Effects of shading on the photosynthetic characteristics and mesophyll cell ultrastructure of summer maize.

A field experiment was conducted to study the effects of shading on the photosynthetic characteristics and mesophyll cell ultrastructure of two summer maize hybrids Denghai605 (DH605) and Zhengdan958 (ZD958). The ambient sunlight treatment was used as control (CK) and shading treatments (40 % of ambient sunlight) were applied at different growth stages from silking (R1) to physiological maturity (R6) (S1), from the sixth leaf stage (V6) to R1 (S2), and from seeding to R6 (S3), respectively. The net photosynthetic rate (P n) was significantly decreased after shading. The greatest reduction of P n was found at S3 treatment, followed by S1 and S2 treatments. P n of S3 was decreased by 59 and 48 % for DH605, and 39 and 43 % for ZD958 at tasseling and milk-ripe stages, respectively, compared to that of CK. Additionally, leaf area index (LAI) and chlorophyll content decreased after shading. In terms of mesophyll cell ultrastructure, chloroplast configuration of mesophyll cells dispersed, and part of chloroplast swelled and became circular. Meanwhile, the major characteristics of chloroplasts showed poorly developed thylakoid structure at the early growth stage, blurry lamellar structure, loose grana, and a large gap between slices and warping granum. Then, plasmolysis occurred in mesophyll cells and the endomembrane system was destroyed, which resulted in the dissolution of cell membrane, karyotheca, mitochondria, and some membrane structures. The damaged mesophyll cell ultrastructure led to the decrease of photosynthetic capacity, and thus resulted in significant yield reduction by 45, 11, and 84 % in S1, S2, and S3 treatments, respectively, compared to that of CK.

Effects of shading on spike differentiation and grain yield formation of summer maize in the field.

A field experiment was conducted to study the effects of shading on tassel and ear development and yield formation of three summer maize hybrids Zhenjie 2 (ZJ2), Denghai 605 (DH605), and Zhengdan 958 (ZD958). The ambient sunlight treatment was used as control (CK) and shading treatments (40 % of ambient sunlight) were applied at different growth stages from silking stage (R1) to physiological maturity stage (R6) (treatment S1), from the sixth extended leaf stage (V6) to R1 (treatment S2) and from seeding to R6 (treatment S3). Shading had no significant effect on the time from seeding to shoot emergence (VE); however, subsequent growth and development were delayed with shading beyond this point. The differentiation time of both tassel and ear delayed, and female spike (tassel) floret differentiation, sexual organ formation time, and anthesis-silking interval (ASI) were lengthened. After shading, the total number of floret, silk, and fertilization floret reduced significantly; the number of abortive seeds increased, and the total setting percentage among different treatments showed that CK>S2>S1>S3; and the total setting percentages in S1, S2, and S3 of ZD958 were 44, 72, and 15 % respectively. The total floret number of tassel primordium differentiation, fertility rate, and seed setting rate of florets in S3 treatment was the minimum; kernels per ear decreased seriously and single ear setting percentage was only 16 %; although floret degeneration number of S2 during ear differentiation stages increased and floret fertility rate reduced than that of CK, fertilization flower seed production increased and abortive seed decreased after canceling shading. Aborted kernel of S1 increased and kernel dry weight reduced, resulting in a significant decrease of kernel number per ear and kernel weight, and the grain abortive rate of 40-62 %. In conclusion, shading changed the growth and development process and caused infertility of tassel and ear; tassel branches decreased, reducing pollen vitality and silks differentiation cut down; and grain dry matter accumulation and setting percentage decreased, causing yield reduction. Grain yield and biomass reduced 66, 36, and 93 % compared to the control by shading treatments of S1, S2, and S3, respectively.

Natural variation of salinity response, population structure and candidate genes associated with salinity tolerance in perennial ryegrass accessions.

Natural variation in salinity response, effects of population structure on growth and physiological traits and gene-trait association were examined in 56 global collections of diverse perennial ryegrass (Lolium perenne L.) accessions. Three population structure groups were identified with 66 simple sequence repeat markers, which on average accounted for 9 and 11% of phenotypic variation for the control and salinity treatment at 300 mm NaCl. Group 1 (10 accessions) had greater plant height, leaf dry weight and water content, chlorophyll index, K(+) concentration and K(+) /Na(+) than group 2 (39 accessions) and group 3 (7 accessions) under salinity stress, while group 3 had higher Na(+) than groups 1 and 2. Eighty-seven single nucleotide polymorphisms were detected from four partial candidate genes encoding aquaporin and Na(+) /H(+) antiporter in both plasma and tonoplast membranes. Overall, rapid decay of linkage disequilibrium was observed within 500 bp. Significant associations were found between the putative LpTIP1 and Na(+) for the control and between the putative LpNHX1 and K(+) /Na(+) under the control and salinity treatments after controlling population structure. These results indicate that population structure influenced phenotypic traits, and allelic variation in LpNHX1 may affect salinity tolerance of perennial ryegrass.

Nitrogen rate impacts on tropical maize nitrogen use efficiency and soil nitrogen depletion in eastern and southern Africa.

Sub-Saharan Africa is facing food security challenges due, in part, to decades of soil nitrogen (N) depletion. Applying N fertilizer could increase crop yields and replenish soil N pools. From 2010 to 2015, field experiments conducted in Embu and Kiboko, Kenya and Harare, Zimbabwe investigated yield and N uptake response of six maize (Zea mays L.) hybrids to four N fertilizer rates (0 to 160 kg N ha-1) in continuous maize production systems. The N recovery efficiency (NRE), cumulative N balance, and soil N content in the upper 0.9 m of soil following the final harvest were determined at each N rate. Plant and soil responses to N fertilizer applications did not differ amongst hybrids. Across locations and N rates, NRE ranged from 0.4 to 1.8 kg kg-1. Higher NRE values in Kiboko and Harare occurred at lower post-harvest soil inorganic N levels. The excessively high NRE value of 1.8 kg kg-1 at 40 kg N ha-1 in Harare suggested that maize hybrids deplete soil inorganic N most at low N rates. Still, negative cumulative N balances indicated that inorganic soil N depletion occurred at all N rates in Embu and Harare (up to - 193 and - 167 kg N ha-1, respectively) and at the 40 kg N ha-1 rate in Kiboko (- 72 kg N ha-1). Overall, maize N uptake exceeded fertilizer N applied and so, while yields increased, soil N pools were not replenished, especially at low total soil N levels (< 10,000 kg N ha-1 in top 0.9 m).

Nitrogen fertilizer rate increases plant uptake and soil availability of essential nutrients in continuous maize production in Kenya and Zimbabwe.

Low fertilizer application rates for several decades have depleted soil nutrients in Sub-Saharan Africa (SSA) and contributed to relatively stagnant maize (Zea mays L.) yields. As maize is a staple crop, nutrient depletion has resulted in major food insecurity. While one potential solution is to apply more nitrogen (N) fertilizer, previous studies in SSA have found maize yield responses to be variable, likely because N is often not the only limiting nutrient. This study aimed to determine the impact of consecutive N fertilizer applications on plant uptake and available soil reserves of non-N nutrients. Maize was grown continuously in 3 sites that were representative of the ecosystem variability found in East/Southern Africa (Embu, Kenya; Kiboko, Kenya; Harare, Zimbabwe) at 4 different N fertilizer rates (0-160 kg N ha-1) from 2010 to 2015. Following the final season, grain, stover, and soil (sampled at different depths to 0.9 m) samples were analyzed for essential plant nutrients. Nitrogen fertilizer increased plant uptake of P, S, Cu, and Zn by up to 280%, 320%, 420%, and 210%, respectively, showing potential for mitigating non-N nutrient deficiencies in 2 of the 3 sites. Cumulatively, however, there was a net negative effect of higher N rates on the P, K, and S soil-plant balances in all sites and on the Mn and Cu soil-plant balance in Kiboko, indicating that applying N fertilizer depletes non-N soil nutrients. While N fertilizer enhances the uptake of non-N nutrients, a balanced application of multiple essential nutrients is needed to sustainably increase yields in SSA.

Crop production kept stable and sustainable with the decrease of nitrogen rate in North China Plain: An economic and environmental assessment over 8 years.

In pursuit of maximum grain yield farmers in the North China Plain usually apply excessive N fertilizer, resulting in wasted resources and environmental pollution. To assess the economic and environmental performances of different nitrogen rates will be conductive to sustain cleaner crop production. An 8-year field experiment was carried out with four treatments, N0 (0 kg ha-1 for winter wheat and summer maize), N1 (168 kg ha-1 for winter wheat and 129 kg ha-1 for summer maize), N2 (240 kg ha-1 for winter wheat and 185 kg ha-1 for summer maize) and N3 (300 kg ha-1 for winter wheat and summer maize), on the double cropping at Dawenkou research field (36°11'N, 117°06'E), Shandong Province, China. The crop production, soil physical-chemical parameters, and greenhouse gas emission are measured and the economic and environmental performances are assessed. The optimal nitrogen rate obtained the highest grain yield of summer maize in 4 of 8 year and was equivalent to conventional N rate in the other years. The nitrogen partial factor productivity and agronomic efficiency of optimal nitrogen rate was 63% and 58% higher than that of conventional nitrogen rate. The optimal nitrogen rate effectively decreased soil bulk density and increased weight percentage of water-stable aggregate and activities of urease and invertase compared to conventional nitrogen rate, which improved soil productivity. The fertilizer nitrogen loss and global warming potential of optimal nitrogen rate reduced by 76% and 35% compared to conventional nitrogen rate. The annual greenhouse gas intensity of optimal nitrogen rate decreased by 14-35% compared to others. The net ecosystem economic budget under optimal nitrogen rate is 252-604 $ ha-1 yr.-1 higher than other addition levels. The optimal nitrogen rate produces more grains and obtains higher economic and environmental benefits.

Applicability of a "Multi-Stage Pulse Labeling" 15N Approach to Phenotype N Dynamics in Maize Plant Components during the Growing Season.

Highlights This work utilizes "multi-stage pulse labeling" 15N applications, primarily during reproductive growth stages, as a phenotyping strategy to identify maize hybrids with superior N use efficiency (NUE) under low N conditions. Research using labeled isotopic N (15N) can precisely quantify fertilizer nitrogen (N) uptake and organ-specific N allocation in field crops such as maize (Zea mays L.). The overall research objective was to study plant N uptake patterns potentially correlated with N use efficiency (NUE) in field-grown maize hybrids using a "multi-stage pulse labeling" 15N phenotyping strategy with an emphasis on the reproductive period. Five hybrids varying in NUE were compared under zero N fertilizer application (0N) plus a moderate rate of 112 kg N ha-1 (112N) in 2013 (2 locations) and 2014 growing seasons. The equivalent of 3.2 (2013) to 2.1 (2014) kg of 15N ha-1, as labeled Ca(15NO3)2, was injected into soil on both sides of consecutive plants at multiple stages between V14 and R5. Aboveground plant biomass was primarily collected in short-term intervals (4-6 days after each 15N application) in both years, and following a single long-term interval (at R6 after 15N injection at R1) in 2014. Averaged across hybrids and site-years, the moderate N rate (112N) increased absolute 15N uptake at all stages; however, plants in the 0N treatment allocated proportionally more 15N to reproductive organs. Before flowering, short-term recovery of 15N (15Nrec) totaled ~0.30 or 0.40 kg kg-1 of the 15N applied, and ~50% of that accumulated 15Nu was found in leaves and 40% in stems. After flowering, plant 15Nrec totaled ~0.30 kg kg-1 of 15N applied, and an average 30% of accumulated 15Nu was present in leaves, 17% in stems, and the remainder-usually the majority-in ears. At the R5 stage, despite a declining overall rate of 15N uptake per GDD thermal unit, plant 15Nrec represented ~0.25 kg kg-1 of 15N applied, of which ~65% was allocated to kernels. Overall long-term 15Nrec during grain filling was ~0.45 and 0.70 kg kg-1 of total 15N applied at R1 with 0 and 112N, respectively, and most (~77%) 15N uptake was found in kernels. The "multi-stage pulse labeling" technique proved to be a robust phenotyping strategy to differentiate reproductive-stage N uptake/allocation patterns to plant organs and maize efficiencies with newly available fertilizer N.

The effect of nitrogen rate on transgenic corn Cry3Bb1 protein expression.

BACKGROUND: Combining herbicide-resistant and Bacillus thuringiensis (Bt) traits in corn (Zea mays L.) hybrids may affect insect resistance management owing to volunteer corn. Some Bt toxins may be expressed at lower levels by nitrogen-deficient corn roots. Corn plants with sublethal levels of Bt expression could accelerate the evolution of Bt resistance in target insects. The present objective was to quantify the concentration of Bt (Cry3Bb1) in corn root tissue with varying tissue nitrogen concentrations. RESULTS: Expression of Cry3Bb1 toxin in root tissue was highly variable, but there were no differences in the overall concentration of Cry3Bb1 expressed between roots taken from Cry3Bb1-positive volunteer and hybrid corn plants. The nitrogen rate did affect Cry3Bb1 expression in the greenhouse, less nitrogen resulted in decreased Cry3Bb1 expression, yet this result was not documented in the field. CONCLUSION: A positive linear relationship of plant nitrogen status on Cry3Bb1 toxin expression was documented. Also, high variability in Cry3Bb1 expression is potentially problematic from an insect resistance management perspective. This variability could create a mosaic of toxin doses in the field, which does not fit into the high-dose refuge strategy and could alter predictions about the speed of evolution of resistance to Cry3Bb1 in western corn rootworm Diabrotica virgifera virgifera LeConte.