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Nutrient management can increase crop yield and income, but its effects on yield components are rarely dissected in on-farm research. In this study, we compared aboveground biomass and yield components of rainfed lowland rice under site-specific nutrient management and farmer management from 69 demonstration sites prone to mild to moderate intermittent drought across 9 Philippine provinces over 3 years (the 2011–2013 wet seasons). The sink size (spikelets m −2 ) was most closely associated with grain yield in all years. Panicle size (spikelets per panicle) increased by 10.4% and 13.0% in 2011 and 2012, respectively, under site-specific nutrient management, with N application around the early reproductive stage of <25 kg N ha −1 in farmer nutrient management versus 30–33 kg N ha −1 in site-specific management. Higher N application during seedling establishment in farmer nutrient management (55 kg N ha −1 ) than in site-specific management (22 kg N ha −1 ) did not increase panicles m −2 in any year. Our results demonstrate the yield advantage of site-specific nutrient management in rainfed lowland rice in relatively fertile and less drought-prone environments: enhancing sink size should be the major target of nutrient management; it is unnecessary to apply high amounts of N during seedling establishment to secure a sufficient panicle number; and N application is most important during the early reproductive stage to increase panicle size. [ABSTRACT FROM AUTHOR]

Differences in crop performance between seasons has raised concerns on whether or not contrasting rice varieties should be grown in each season in the tropics. Solving this question requires evaluation if key plant traits subtending grain yield are expressed similarly across seasons. Within each dry season (DS) and wet season (WS), one approach to address the issue was to group 32 high-yielding and contrasted cultivars with respect to crop duration, and to consider the crop cycle as two distinct phases: the first one from early vegetative to flowering (pre-flowering) and the second one from flowering to maturity (post-flowering). For each of these two phases, one key descriptive parameter was identified, the potential sink size (PSS) at flowering for the pre-flowering phase, and the grain filling rate (FR) for the post-flowering phase. Significant correlations were observed for grain yield with both PSS and FR within maturity groups in both seasons, indicating the strong effect of both phases in grain yield establishment. Finally, relevant traits in the DS were those favoring light capture along the whole crop cycle in both maturity groups. Traits for the WS differed among maturity groups. Traits for the early maturity group were also those favoring light capture, while the same traits were non desirable for the medium and late maturity groups, for which delayed leaf senescence and short plant height were identified. The difference between relevant traits highlights the need of developing distinct breeding programs respective of the growing seasons and maturity groups. [ABSTRACT FROM AUTHOR]

Fertilization sensitivity to heat in rice is a major issue within climate change scenarios in the tropics. A panel of 167 indica landraces and improved varieties was phenotyped for spikelet sterility (SPKST) under 38°C during anthesis and for several secondary traits potentially affecting panicle micro-climate and thus the fertilization process. The panel was genotyped with an average density of one marker per 29 kb using genotyping by sequencing. Genome-wide association analyses (GWAS) were conducted using three methods based on single marker regression, haplotype regression and simultaneous fitting of all markers, respectively. Fourteen loci significantly associated with SPKST under at least two GWAS methods were detected. A large number of associations was also detected for the secondary traits. Analysis of co-localization of SPKST associated loci with QTLs detected in progenies of bi-parental crosses reported in the literature allowed to narrow -down the position of eight of those QTLs, including the most documented one, qHTSF4.1. Gene families underlying loci associated with SPKST corresponded to functions ranging from sensing abiotic stresses and regulating plant response, such as wall-associated kinases and heat shock proteins, to cell division and gametophyte development. Analysis of diversity at the vicinity of loci associated with SPKST within the rice three thousand genomes, revealed widespread distribution of the favourable alleles across O. sativa genetic groups. However, few accessions assembled the favourable alleles at all loci. Effective donors included the heat tolerant variety N22 and some Indian and Taiwanese varieties. These results provide a basis for breeding for heat tolerance during anthesis and for functional validation of major loci governing this trait. [ABSTRACT FROM AUTHOR]

The traditional transplanted and flooded rice cropping system is threatened by water shortage and labor cost increase in many Asian countries and is currently evolving toward direct-seeded and water-saving systems. However, yield penalties generally accompany this evolution and ongoing research targets their mitigation. In a previous research, slower plant development rate was observed in aerobic crops, possibly caused by the increased plant density. This possible factor was tested in experiments conducted at IRRI’s farm, Los Baños, Philippines, in 2012 and 2013. One elite variety was direct-seeded at three plant distances (6, 10, and 20 cm × 20 cm) and sprinkler-irrigated to keep the soil water potential above −10 kPa. Weekly measurements were made to dissect crop and plant physiological differences. Data from previous experiments on tillering in the greenhouse of four varieties and in fields in aerobic and flooded conditions were added to strengthen the results from the current experiment. The last upper leaves of the main stem appeared faster at higher plant density, thus increase plant density was not the cause for the slower leaf appearance in aerobic than in flooded crops. Plants at higher plant density also had earlier panicle initiation, earlier earing, and 0.8 leaf less than plants at low density. The number of outgrowing tillers was linked to the number of leaves on the main stem by a variety-specific relationship that is independent from plant density and water management. Tiller density was regulated by the cessation of the outgrowth of tillers triggered by the onset of stem elongation at 20-cm plant distance and occurred earlier at shorter distances probably in response to the high root density. Grain yields were significantly higher at higher plant density (6-cm and 10-cm plant distances) and correlated with a higher tiller density. Thus for aerobic crops, seedling density higher than 50 plants m −2 remains the way to get high yields for the type of variety used in this study. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rice yielded 5.7tons/ha grain in aerobic soil and 7.8tons/ha in flooded conditions. [•] Plants in aerobic crops grew slower and had fewer leaves of smaller size per tiller. [•] Aerobic plants flowered later and contained less nitrogen. [•] RUE was nil in both environments during 10 days before flowering. [•] RUE was higher than previously published in flooded crops when biomass accumulated. [Copyright &y& Elsevier]

Rice has generally been reported to yield less in aerobic soil conditions than in flooded soil conditions, in association with delayed anthesis and a reduction in height. A greenhouse experiment was conducted and repeated twice with four rice varieties grown in either flooded or nearly-saturated aerobic soil, in either large or small pots. The rate of leaf appearance was recorded weekly until heading time, when plants were harvested for shoot and root biomass. The kinetics of leaf appearance was generally trilinear with longer phyllochrons in the May sowing. Pot size had only a small effect whereas aerobic soil conditions slowed down the rate of leaf initiation, which consequently delayed panicle initiation and heading date and thus increased the duration of the tillering phase and finally the number of tillers. Surprisingly, the four varieties showed a significant response to sowing date, especially IR72, which headed 21 days later and after the expansion of two more leaves in the May sowing. The aerobic soil conditions and sowing date have thus changed the rate of development of rice plants and this trait has appeared to be the initial link of a chain of consequences in a series of traits known to be affected by these factors. [ABSTRACT FROM AUTHOR]

Abstract: Higher harvest index, widely reported as a main trait supporting major improvements in rice yield potential, also supports the higher performance of hybrids over inbreds. Although higher sink size is generally claimed for being the driver of higher grain filling in hybrid rice, it is relevant to question whether efficient sink regulation, that is timing and magnitude of the successive sink activities over crop growth from early stage, plays a role in supporting higher harvest index and is a key feature ensuring higher performance of hybrid rice. The partitioning coefficient (PC), as the increase in dry matter of one single organ over that of shoot, was calculated over short and successive periods to quantify the dynamics of dry matter partitioning. Four hybrid and four inbred genotypes of similar crop duration were grown under the same cropping management on the IRRI farm during a wet and a dry seasons. While PC to blade and sheath was similar for both plant types in both seasons at the early stage, PC to culm of hybrids was higher than that of inbreds, and PC to blade was lower, in the late vegetative and early reproductive phases. During the late reproductive phase, PC to panicle of hybrids was higher than that of inbreds, and PC to culm was lower whereas culm elongation was similar for both plant types. During grain filling, PC to culm was significantly more negative with hybrids which indicated stronger remobilization with hybrids. Specific culm length before anthesis, as low as 40cmg−1, accounted for storage ability, and at maturity, as high as 130cmg−1, for remobilization ability. Sink strength index, as a better indicator of dry matter partitioning efficiency than harvest index, and unfilled grain size, as an indicator of assimilate wastage, were suggested as key indices to account for the better sink regulation in hybrid rice and to improve screening protocols for increasing yield potential and tolerance to lodging. [Copyright &y& Elsevier]

Abstract: The better performance of hybrid rice compared with elite inbreds has been associated in previous studies with higher harvest index and higher shoot biomass at maturity, with the occurrence of one excluding the other and depending on weather conditions. These outputs were examined here by comparing high-yielding genotypes of similar crop duration and through an intense sampling schedule to identify key crop stages and processes involved in hybrid rice performance. Four contrasting hybrids and four contrasting inbreds were grown on the IRRI farm during a wet and a dry season under the same cultural practices. The grain yield of hybrids was significantly higher than that of inbreds by 14% in the wet season and 18% in the dry season. Crop growth rate (CGR) during each phenological phase, blade growth rate during the vegetative phase, stem growth rate during the reproductive phase and panicle growth rate during the ripening phase were consistently higher for hybrids than for inbreds. As a consequence, the shoot biomass of hybrids increased quicker with time than that of inbreds. And the larger biomass accumulation observed for hybrids at maturity, 2139 and 1906gm−2 vs. 1952 and 1736gm−2 for the dry and wet seasons, respectively, was systematically associated with a higher harvest index. In contrast to most of the previous studies aiming at characterizing the higher performance of hybrid rice, differences in leaf area production and in specific leaf area among plant types during the vegetative phase were inconsistent and rarely significant in this study and could not support the higher performance of hybrids. The tillering dynamics was similar between both plant types. This work highlighted the key role of each phenological phase in contributing to the higher biomass production of hybrid rice regardless of the cropping season, dry or wet, while the processes underlying this higher performance are still unclear. [Copyright &y& Elsevier]

Abstract: Higher harvest index, widely reported as a main trait supporting major improvements in rice yield potential, also supports the higher performance of hybrids over inbreds. Although higher sink size is generally claimed for being the driver of higher grain filling in hybrid rice, it is relevant to question whether efficient sink regulation, that is timing and magnitude of the successive sink activities over crop growth from early stage, plays a role in supporting higher harvest index and is a key feature ensuring higher performance of hybrid rice. The partitioning coefficient (PC), as the increase in dry matter of one single organ over that of shoot, was calculated over short and successive periods to quantify the dynamics of dry matter partitioning. Four hybrid and four inbred genotypes of similar crop duration were grown under the same cropping management on the IRRI farm during a wet and a dry seasons. While PC to blade and sheath was similar for both plant types in both seasons at the early stage, PC to culm of hybrids was higher than that of inbreds, and PC to blade was lower, in the late vegetative and early reproductive phases. During the late reproductive phase, PC to panicle of hybrids was higher than that of inbreds, and PC to culm was lower whereas culm elongation was similar for both plant types. During grain filling, PC to culm was significantly more negative with hybrids which indicated stronger remobilization with hybrids. Specific culm length before anthesis, as low as 40cmg−1, accounted for storage ability, and at maturity, as high as 130cmg−1, for remobilization ability. Sink strength index, as a better indicator of dry matter partitioning efficiency than harvest index, and unfilled grain size, as an indicator of assimilate wastage, were suggested as key indices to account for the better sink regulation in hybrid rice and to improve screening protocols for increasing yield potential and tolerance to lodging. [Copyright &y& Elsevier]

Highlights • Farmers' mean seeding depth under DDSR in rainfed lowlands ranges from 16 to 35 mm. • Seedling emergence from 85 mm was >80% in tolerant cultivars, but <1% in popular cultivars. • Deep-sowing tolerance in the field was attributable to the mesocotyl's elongation. • Deep-sowing tolerance will be required to improve seedling establishment. Abstract Dry direct seeding of rice (DDSR) is practiced as a technique to address drought in tropical Asia, as it requires less water than transplanting. However, rice has a narrow range of optimal sowing depth, and deep sowing often causes poor seedling emergence. The objective of this study was to elucidate differences among rice cultivars in their tolerance to deep sowing (i.e., the ability to show vigorous seedling emergence from deep soil) and the traits associated with this vigor. Two-year on-farm research in rainfed lowlands showed mean seeding depths of 16.1–35.0 mm (a coefficient of variation of 27%–49%). The deep-sowing tolerance differed significantly among 16 cultivars in research station field experiments. The emergence of seedlings from sowing at a depth of 85 mm was >80% in tolerant cultivars versus <1% in cultivars popular with farmers. Deep-sowing tolerance was attributable primarily to the elongation ability of the mesocotyl and, to a lesser extent, of the 1st internode. In a pot experiment that simulated drought around the time of sowing, seedlings that germinated from deeper soil had greater emergence and greater shoot and root growth than those that emerged from shallow soil, as the deeper seeds could acquire residual soil moisture. Our results suggest that improving the tolerance of rice to deep sowing should stabilize seedling establishment in DDSR. Cultivars that tolerate deep sowing will allow further development of the deep sowing technique and reduce the risk of sowing failure caused by drought during seedling emergence in water-scarce environments. [ABSTRACT FROM AUTHOR]

Dry direct seeding of rice (DDSR) is becoming a common practice in drought-prone lowland areas where there is insufficient labor for transplanting, but early-season drought often causes poor crop establishment, which allows subsequent weed infestation. Although early-maturing drought-tolerant cultivars have been released in tropical Asia in the last decade, almost all farmers in these areas still use cultivars selected for irrigated lowlands. The objective of our study was to compare a new drought-tolerant cultivar (Rc348) and a popular cultivar of farmers (Rc10) in DDSR under rainfed lowland conditions. On-farm experiments in three villages in northern Luzon, the Philippines, showed that the yield of Rc10, but not Rc348, was negatively associated with soil drying. Although their average yields were comparable (Rc348, 3.03 t ha −1 ; Rc10, 3.00 t ha −1 ), Rc348 yielded 34% more with moderate to severe weed infestation or soil dryness (3.12 vs. 2.33 t ha −1 ). Weed infestation increased with increasing soil dryness, but the weed pressure was more severe for Rc10 than for Rc348. Rc348 had a higher seedling emergence percentage, number of seedlings m −2 , and ground cover at 30 days after sowing than Rc10 in drought-prone fields at upper (drier) positions in the toposequence. These results were validated by an on-station experiment with controlled drought stress at the International Rice Research Institute. We suggest that the adoption of newly released cultivars from the breeding programs for rainfed rice with reliable seedling emergence and early vigor in the presence of fluctuating soil moisture would stabilize the yield of DDSR in the target drought-prone areas. [ABSTRACT FROM AUTHOR]

Dry direct-seeded rice (DDSR) is an adaptive practice used in rainfed lowlands of tropical Asia where insufficient rainfall at the onset of the wet season makes it difficult to successfully transplant rice seedlings. However, basic guidelines for nutrient management to support DDSR are lacking. Here, we evaluated crop growth and yield in DDSR with different N fertilizers (ammonium sulfate, urea, and coated compound fertilizer) applied at different timings. The total N rate was fixed at 100 kg ha−1. On-station trials were conducted at the International Rice Research Institute, in the Philippines, during the wet seasons of 2017 and 2018, and 17 on-farm trials were conducted in nine provinces in 2018 and 2019. In the on-station trials, ammonium sulfate did not increase yield compared with urea and coated compound fertilizer in either year. Application of N fertilizers at sowing promoted plant N uptake and early shoot growth, but the effect of the timing of N application on yield was not significant. The on-farm trials showed that a uniform N management recommendation is unlikely to be effective in DDSR. Yield increased significantly with coated compound fertilizer relative to the uncoated fertilizers in clay and silty clay soils, but not in other soil types. In general, flexible timing of the first application of N, ranging from 0 to 30 days after sowing, had no significant effect on yield, but the omission of N application at sowing reduced yield by 13–25% in two trials on sandy soils with a low nutrient-holding capacity. • Three N fertilizers were applied in three regimes for dry direct-seeded rice. • Ammonium sulfate didn't give higher yield than urea and coated fertilizer. • Coated fertilizer gave highest yield on clayey soils but not on other soils. • First N application could be delayed by 30 days except on highly sandy soils. [ABSTRACT FROM AUTHOR]

Shortening the period of rice cropping enables farmers to reduce irrigation water in tropical lowlands. However, grain yield of tropical short-duration rice is often restricted by poor grain filling, whose causes are yet unknown. This study evaluated the source–sink balance, which is closely associated with grain filling, in tropical rice. We compared the percentage of filled grains and stem nonstructural carbohydrates in a popular short-duration cultivar and a high-yielding hybrid cultivar over a wide range of spikelets m−2 by differentiating the N inputs and planting densities on a lowland farm of the International Rice Research Institute, the Philippines, during four seasons. Grain yield ranged from 2.2 to 8.6 t ha−1, with the hybrid cultivar producing consistently higher yield (by 27% on average). Compared with the hybrid cultivar, the short-duration cultivar had less stem nonstructural carbohydrates at heading per spikelet (by 24% on average) and a lower percentage of filled grains in the wet season (by 13% on average), when the source capacity was lowest. Source capacity per spikelet and stem nonstructural carbohydrates at heading per spikelet were significantly associated with the percentage of filled grains, suggesting that these indicators of the source–sink ratio modulate grain filling in tropical rice. Although the sink capacity of the short-duration cultivar was similar to, or smaller than, that of the hybrid cultivar, its limited source capacity resulted in poorer grain filling. We suggest that the role of stem nonstructural carbohydrates should be emphasized in the breeding of tropical short-duration rice. • Increase in spikelets/m2 does not increase yield in tropical short-duration rice. • Grain filling was poorer in short-duration rice than hybrid rice in wet seasons. • Source capacity and stem nonstructural carbohydrate were related to grain filling. • A high source-to-sink ratio with high stem nonstructural carbohydrates is needed. [ABSTRACT FROM AUTHOR]