Optimal NH4+ /NO3- ratios enhanced the shade tolerance of soybean seedlings under low light conditions.

In maize-soybean intercropping system, shade is the major chronic restrain that effected the normal growth of soybean. Different spatial patterns of this system have affected the microclimate of soybean by shading of maize plants. However, the negative impacts of shading stress can be mitigated by providing the optimal ratios of different fertilizers. Therefore, to test this hypothesis, the soybean plants were grown under different light conditions (normal light and shade) to evaluate the response to varying NH4+ /NO3- ratios. The seed of soybean (Glycine max L. cultivar of "Nan-99-6") were grown in nutrient solution at a total concentration of 5mM N using different NH4+ /NO3- ratios (To= 0:0, T1= 0:100, T2= 25:75, T3=50:50, and T4= 75:25) for 40 days in greenhouse at PPFD 320.95 μmolm-2 s-1 (low light intensity) and 967.53 μmolm-2 s-1 (normal light intensity). Under low light the growth and photosynthesis of soybean seedlings had significantly decreased as compared to normal light conditions. Nonetheless, the optimal ratios of NH4+ /NO3- had improved the growth and photosynthesis activities 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 showed the maximum biomass yield, chlorophyll pigments, leaf gas exchange, photochemical activities and root growth as compared to high NH4+ /NO3- ratios (T1 and T4). Since, the higher ratios of NH4+ /NO3- resulted in lower plant growth due to accumulation in plant tissues; therefore, we suggested that optimal ratios of NH4+ /NO3- can enhance the shade resistance of soybean seedlings.