Remobilization Efficiency Research Articles

Integrating density and fertilizer management to optimize the accumulation, remobilization, and distribution of biomass and nutrients in summer maize

Improved the utilization of fertilizer while maintaining the increased of grain yield was the focus of Chinese researchers. Nutrient uptake, distribution, and remobilization are important factors affecting the fertilizer utilization and grain yield of maize. This study aimed to provide a theoretical and practical basis for science-based, high-yielding, and high-efficiency cultivation practices by examining differences in biomass and nutrient uptake, distribution, and remobilization characteristics under three cultivation patterns. We set 12 treatments as follows: super high-yielding cultivation pattern (SH), optimized nutrient management cultivation pattern (ONM), local farmer's practice cultivation pattern (FP), and a series of nutrient omission plots, which excluded nitrogen (N), phosphorus (P), or potassium (K) from the three patterns. The results demonstrated that SH and ONM increased the yield and actual harvested ears by 35.4, 20.7 and by 20.2, 17.6%, respectively. Compared with the FP, SH and ONM increased biomass, N, P, and K accumulation at silking (R1 stage) by 24.4, 31.2, 39.4, and 34.8%, and by 21.7, 22.2, 31.7, and 34.8%, respectively. SH and ONM significantly increased biomass and nutrient distribution to the grains. ONM significantly increased N use efficiency. P and K use efficiency under the ONM pattern was significantly higher than under SH, but was lower than under the FP pattern over two years. This research demonstrates that ONM may significantly reduce fertilizer rates, effectively improve the nutrient remobilization efficiency and uptake at post-silking without negatively affecting grain yield, thereby increasing N use efficiency.

Combined physiological, transcriptome, and genetic analysis reveals a molecular network of nitrogen remobilization in maize.

In plants, nitrogen remobilization from source to sink organs is an important process regulated by complex transcriptional regulatory networks. However, the relationship between nitrogen remobilization and leaf senescence and the molecular regulatory network that controls them are unknown in maize. Here, using 15N labeling and a transcriptome approach, a dynamic analysis of the nitrogen remobilization process was conducted in two elite maize inbred lines (PH4CV and PH6WC) with contrasting leaf senescence. PH4CV showed higher nitrogen remobilization efficiency (NRE) than PH6WC, mainly in the middle and lower leaves from 15 d to 35 d after silking. The co-expression network analysis revealed that ethylene and cytokinin metabolism-related genes triggered the onset of nitrogen remobilization, while abscisic acid and jasmonic acid biosynthesis-related genes controlled the progression of nitrogen remobilization. By integrating genetic analysis, functional annotation, and gene expression, two candidate genes underlying a major quantitative trait locus of NRE were identified, namely an early senescence acting gene (ZmASR6) and an ATP-dependent Clp protease gene (GRMZM2G172230). Hormone-coupled transcription factors and downstream target genes reveal a gene regulatory network for the nitrogen remobilization process after silking in maize. These results uncovered a sophisticated regulatory mechanism for nitrogen remobilization, and further provided characterization of valuable genes for genetic improvement of nitrogen use efficiency in maize.

Grain Yields and Nitrogen Use Efficiencies in Different Types of Stay-Green Maize in Response to Nitrogen Fertilizer.

The stay-green leaf phenotype is typically associated with increased yields and improved stress resistance in maize breeding, due to higher nitrogen (N) nutrient levels that prolong greenness. The application of N fertilizer can regulate the N status of plants, and furthermore, impact the photosynthetic rates of leaves at the productive stage; however, N deficiencies and N excesses will reduce maize yields. Consequently, it is necessary to develop N fertilizer management strategies for different types of stay-green maize. For this study, the senescent cultivar Lianchuang 808 (LC808), moderate-stay-green cultivar Zhengdan 958 (ZD958), and over stay-green cultivar Denghai 685 (DH685) were selected as experimental models. Our results revealed that yields of ZD958 were slightly higher than DH685 and notably improved over than LC808. Compared with a non-stay-green cultivar LC808, ZD958 and DH685 still maintained higher chlorophyll contents and cell activities following the silking stage, while efficiently slowing the senescence rate. The supply of N fertilizer significantly prolonged leaf greenness and delayed senescence for ZD958 and DH685; however, the effect was not obvious for LC808. The stem remobilization efficiency of N was higher in the moderate-stay-green cultivar ZD958, in contrast to LC808, while the transfer of leaf N was lower than LC808, which guaranteed high leaf N levels, and that sufficient N was transferred to grains in ZD958. To obtain the highest yields, the optimal N fertilizer rates were 228.1 kg hm−2 for LC0808, 180 kg hm−2 for ZD958, and 203.8 kg hm−2 for DH685. In future, the selection of stay-green type crops might serve as an important agricultural strategy to reduce the quantity of N fertilizer and increase N efficiency.

Nutrient accumulation and remobilization in relation to yield formation at high planting density in maize hybrids with different senescent characters

ABSTRACT To test the hypothesis that plants could remobilize more nutrients from vegetative organs to fulfill the requirement of grain formation under high planting density, eight maize cultivars with different leaf senescence characters were chosen to study the relationship between stay-green character, nutrient (N, P, K) accumulation, post-silking nutrient remobilization and grain yield under three planting densities (60,000, 75,000, and 90,000 plants per ha) in 2 years. We found that increased plant density aggravated leaf senescence, reduced concentration of N, P and K in all organs, reduced post-silking N, P, K accumulation per ha, increased nutrient remobilized efficiency. Stay-green degree was positively correlated to yield reduction at high planting density (R2 = 0.35, P < 0.01), and negatively correlated to grain yield across planting densities and experimental years. Grain yield reduction at high planting density was negatively correlated to N (R2 = 0.29, P < 0.01), P (R2 = 0.51, P < 0.01), K (R2 = 0.17, P < 0.05) remobilization efficiency. Thus, we conclude that efficient nutrient remobilization is important for achieving higher yield at high planting density. Stay-green too much may not be a favorable trait for high-yield breeding at high planting density.

The impact of high plant density on dry matter remobilization and stalk lodging in maize genotypes with a different stay-green degree

ABSTRACT To test the hypothesis that stay-green genotypes help to increase stalk lodging resistance under high planting density by reducing dry matter remobilization from stalk, a two-year field study with three planting densities (60,000, 75,000, and 90,000 plants per ha) and six hybrids was conducted. When planting density changed from 75,000 to 90,000, stalk dry matter remobilization efficiency increased by 4.1–191%. Stem dry weight per length, rind penetration strength, average crushing strength for the first to fifth internode, and average bending strength for the third to fifth internode decreased by 25.4–47.2%, 5.1–24.6%, 1.4–29.9%, and 14.0–36.0%, respectively. Stalk lodging rate increased by 17–275%. Stalk dry matter remobilization efficiency was negatively correlated to either stem dry weight per length (r = 0.23, p < 0.05), rind penetration strength (r = 0.75, p < 0.01), crushing strength (r = 0.61, p < 0.01), or bending strength (r = 0.32, p < 0.01). Cultivars with higher stay-green degree (SR999, LY99, and NH101) had lower stalk lodging rate compared to the senescent cultivars XY335 and LY918. It is concluded that stay-green genotypes with low dry matter remobilization help to strengthen stem and reduce stalk lodging risk.

Improving grain yield and protein concentration of maize (Zea mays L.) simultaneously by appropriate hybrid selection and nitrogen management

The simultaneous improvement of grain yield and protein concentration in crop production is an important global challenge. In maize, genotype selection and nitrogen (N) management are potential methods to improve grain yield and protein concentration. A field experiment was conducted in 2015–2016 in the North China Plain to evaluate the response of grain yield and protein concentration of different modern hybrids (XY1266, ZD958 and DH618) under different N rates (0–259 kg ha−1). The highest grain dry matter and protein concentration were observed in DH618. The averaged maximal grain dry matter over the 2 years was 10.3 Mg ha−1 for DH618, 6.1 % higher than for ZD958 (9.7 Mg ha−1) and 9.9 % higher than for XY1266 (9.4 Mg ha−1), and the maximal grain protein concentration in DH618 was 8.6 %, 11.1 % higher than for XY1266 (7.8 %) and 10.1 % higher than for ZD958 (7.9 %). DH618 had higher post-silking biomass accumulation, a stable high harvest index (HI), and higher post-silking N uptake as well as N harvest index (NHI) compared to the other two hybrids, meanwhile without reducing N remobilization efficiency (NRE). In DH618, high leaf N concentration after silking facilitated a higher rate of photosynthesis, further leading to greater post-silking biomass and N accumulation, and quickly remobilization of leaf N at late grain-filling stage improved the NRE and NHI. Together with choosing the right hybrid, optimisation of N management with split N application as well as appropriate N rate taking into account the residual soil mineral N could ensure both high grain dry matter and increased protein concentration in maize.

Zinc, iron, and copper isotopic fractionation in Elsholtzia splendens Nakai: A study of elemental uptake and (re)translocation mechanisms

Zinc, Fe and previously reported Cu isotopic compositions were applied in a systematic investigation of uptake and translocation in Cu-tolerant Elsholtzia splendens Nakai. They exhibit characteristic isotopic fractionation during uptake: non-specific uptake for Zn, direct uptake as Fe(Ш)-complexes as strategy II species and a prerequisite of Cu(II) reduction to Cu(I) before uptake of Cu. There are, however, differences during translocation from stem and branch to leaf and flower. Light Zn and Fe isotopic compositions of flowers are dependent on their phloem pool, with enrichment in branch and stem. The similarity of Zn and Fe isotopic compositions in flowers, regardless of height, implies uniform mobility in phloem. Cu isotopic composition displays marked variations in leaves and flowers with height. Re-translocation from old tissue to new tissue has a significant influence on metal isotopic compositions in leaves and flowers during the late life stage. Light Zn and Fe isotopes are exported from leaves by transporters, leaving heavy isotopes in the remaining fraction. Cu readily forms complexes, and its heavy isotope is re-translocated preferentially and then light isotope by transporter proteins from old leaves dependent on the degree of remobilization. The remobilization efficiency follows the trend Zn < Fe < Cu, opposite to the trend of metal mobility in xylem and phloem (Zn > Fe > Cu).

Post-flowering biotic and abiotic stresses impact nitrogen use efficiency and seed filling in Arabidopsis thaliana.

Nitrogen (N) is an essential nutrient that plants require for the synthesis of amino acids, proteins, and many other important metabolites. Plant metabolism and growth are consequently dependent on the amount of N that is assimilated and distributed from source leaves to developing sinks, such as fruits and seeds. The environmental stresses enhanced by climate change deeply influence seed yield and seed composition, and may disturb N use efficiency (NUE) in pants. We aimed to investigate plant responses to extreme climates with regard to NUE, N remobilization efficiency, and seed composition. By studying a collection of Arabidopsis genotypes showing a range of C:N ratios in seeds, we investigated the impact of different post-flowering growth conditions (control, heat, drought, low nitrate availability, induced senescence, and induced plant defense) on seed yield, N allocation in organs, NUE, and N remobilization efficiency. We analysed how post-flowering stresses could change seed filling and showed that post-flowering stresses change both the range of N and C concentrations and the C:N stoichiometry in seeds. Using a new trait, called delta seed composition, we measured the deviation in C:N stoichiometry of each genotype and revealed the genetic determinism of the C:N stoichiometry. Altogether, the results indicate that extreme climate impacts NUE dramatically in plants and generates different bottlenecks in N fluxes during seed filling.

Responses of photosynthetic characteristics and enzyme activity of nitrogen metabolism to low nitrogen in maize with different nitrogen tolerance

Understanding the physiological processes associated with leaf photosynthetic characteristics and nitrogen (N) assimilation during grain-filling stage are helpful for enhancing nitrogen utilization efficiency (NUtE) of maize. In this study, the leaf photosynthetic and N assimilation parameters in maize, including Zhengdan 958 (ZD958), a low-N tolerance cultivar and Huanong 138 (HN138), a low-N sensitive cultivar under different N rates were examined. Results showed that ZD958 displayed significant increases on grain yield and NUtE than that in HN138. Analyses on the leaf photosynthetic and N assimilation-associated processes indicated that ZD958 had higher leaf N remobilization (Rem N), net photosynthetic rate (Pn) and photosynthetic N use efficiency (PNUE) with respect to those of HN138 during grain-filling stage. In addition, ZD958 was also shown to be higher activities of leaf nitrate reductase (NR), glutamine synthetase (GS), nitrate reductase (GDH) and glutamine synthetase (GAGOT) than those of HN138. The leaf PNUE was significantly positively correlated with NR, GS, GDH, GOGAT suggesting that leaf PNUE and NR, GS, GDH, GOGAT jointly determined the N remobilization efficiency and the leaf N remobilization during post-silking. These results suggested that ZD958 possessed improved PNUE, NR and GS activities in leaves during grain-filling stage that contributes improve grain weights and yield formation capacities upon under low-N conditions. Keywords: low-N, photosynthesis, nitrogen assimilation, nitrogen utilization efficiency, maize DOI: 10.25165/j.ijabe.20201306.5891 Citation: Ji P T, Cui Y W, Li X L, Xiao K, Tao P J, Zhang Y C. Responses of photosynthetic characteristics and enzyme activity of nitrogen metabolism to low nitrogen in maize with different nitrogen tolerance. Int J Agric & Biol Eng, 2020; 13(6): 133–143.

Nitrogen dynamic in wheat (Triticum aestivum L.) agroecosystem as influenced by intra- and interspecific competition

Abstract Background: The study of both intra- and interspecific competitions in Wheat (Triticum aestivum L.) agroecosystem is quite complex and it is essential to understand the influence of nitrogen in these competitions. Objective: The aim of this study was to evaluate the effect of different competitive models on nitrogen balance in wheat ecosystem agriculture (Chamran cultivar). Methods: A field experiment was performed using split-plot based on randomized complete block design (RCBD) with three replications. The main and sub-plots consisted of different nitrogen rates (control, 50, 100 and 150 kg nitrogen ha-1) and different competitive patterns (no, intraspecific, interspecific competition and intra- and interspecific in combination) were applied to the sub-plots, respectively. Weed density in the interspecies competition was applied through the equal planting of two narrow leaves (Avena sativa L.) and broadleaf (Sinapis arvensis L.) weed species. Results: The dry matter (DM) accumulation and nitrogen content (NC) at anthesis was higher than that at maturity stage for all competition treatments as well as nitrogen rates. The trend of nitrogen translocation efficiency (NTE) and nitrogen harvest index (NHI) was different from dry matter translocation efficiency (DMTE), and both were highest in interspecific competition treatments. Moreover, the highest grain yield in no competition treatment resulted in a lower dry matter and NTE. Conclusions: It was concluded that the capacity for nitrogen accumulation in the stem, was associated with a high nitrogen uptake, nitrogen assimilation and high post-anthesis nitrogen remobilization efficiency. In additions, high NHI could be used in the development of cultivars with the desired N balance.

Characterization of low-N responses in maize (Zea mays L.) cultivars with contrasting nitrogen use efficiency in the North China Plain

Over-use of N fertilizer in crop production has resulted in a series of environmental problems in the North China Plain (NCP). Thus, improvement of nitrogen use efficiency (NUE) in summer maize has become an effective strategy for promoting sustainable agriculture in this region. Using twenty maize cultivars, plant dry matter production, N absorption and accumulation, yield formation, and NUE in summer maize were investigated under three N levels in two growing seasons. Based on their yield and yield components, these maize cultivars were categorized into four groups including efficient-efficient (EE) cultivars, high-nitrogen efficient (HNE) cultivars, low-nitrogen efficient (LNE) cultivars and nonefficient-nonefficient (NN) cultivars. In both two seasons, the EE cultivars improved grain yield together with increased plant biomass, and enhanced accumulative amounts as well as higher average grain yields than the other cultivar groups under deficient-N conditions. Significant correlations were observed between yield and kernel numbers (KN), dry matter (DM) amount and N accumulation at both post-silking and maturity stages. DM and N accumulation at late growth stage (i.e., from silking to maturity) contributed largely to the enhanced yield capacity and improved NUE under N-deficient conditions. Compared with the NN cultivars, the EE cultivars also showed increased N assimilation amount (NAA) and N remobilization content (NRC), and elevated N remobilization efficiency (NRE), NUE and nitrogen partial factor productivity (PFPN). Our investigation has revealed N-associated physiological processes and may provide guidance for cultivation and breeding of high yield and NUE summer maize under limited N conditions in the NCP.

Nitrogen fertilization and fungicide mixtures in wheat: how do they affect the severity, yield and dynamics of nitrogen under leaf rust infections?

Nitrogen (N) fertilization and fungicides are important tools in order to achieve high yields and quality in wheat (Triticum aestivum L.), although its use may affect the expression of foliar diseases such as leaf rust (LR) (Puccinia triticina Eriks.). An approach focused on the impact of LR and its interaction with fungicides mixtures and N fertilization rates could be useful to improve the modelling of crop N dynamics and enhance grain yield and quality in the context of the integrated management. Our study was designed to determine the additive effects of carboxamide fungicides (TSC) to a double triazole-strobilurin (TS) mixture under three N fertilization rates on the LR disease progress, healthy area duration (HAD), grain yield and N dynamics during two years. Main plots corresponded to three fungicide treatments and three N doses were the sub-plots. LR disease progress, HAD, yield, N remobilization (NREM), N post-anthesis absorption (NPA), N stored in grains (NG) and %N in grains (%Ng) were evaluated. LR severity increased at higher N rates. TSC applications produced the major decreases on LR severity and the main increases in HAD, which resulted in the higher yield increases (+2726 kg/ha). The %Ng increased with the application of fungicides when LR was controlled. LR infections generated reductions in NREM, NPA and NG, a response that could be explained by decreases in aboveground biomass and yield, coupled with reductions on N harvest index and N remobilization efficiency. Combined effect of TSC fungicides and increases in N dose showed the largest increases in NREM, NPA and NG.

Variabilidad en carbohidratos solubles del tallo y su contribución al rendimiento de grano en trigo

En trigo la capacidad para almacenar carbohidratos solubles (CHS) en el tallo y remobilizarlos hacia el grano, constituye una característica deseable a incorporar en el germoplasma dirigido a regiones donde ocurre con frecuencia sequía terminal. El objetivo del trabajo fue evaluar la variabilidad en la acumulación y remobilización de los carbohidratos solubles en agua (CHS) y su contribución al rendimiento de grano cuando la fuente postantesis es alterada por defoliación y estrés hídrico. Los experimentos se llevaron a cabo en el ciclo verano-otoño 2013 (temporal, exp. 1) e invierno-primavera 2013-2014 (riego, exp. 2) en Toluca, México, a una densidad de 350 semillas m-2, en el exp. 1 la defoliación se realizó 7 días después de antesis y en el exp. 2 no se regó después de antesis. En el exp. 1 se estudiaron 60 líneas élite de trigo y dos tratamientos de reducción de fuente (defoliado y sin defoliar, llamados ambientes en adelante), bajo un diseño de parcelas divididas con 2 repeticiones. El exp. 2 consistió en 20 genotipos seleccionados del exp. 1 y sometidos a dos regímenes hídricos (riego y sequía postfloración) en un diseño de bloques completos al azar con 3 repeticiones en donde cada régimen hídrico se consideró un ambiente. Se observó gran variabilidad para rendimiento de grano (RG) y las características fisiológicas asociadas. El RG fue reducido en promedio 14% por efecto de la defoliación y sequía pos-antesis. La mayor contribución de CHS estuvo en línea con una mayor eficiencia de remobilización. Existen genotipos promisorios para usarse en programas de mejoramiento genético con miras a incorporar características fisiológicas para incrementar el rendimiento potencial bajo condiciones de estreses bióticos o abióticos en pos-antesis.

Correlation between nitrogen fruiting efficiency and nitrogen utilization and remobilization in winter wheat at different sowing dates

To understand the correlation between nitrogen fruiting efficiency and nitrogen utilization and remobilization in winter wheat, the differences and mutual relationships of nitrogen fruiting efficiency, nitrogen utilization and remobilization of winter wheat at different sowing dates (S1:24 September, S2:1 October, S3:8 October, S4:15 October and S5:22 October) were analyzed in two growing seasons from 2014 to 2016. The results showed that there was no significant difference in grain yield and grain number per unit area among different sowing dates. Delayed sowing date decreased nitrogen accumulation in both shoots and spikes, and then reduced nitrogen uptake efficiency and increased nitrogen utilization efficiency and nitrogen fruiting efficiency. Nitrogen fruiting efficiency was positively correlated with nitrogen utilization efficiency, negatively correlated with nitrogen uptake efficiency, but not significantly correlated with nitrogen use efficiency. Nitrogen nutrition index tended to be optimum with delayed sowing dates, showed synchronicity with the improvement of nitrogen fruiting efficiency. Pre-anthesis nitrogen remobilization amount in vegetative organs and post-anthesis nitrogen accumulation amount significantly decreased with postpone of sowing dates, but pre-anthesis nitrogen remobilization efficiency remarkably rise. There was a positive correlation between nitrogen fruiting efficiency and nitrogen remobilization efficiency, indicating that the improvement in nitrogen remobilization efficiency facilitated the increment in nitrogen frui-ting efficiency. Taken together, properly delayed sowing date reduced nitrogen uptake, but increased nitrogen utilization efficiency and nitrogen fruiting efficiency and improved nitrogen supply status, which provided a theoretical basis for the implementation of reducing input and improving nitrogen utilization efficiency in wheat production in this region.

Decreased Temperature with Increasing Elevation Decreases the End-Season Leaf-to-Wood Reallocation of Resources in Deciduous Betula ermanii Cham. Trees

Global air temperature has increased and continues to increase, especially in high latitude and high altitude areas, which may affect plant resource physiology and thus plant growth and productivity. The resource remobilization efficiency of plants in response to global warming is, however, still poorly understood. We thus assessed end-season resource remobilization from leaves to woody tissues in deciduous Betula ermanii Cham. trees grown along an elevational gradient ranging from 1700 m to 2187 m a.s.l. on Changbai Mountain, northeastern China. We hypothesized that end-season resource remobilization efficiency from leaves to storage tissues increases with increasing elevation or decreasing temperature. To test this hypothesis, concentrations of non-structural carbohydrates (NSCs), nitrogen (N), phosphorus (P), and potassium (K) during peak shoot growth (July) were compared with those at the end of growing season (September on Changbai Mt.) for each tissue type. To avoid leaf phenological effects on parameters, fallen leaves were collected at the end-season. Except for July-shoot NSC and July-leaf K, tissue concentrations of NSC, N, P, and K did not decrease with increasing elevation for both July and September. We found that the end-season leaf-to-wood reallocation efficiency decreased with increasing elevation. This lower reallocation efficiency may result in resource limitation in high-elevation trees. Future warming may promote leaf-to-wood resource reallocation, leading to upward shift of forests to higher elevations. The NSC, N, P, and K accumulated in stems and roots but not in shoots, especially in trees grown close to or at their upper limit, indicating that stems and roots of deciduous trees are the most important storage tissues over winter. Our results contribute to better understand the resource-related ecophysiological mechanisms for treeline formation, and vice versa, to better predict forest dynamics at high elevations in response to global warming. Our study provides resource-related ecophysiological knowledge for developing management strategies for high elevation forests in a rapidly warming world.

Sowing date effects on dry matter remobilization and yield of triticale (Triticosecale wittmack) under late season drought stress

A 2-year field experiment using a randomized complete block design with the treatments arranged as split-split-plot with three replicates was conducted to investigate the effects of different sowing dates. Different irrigation regimes and different triticale cultivars were tested during 2014 and 2015 growing seasons. Under cutting off irrigation at the milk development stage, Sanabad with 46.2% had higher relative water content on December 3 sowing date. Assimilate remobilization in cutting off irrigation at dough development was more than that at milk development and Sanabad had the highest assimilate remobilization on 3 December sowing date in both years. In both years, Sanabad had the highest remobilization efficiency. The highest contribution of pre-anthesis assimilates to grain was obtained on 3 December sowing date in both years in Sanabad. Lower grain yield in Juanillo cultivar under cutting off irrigation, appeared to be due to reduction in remobilization efficiency, especially by cutting off irrigation at milk development stage in late sowing date. Overall, Sanabad was more tolerant to cutting off irrigation than Juanillo.

A Genotypic Comparison Reveals That the Improvement in Nitrogen Remobilization Efficiency in Oilseed Rape Leaves Is Related to Specific Patterns of Senescence-Associated Protease Activities and Phytohormones.

Oilseed rape (Brassica napus L.) is an oleoproteaginous crop characterized by low N use efficiency (NUE) that is mainly related to a weak Nitrogen Remobilization Efficiency (NRE) during the sequential leaf senescence of the vegetative stages. Based on the hypothesis that proteolysis efficiency is crucial for the improvement of leafNRE, our objective was to characterize key senescence-associated proteolytic mechanisms of two genotypes (Ténor and Samouraï) previously identified with contrasting NREs. To reach this goal, biochemical changes, protease activities and phytohormone patterns were studied in mature leaves undergoing senescence in two genotypes with contrasting NRE cultivated in a greenhouse under limiting or ample nitrate supply. The genotype with the higher NRE (Ténor) possessed enhanced senescence processes in response to nitrate limitation, and this led to greater degradation of soluble proteins compared to the other genotype (Samouraï). This efficient proteolysis is associated with (i) an increase in serine and cysteine protease (CP) activities and (ii) the appearance of new CP activities (RD21-like, SAG12-like, RD19-like, cathepsin-B, XBCP3-like and aleurain-like proteases) during senescence induced by N limitation. Compared to Samouraï, Ténor has a higher hormonal ratio ([salicylic acid] + [abscisic acid])/([cytokinins]) that promotes senescence, particularly under low N conditions, and this is correlated with the stronger protein degradation and serine/CP activities observed during senescence. Short statement: The improvement in N recycling during leaf senescence in a genotype of Brassica napus L. characterized by a high nitrogen remobilization efficiency is related to a high phytohormonal ratio ([salicylic acid] + [abscisic acid])/([cytokinins]) that promotes leaf senescence and is correlated with an increase or the induction of specific serine and cysteine protease activities.

Radiochemical evidence for the contribution of iron (using 59Fe) remobilization efficiency towards nitrogen (N) and Fe deficiency tolerance in wheat

Abstract Widespread deficiency of soil macro (N) and micro nutrient (Fe) limit crop productivity and nutritional quality of the produce worldwide. Iron is relatively immobile in plants however, its retranslocation can be facilitated by naturally produced chelator in the leaves. Synthesis of these metal-chelators depends on N nutrition of crops. The present study uses 59Fe tracer to measure the contribution of in-plant remobilization (from fully developed 2nd older leaf (OL) to a younger developing 3rd leaf (YL)) of relatively immobile iron towards N and Fe deficiency stress tolerance in Fe efficient bread and Fe inefficient durum wheat. Dual nutrient deficiency of nitrogen and iron induced senescence and hastened the process of chlorophyll degradation and induced a higher 59Fe remobilization from OL to YL. Further, 59Fe retranslocation was also highest under dual nutrient deficiency (N−Fe−). Percent 59Fe retranslocated into the YL of bread and durum wheat, respectively was higher in Fe deficient (N+Fe− and N−Fe−) than Fe sufficient (N+Fe+ and N+Fe−) treatments. Results clearly reveal that Fe deficiency tolerance response under N and Fe deficiency is chiefly determined an efficient Fe uptake and Fe retranslocation, respectively in the bread and the durum wheat.

Overexpression of ATG8 in Arabidopsis Stimulates Autophagic Activity and Increases Nitrogen Remobilization Efficiency and Grain Filling

Autophagy knock-out mutants in maize and in Arabidopsis are impaired in nitrogen (N) recycling and exhibit reduced levels of N remobilization to their seeds. It is thus impoortant to determine whether higher autophagy activity could, conversely, improve N remobilization efficiency and seed protein content, and under what circumstances. As the autophagy machinery involves many genes amongst which 18 are important for the core machinery, the choice of which AUTOPHAGY (ATG) gene to manipulate to increase autophagy was examined. We choose ATG8 overexpression since it has been shown that this gene could increase autophagosome size and autophagic activity in yeast. The results we report here are original as they show for the first time that increasing ATG8 gene expression in plants increases autophagosome number and promotes autophagy activity. More importantly, our data demonstrate that, when cultivated under full nitrate conditions, known to repress N remobilization due to sufficient N uptake from the soil, N remobilization efficiency can nevertheless be sharply and significantly increased by overexpressing ATG8 genomic sequences under the control of the ubiquitin promoter. We show that overexpressors have improved seed N% and at the same time reduced N waste in their dry remains. In addition, we show that overexpressing ATG8 does not modify vegetative biomass or harvest index, and thus does not affect plant development.

Physiological mechanisms underlying post‐silking nitrogen use efficiency of high‐yielding maize hybrids differing in nitrogen remobilization efficiency

AbstractIt is a great challenge to increase nitrogen (N) remobilization from leaves while ensuring high grain yield of maize (Zea mays L.). One possible way is to increase photosynthetic N use efficiency (PNUE) of leaves during post‐silking. This study aimed to understand the strategies required to maintain high PNUE during post‐silking in a N‐remobilizing high‐yielding maize hybrid. In this study, the high‐yielding hybrid XY335 with high N remobilization efficiency (NRE) and the high‐yielding hybrid ZD958 with low NRE were used in a three‐year field experiment under normal N levels. The results show that specific leaf N contents were similar in lower, middle, and upper leaves of the two hybrids from silking stage to maturity. Throughout the grain filling period, net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 concentration, and PNUE of XY335 were similar to that in ZD958 in lower leaves, but higher in middle and upper leaves. Stomatal density and maximum rate of carboxylation in upper leaves and maximum rate of electron transport in middle leaves was higher in XY335 than that in ZD958. It is concluded that maintaining the PNUE of lower leaves together with increasing the PNUE of middle and upper leaves at canopy‐level are the possible regulatory mechanisms to improve NRE without penalty in whole‐plant photosynthesis during post‐silking in modern high‐yielding cultivars. The higher PNUE is at least partly related to the higher proportion of N allocation to the carboxylation and bioenergetics systems and the higher stomatal density and greater gs. Therefore, genetic improvement in N allocation to the carboxylation and bioenergetics systems and stomatal density and gs might increase PNUE.