Large-Scale Evaluation of Maize Germplasm for Low-Phosphorus Tolerance

<div><p>Low-phosphorus (LP) stress is a global problem for maize production and has been exacerbated by breeding activities that have reduced the genetic diversity of maize. Although LP tolerance in maize has been previously evaluated, the evaluations were generally performed with only a small number of accessions or with samples collected from a limited area. In this research, 826 maize accessions (including 580 tropical/subtropical accessions and 246 temperate accessions) were evaluated for LP tolerance under field conditions in 2011 and 2012. Plant height (PH) and leaf number were measured at three growth stages. The normalized difference vegetation index (NDVI) and fresh ear weight (FEW) were also measured. Genetic correlation analysis revealed that FEW and NDVI were strongly correlated with PH, especially at later stages. LP-tolerant and -sensitive accessions were selected based on the relative trait values of all traits using principal component analysis, and all the 14 traits of the tolerant maize accessions showed less reduction than the sensitive accessions under LP conditions. LP tolerance was strongly correlated with agronomic performance under LP stress conditions, and both criteria could be used for genetic analysis and breeding of LP tolerance. Temperate accessions showed slightly better LP tolerance than tropical/subtropical ones, although more tolerant accessions were identified from tropical/subtropical accessions, which could be contributed by their larger sample size. This large-scale evaluation provides useful information, LP-tolerant germplasm resources and evaluation protocol for genetic analysis and developing maize varieties for LP tolerance.</p></div

<i>C. parasiticum</i> growth as affected by root exudates.

<p>A, growth performance, B, colony diameter, C, sporulation. LP, 15 µM P; HP, 500 µM P. MS: soybean monoculture; MC: maize monoculture; ISC: maize/soybean intercropping. F value from Two-way ANOVA: colony diameter, 685.62 for P treatment (<i>P</i><0.001), 1119.75 for cultivation mode (<i>P</i><0.001), 12.73 for interaction (<i>P</i><0.001); sporulation, 38.33 for P treatment (<i>P</i><0.001), 116.49 for cultivation mode (<i>P</i><0.001), 1.02 for interaction (not significant). Each bar represents the mean of four replicates ± SE. Bars with different letter(s) vary significantly among treatments as determined by Duncan's multiple range test (<i>P</i><0.05).</p

Classification of 826 maize accessions for their responses to LP stress based on LPTI and LPPI.

<p>LPPI: LP performance index; LPTI: LP tolerance index; TG: tolerant and good-performance group; SG: sensitive and good-performance group; TP: tolerant and poor-performance group; SP: sensitive and poor-performance group. The number in the bracketsis the number of maize accessions in each group.</p

Disease severity of soybean caused by <i>C. parasiticum</i> as affected by cultivation mode, P level and planting distance in the two years of field experiments on acid soils.

<p>Note: Disease incidence and index were measured as described in Materials and Methods. HP: 80 kg P₂O₅ ha⁻¹ added as calcium superphosphate, LP: no P fertilizer added. MS: soybean monoculture, ISC1: maize/soybean intercropping with 20 cm spacing; ISC2: maize/soybean intercropping with 5 cm spacing. All the data are the mean of four replicates ± SE. The same upper-case letter after numbers in the same column for the same trait in the same year indicates no significant difference among cultivation modes at 0.05 (<i>P</i><0.05); The same lower-case letter after numbers in the same row for the same trait in the same year indicates no significant difference between two P levels at 0.05 (<i>P</i><0.05).</p

Root Interactions in a Maize/Soybean Intercropping System Control Soybean Soil-Borne Disease, Red Crown Rot

<div><p>Background</p><p>Within-field multiple crop species intercropping is well documented and used for disease control, but the underlying mechanisms are still unclear. As roots are the primary organ for perceiving signals in the soil from neighboring plants, root behavior may play an important role in soil-borne disease control.</p><p>Principal Findings</p><p>In two years of field experiments, maize/soybean intercropping suppressed the occurrence of soybean red crown rot, a severe soil-borne disease caused by <i>Cylindrocladium parasiticum</i> (<i>C. parasiticum</i>). The suppressive effects decreased with increasing distance between intercropped plants under both low P and high P supply, suggesting that root interactions play a significant role independent of nutrient status. Further detailed quantitative studies revealed that the diversity and intensity of root interactions altered the expression of important soybean <i>PR</i> genes, as well as, the activity of corresponding enzymes in both P treatments. Furthermore, 5 phenolic acids were detected in root exudates of maize/soybean intercropped plants. Among these phenolic acids, cinnamic acid was released in significantly greater concentrations when intercropped maize with soybean compared to either crop grown in monoculture, and this spike in cinnamic acid was found dramatically constrain <i>C. parasiticum</i> growth <i>in vitro</i>.</p><p>Conclusions</p><p>To the best of our knowledge, this study is the first report to demonstrate that intercropping with maize can promote resistance in soybean to red crown rot in a root-dependent manner. This supports the point that intercropping may be an efficient ecological strategy to control soil-borne plant disease and should be incorporated in sustainable agricultural management practices.</p></div

Basic chemical characteristics of the soil (0 to 20 cm depth) used to evaluate 826 maize accessions in Guangzhou, China.

<p>Basic chemical characteristics of the soil (0 to 20 cm depth) used to evaluate 826 maize accessions in Guangzhou, China.</p

Disease severity of soybean red crown rot in sand culture.

<p>A, disease incidence, B, disease index, C, CFU. LP, 15 µM P; HP, 500 µM P. All of the data are the means of four replicates ±SE. Bars with different letter(s) vary significantly among the different inoculation treatments as determined by Duncan's multiple range test (<i>P</i><0.05).</p

Expression of eight defense-related (<i>PR</i>) genes in soybean roots.

<p>LP: 15 µM P; HP: 500 µM P. Soybean roots were inoculated with <i>C. parasiticum</i> (see Materials and Methods for details). Solid barrier: eliminate root interactions and exudates movement between the roots of the two plant species; mesh barrier: prevent root intermingling of two species while permitting root exudates exchange; no barrier: allow roots and exudates to completely interact. Each bar represents the mean of three replicates ± SE.</p

Distribution of all traits measured for 826 maize accessions.

<p>PH: plant height; LN: leaf number; NDVI: normalized difference vegetation index; FEW: fresh ear weight; DAP: days after planting; LP: low-P; NP: normal-P; Percentage: the number of accessions in each interval divided by the total number of accessions.</p

Root interactions between soybean and maize plants in the field.

<p>Root interactions were measured as the total root length in the upper 20: 80 kg P₂O₅ ha⁻¹ added as calcium superphosphate, LP: no P fertilizer added. ISC1: maize/soybean intercropping with 20 cm spacing; ISC2: maize/soybean intercropping with 5 cm spacing. All the data are the mean of four replicates ± SE. F value from Two-way ANOVA: 8.35 for P treatment (<i>P</i><0.05), 159.08 for cultivation mode (<i>P</i><0.001), 1.99 for interaction (not significant). Bars with different letter(s) vary significantly among treatments as determined by Duncan's multiple range test (<i>P</i><0.05).</p