Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture. Plant NUE is inherently complex, as each step—including N uptake, translocation, assimilation, and remobilization—is governed by multiple interacting genetic and environmental factors. The limiting factors in plant metabolism for maximizing NUE are different at high and low N supplies, indicating great potential for improving the NUE of current cultivars, which were bred in well-fertilized soil. Decreasing environmental losses and increasing the productivity of crop-acquired N requires the coordination of carbohydrate and N metabolism to give high yields. Increasing both the grain and N harvest index to drive N acquisition and utilization are important approaches for breeding future high-NUE cultivars.

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Plant nitrogen assimilation and use efficiency.

Rice ( Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. Elsewhere, it represents a high‐value commodity crop. Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto a puddled soil surface (wet‐seeding) or into shallow standing water (water‐seeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe, Australia, and the United States, direct‐seeding is highly mechanized. The risk of crop yield loss due to competition from weeds by all seeding methods is higher than for transplanted rice because of the absence of the size differential between the crop and weeds and the suppressive effect of standing water on weed growth at crop establishment. Of 1800 species reported as weeds of rice, those of the Cyperaceae and Poaceae are predominant. The adoption of direct‐seeding has resulted in a change in the relative abundance of weed species in rice crops. In particular, Echinochloa spp., Ischaemum rugosum, Cyperus difformis , and Fimbristylis miliacea are widely adapted to conditions of DSR. Species exhibit variability in germination and establishment response to the water regime postsowing, which is a major factor in interspecifically selecting constituents of the weed flora. The relatively rapid emergence of “weedy” (red) rice, rice phenotypically similar to cultivars but exhibiting undesirable agronomic traits, has been observed in several Asian countries practicing DSR, and this poses a severe threat to the sustainability of the production system. Stale seedbeds, tillage practices for land leveling, choice of competitive rice cultivars, mechanical weeders, herbicides, and associated water management are component technologies essential to the control of weeds in DSR. Herbicides in particular are an important tool of weed management, but hand weeding is either partially or extensively practiced in countries of Asia, Africa, and Latin America. Though yet to be globally commercialized, transgenic rice varieties engineered for herbicide resistance are a potential means of weed control. The release of herbicide‐resistant rice for red rice control in the United States has indicated the need to critically examine mitigation methods for the control of gene flow. Integrating preventive and interventional methods of weed control remains essential in managing weed communities in DSR, both to prohibit the evolution of herbicide resistance and to maximize the relative contributions of individual components where herbicides are not widely used. There remains a need to further develop understanding of the mechanisms and dynamics of rice weed competition and of the community dynamics of weed populations in DSR to underpin sustainable weed management practices.

Weed Management in Direct‐Seeded Rice

Monocarpic plants require the initiation of whole-plant senescence to remobilize and transfer assimilates pre-stored in vegetative tissues to grains. Delayed whole-plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging-resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain-filling period induces early senescence, reduces photosynthesis, and shortens the grain-filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain-filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole-plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain-filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain-filling rate can outweigh the loss of reduced photosynthesis and the shortened grain-filling period, leading to an increased grain yield, better harvest index and higher water-use efficiency.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2005.01597.x

Grain filling of cereals under soil drying.

Nitrogen-use efficiency of indica varieties of rice is superior to that of japonica varieties. We apply 16S ribosomal RNA gene profiling to characterize root microbiota of 68 indica and 27 japonica varieties grown in the field. We find that indica and japonica recruit distinct root microbiota. Notably, indica-enriched bacterial taxa are more diverse, and contain more genera with nitrogen metabolism functions, than japonica-enriched taxa. Using genetic approaches, we provide evidence that NRT1.1B, a rice nitrate transporter and sensor, is associated with the recruitment of a large proportion of indica-enriched bacteria. Metagenomic sequencing reveals that the ammonification process is less abundant in the root microbiome of the nrt1.1b mutant. We isolated 1,079 pure bacterial isolates from indica and japonica roots and derived synthetic communities (SynComs). Inoculation of IR24, an indica variety, with an indica-enriched SynCom improved rice growth in organic nitrogen conditions compared with a japonica-enriched SynCom. The links between plant genotype and root microbiota membership established in this study will inform breeding strategies to improve nitrogen use in crops.

NRT1.1B is associated with root microbiota composition and nitrogen use in field-grown rice

Asian cultivated rice (Oryza sativa L.) consists of two main subspecies, indica and japonica. Indica has higher nitrate-absorption activity than japonica, but the molecular mechanisms underlying that activity remain elusive. Here we show that variation in a nitrate-transporter gene, NRT1.1B (OsNPF6.5), may contribute to this divergence in nitrate use. Phylogenetic analysis revealed that NRT1.1B diverges between indica and japonica. NRT1.1B-indica variation was associated with enhanced nitrate uptake and root-to-shoot transport and upregulated expression of nitrate-responsive genes. The selection signature of NRT1.1B-indica suggests that nitrate-use divergence occurred during rice domestication. Notably, field tests with near-isogenic and transgenic lines confirmed that the japonica variety carrying the NRT1.1B-indica allele had significantly improved grain yield and nitrogen-use efficiency (NUE) compared to the variety without that allele. Our results show that variation in NRT1.1B largely explains nitrate-use divergence between indica and japonica and that NRT1.1B-indica can potentially improve the NUE of japonica.

Variation in NRT1.1B contributes to nitrate-use divergence between rice subspecies

Dry matter and N accumulation and translocation for Indica and Japonica rice under Mediterranean conditions

Genotypic differences for grain yield and nitrogen utilization in Indica and Japonica rice under Mediterranean conditions

Grain quality variation and relationships with morpho-physiological traits in rice (Oryza sativa L.) genetic resources in Europe

: Field experiments were conducted in 1996 and 1997 to evaluate barnyardgrass (Echinochloa crus-galli) control, rice (Oryza sativa) yield, and grain quality with cyhalofop-butyl applied early postemergence (EPOST) or late postemergence (LPOST) in drained or flooded plots at rates of 0.15 to 0.4 kg ai/ha and standard treatments of molinate or propanil. Control with cyhalofop-butyl at 0.15 and 0.2 kg/ha applied in drained plots was greater compared to propanil and generally increased when treatments were applied EPOST to plots reflooded 2 d after treatment. Barnyardgrass control with cyhalofop-butyl at 0.15 and 0.2 kg/ha applied in flooded plots was similar to or better than the control obtained with molinate. Rice yields generally reflected differences in barnyardgrass control and were greatest with cyhalofop-butyl applied EPOST in drained plots. The 0.4 kg/ha rate of cyhalofop-butyl resulted in rice foliar injury and significant reductions in grain yield. Grain qualitative characteristics (total m...

https://bioone.org/journals/weed-technology/volume-14/issue-2/0890-037X(2000)014[0383:BECGCI]2.0.CO;2/Barnyardgrass-Echinochloa-crus-galli-Control-in-Water-Seeded-Rice-Oryza/10.1614/0890-037X(2000)014[0383:BECGCI]2.0.CO;2.pdf

Barnyardgrass (Echinochloa crus-galli) Control in Water-Seeded Rice (Oryza sativa) with Cyhalofop-butyl1

The effects of Pyricularia oryzae, the causal agent of blast disease in rice, on plant growth, grain yield, and grain quality under Mediterranean conditions were studied in 2002 and 2003. Four rice varieties were grown under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Results showed that inoculation affected the overall plant growth and yield and resulted in a reduction in plant height, number of productive tillers per plant, grain weight, and grain yield. The magnitude of the reduction was dependent on the varietals' susceptibility to the disease. Both types of the disease (leaf blast and neck blast) were negatively correlated with grain yield. It was estimated that each unit increase in leaf blast rating and neck blast resulted in 5.97 and 0.23 g plant -1 yield reduction, respectively. Grain quality traits were less affected by the disease treatments compared to the agronomic traits. The most pronounced effect of the disease was on total milling yield, which in some cases was reduced by inoculation up to 11%.

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D. A. Ntanos

Papers

Dry matter and N accumulation and translocation for Indica and Japonica rice under Mediterranean conditions

Genotypic differences for grain yield and nitrogen utilization in Indica and Japonica rice under Mediterranean conditions

Grain quality variation and relationships with morpho-physiological traits in rice (Oryza sativa L.) genetic resources in Europe

Barnyardgrass (Echinochloa crus-galli) Control in Water-Seeded Rice (Oryza sativa) with Cyhalofop-butyl1

Blast disease influence on agronomic and quality traits of rice varieties under Mediterranean conditions