Comparative Analysis Of Agricultural Traits Of Improved Maize Inbred Line224and Its Parental Lines And Effects Of Morphological Plasticity Of The Root In Response To Low Phosphorus Stress

The physiological and biological potential of crops to take up and utilize nutrients underlies efficient nutrient utilization and high grain yield in crop production. A critical step is to breed crop varieties with high yield and high nutrient use efficiency. Molecular marker assisted breeding paves the way for modern crop breeders. Here, we carried out comparative studies of improved inbred line224, donor parent478, and recurrent parent312, in terms of leaf area, biomass, ear traits, grain yield, phosphorus (P) uptake, and root morphology via two consecutive year’s greenhouse and field experiments under different P regimes. Additionally, we hypothesized that maize responds to low P stress via root morphological adaptation that enhances P uptake. To test this hypothesis, we compared the root/shoot ratio, root morphological traits, and related physiological parameters between maize and faba bean under different P levels in a two-year filed experiment. The main results were as follows:(1) The improved inbred line224, similar to its recurrent parent312, was inferior to donor parent478at the seedling stage in terms of root morphological adaptation, compensatory root growth, biomass accumulation, and P uptake in hydroponic experiments under low P (5μmol/L) and high P (250μmol/L) conditions and after root-restriction treaement (removing seminal roots and brace roots), suggesting that introgressed DNA segments in line224had no detectable effects on plant growth and P uptake at seedling stages.(2) Similar to the results derived from the hydroponic experiments, the total root length, lateral root length, and vertical distribution of roots along the0-40cm soil profile of line224were statistically similar to those of its recurrent parent312at silking and maturity under different P conditions in a two-year field experiment, but lower or less than those of donor parent478. However, the total leaf area, the whole plant dry weight, grain yield and P uptake of line224were significantly higher than its recurrent parent312, especially under the low P condition, indicating that the introgressed DNA segment in224enhances post-seedling growth and P uptake in the field. Also, P uptake after silking in line224, similar to the donor parent478, is significantly higher than that of line312according to two year’s results, suggesting that certain unknown DNA segments mediating P uptake after silking may be introgressed into line224. In addition, there was no significant difference in the Olsen-P concentration in the0-40cm soil profile at silking and maturity among three inbred lines grown under the same P level; we also found no distinctive patterns of Olsen-P variation in the rhizospheric and bulk soil of each of the three lines.(3) We analyzed root distribution patterns along the0-60cm soil profile and found that the fine root of inbred line478increased in length in the40-50cm soil profile at jointing and silking stages. There was no significant difference in the inorganic P fraction between the rhizospheric and bulk soil.(4) Maize and faba bean root systems had different adaptive responses to low P according to two year’s field experiments. There was a significantly increase in the root/shoot ratio in maize and maize had larger total root length in the0-20cm soil profile under low P, in comparison with faba bean. On the contrary, the activity of acid phosphatase significantly increased and the pH had a significant decrease in the rhizosphere of faba bean.In conclusion, our results provided theoretical basis for P efficient high-yield crop breeding with appropriate P supplies. However, there were no obvious difference between224and312in either total root length or root vertical distribution within0-40cm soil profile at field and hydroponic experiments, although some important QTLs associated with root traits have been introgressed from the donor parent478into224; the results implied that DNA segment controlling post-silking P uptake was introgressed from the donor parent478into224, which had not been identified yet. In addition, maize, in contrast to faba bean, increased P uptake mainly via root morphological adaptation upon low P stress. The results suggest that it is more important in maize production to increase root growth by P fertilizer application and rhizospheric management, and thus to increase P use efficiency.