Abstract
Water, nutrient, light, and interspecific facilitation regulation of soil physicochemical properties and root morphology modulate nitrogen (N) uptake in cereal and legume intercropping systems. However, maize root morphological plasticity and N uptake capability response to gravity in the intercropping system remains to be determined. In this study, maize was grown under 20 cm (I20), 40 cm (I40), and 60 cm (I60) of narrow row spacing in an intercropping system (maize–soybean strip relay intercropping) and equal row spacing of monoculture (M) in a 2-year field experiment. As a supplementary for the field experiment, maize root barrier and plant inclination experiments were conducted. Plant inclination, brace root morphology, N uptake, indole-3-acetic acid (IAA) level, IAA synthesis genes, and grain yield were assessed. The result showed that the plant inclination increased with decreasing narrow row spacing in intercropping system. Also, the brace unilateral root growth ratio (BURR) increased with increasing plant inclination in intercropping treatments. The plant inclination experiment showed the BURR achieved 94% after inclination at 45°. BURR tended to be positively correlated (p = 0.00) with plant inclination. Thus, gravity (plant inclination) causes brace unilateral root growth. The IAA concentration of stem nodes in the wide row increased with increasing plant inclination, while the IAA accumulation decreased in the narrow row. The Zmvt2 and ZM2G141383 genes (associated with IAA biosynthesis) were highly expressed in a wide row. There was a strong correlation (p = 0.03) between the IAA concentration of wide row and the BURR. Therefore, gravity regulates the IAA level, which affects BURR. In addition, the brace root number, volume, and surface area were decreased when BURR was increased. Subsequently, the leaf N, cob N, and kernel N accumulation were reduced. These organs N and grain yield in I60 were not significantly different as compared to the control treatment. The excessive brace unilateral root growth was not conducive to N uptake and increased yield. Our results suggest that gravity is essential in regulating root morphology plasticity by regulating IAA levels and decreasing N uptake capacity. Furthermore, these results indicate that plant inclination can regulate root phenotype and N uptake of maize and by adjusting the spacing of narrow maize row, we can improve the N uptake and yield of the maize–soybean strip relay-intercropping system.
Highlights
It is assumed that the world population might reach 9.3 billion by the year 2050, which indicates that an increase in food production with limited resources is urgent for the growing population (Xie et al, 2017)
Intercropping system enhances N uptake by highly plastic root morphology and fungal diversity through interspecific interactions between cereal and legume (Ramirez-Garcia et al, 2015; Zhang et al, 2020)
It should be mentioned that we only examined the effect of plant inclination on the distribution of brace root and the associations between gravity and N uptake after the flowering stage when the first layer brace root of aboveground started to grow, and the plant began to incline at stage V14 (Supplementary Figure 5)
Summary
It is assumed that the world population might reach 9.3 billion by the year 2050, which indicates that an increase in food production with limited resources is urgent for the growing population (Xie et al, 2017). Intercropping maize enhances nitrogen fixation of faba bean by maize root exudates, enabling flavonoid synthesis in faba bean that increases nodulation (Li et al, 2016). Intercropping system enhances N uptake by highly plastic root morphology and fungal diversity through interspecific interactions between cereal and legume (Ramirez-Garcia et al, 2015; Zhang et al, 2020). Except for these findings, detailed investigations on how the environment (excluding the interspecific interactions) regulates root developmental plasticity to increase cereal N uptake is lacking. Water, and nutrient spatial distribution in intercropping are different from monoculture (Gao et al, 2010b; Liu et al, 2017; Rahman et al, 2017; Zhou et al, 2019a)
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