and ammonium-nitrate ratio on yield and nitrate accumulation in lettuce (Lactuca sativa var. longifolia)

“…In another study on cabbage, it is reported that NH4 + :NO3ratio has no impact on leaf count (Assimakopoulou et al, 2019). Raza et al (2021) demonstrate that only NH4 + or NO3applied to soybeans under normal or low light availability, leaf fresh and dry weights decreased. However, they also reported that if both N forms are applied together, these levels increase.…”

“…The NH4 + :NO3ratio can be used as a strategy to eliminate abiotic stresses such as low light availability, salinity, drought, and heat. Numerous studies have suggested that growing plants with only NH4 + or NO3with various NH4 + :NO3ratios can affect tolerance of plants low light availability (Raza et al, 2021), water stress (Shang and Shen, 2018) or salinity (Meng et al, 2016;Ashraf et al, 2021).…”

Ammonia–Nitrate Mixture Dominated by NH4+–N Promoted Growth, Photosynthesis and Nutrient Accumulation in Pecan (Carya illinoinensis)

“…Nitrogen (N) is a nutrient that plays a key role in plant growth and development, and ammonia-nitrogen (NH 4 + -N) and nitrate-nitrogen (NO 3 − -N) are the two main N forms absorbed and utilized by plants [1]. NH 4 + enters the plant and combines with organic acids to form amino acids and amides [2]; NO 3 − is absorbed into plants and cannot be used directly by plants: part of it is reduced to NH 4 + [3], and part is stored in vacuoles [4]. Different plants have different preferences for the uptake of NH 4 + and NO 3 − .…”