ABSTRACT Spikelet number per unit area is the most important yield component of rice (Oryza sativa L.). While spikelet number in rice is often related to total N content in whole plant, several studies have reported that it is more strongly correlated with dry weight (DW). The aims of this study are to identify the most critical factor determining spikelet number and to evaluate cultivar differences in spikelet production efficiency. In a simple yet unique experiment, we investigated the relationships between spikelet number, DW, and N content of the same plants at heading stage in 27 cultivars. In all cultivars except Tsukisuzuka, with an extremely short panicle, DW at heading stage had a clear proportional relationship with spikelet number. N content had no consistent relationship with spikelet number. The constant of the proportionality between DW and spikelet number estimated from our 2-year dataset at a single site agreed with that estimated from multisite data in previous studies. We concluded that DW at heading was the most critical factor determining spikelet number. Its proportional relationship can be used as a simple model for predicting spikelet number. The proportionality constant reflected cultivar differences in spikelet production efficiency. This study not only settles the question of the most critical factor determining spikelet number, but also efficiently evaluated cultivar differences in spikelet production efficiency, which we expect to contribute significantly to regulating spikelet number in rice.

ABSTRACT In devising a suitable nitrogen (N) fertilisation method to reduce environmental impacts, the N supply must be optimized to meet the demand for crop production. Sweet corn (Zea mays L.) was grown using different fertilisation methods and winter cover crops from 2021 to 2023. Hairy vetch (HV; Vicia villosa Roth.) and rye (Secale cereale L.) were mixed-sown at the rate of HV:rye = 50:50 and 70:30 kg∙ha−1 in September 2021 and 2022, respectively. In addition, different fertiliser treatments – a mixture of poultry manure and bark manure (manure), polymer-coated urea fertiliser (slow-release fertiliser), and ammonium sulfate (fast-release fertiliser) – were applied before seeding sweet corn in May 2022 and 2023. Soil inorganic N, microbial biomass nitrogen (MBN), and N absorption of corn were measured during the corn growing seasons. In both years, inorganic N in the soil treated with fast-release fertiliser increased drastically and leached into the 30–60 cm soil layer at the corn seeding stage. The N supply from manure or slow-release fertiliser gradually increased inorganic N in the soil. MBN was highest in the soil surface layer where manure was applied. The N uptake rate was the highest just before the reproductive stage. The 100-grain weight was slightly lower when slow-release fertiliser was applied because of the delay in N supply. The combination of cover crops and manure was found to be preferable to the application of commonly used fast-release fertiliser to maximize corn yield while reducing environmental impacts.

ABSTRACT The source capacity affects grain yields. Many efforts have been made to enhance grain yields by introducing quantitative trait loci (QTLs) responsible for shoot-related traits associated with source capacity. Root-related traits could improve the source capacity by improving the uptake of water and nutrients; however, studies have been limited. In this study, to elucidate the root traits related to source capacity in paddy fields, we conducted a physio-morphological analyses and field trials for yield-related traits for three years. We evaluated high-yielding lowland rice, IR64, along with four near-isogenic lines (NILs) and pyramiding lines (PYLs) that introduced upland rice alleles for four root QTLs (root growth angle, thickness, length, and volume). DRO1-NIL with the root growth angle QTL, DEEPER ROOTING 1 (DRO1), exhibited a larger rooting depth index (RDI) but a shorter total root length (TRL) than IR64. qRL6.1-NIL with a root length QTL, qRL6.1, displayed a similar RDI but a longer TRL than IR64. In contrast, qRL6.1+DRO1-PYL with both DRO1 and qRL6.1 not only had a larger RDI than IR64 but also a TRL comparable to that of IR64. qRL6.1+DRO1-PYL significantly increased grain weight per plant compared to IR64; however, neither of the NILs improved grain weight per plant. Furthermore, qRL6.1+DRO1-PYL exhibited a higher bleeding rate and nitrogen content, along with an integrated net CO2 assimilation rate (A all ) superior to that of IR64, which may have contributed to its enhanced source capacity. Our study highlights that pyramiding root QTLs has the potential to enhance source capacity, including photosynthetic ability.

ABSTRACT Drought stress poses a major threat to the morphological and physiological processes of wheat. Vermicompost application can help alleviate these effects under water scarcity. This study investigated the protective role of rice straw vermicompost on the nutritional, biochemical, morphological, and physiological traits of wheat seedlings exposed to different drought levels. Three drought regimes – control (CK, 70% field capacity), moderate drought (MD, 45% field capacity), and severe drought (SD, 30% field capacity) – were combined with four vermicompost treatments – RVC-0 (control, no vermicompost), RVC-1 (4 t ha− 1), RVC-2 (6 t ha− 1), and RVC-3 (8 t ha− 1), applied to two wheat cultivars, Faisalabad-08 and Galaxy-13. Vermicompost application significantly improved growth traits, including root and shoot fresh and dry weights, with the most pronounced effects observed at RVC-2, under both moderate and severe drought conditions. RVC-2 also resulted in the highest stomatal conductance and chlorophyll content in both cultivars. Under severe drought, RVC-2 increased stomatal conductance by 14.38%, sub-stomatal CO2 concentration by 11.36%, total chlorophyll by 11.55%, carotenoids by 7.63%, and ascorbate peroxidase activity by 26.74% compared with the control. Moreover, RVC-2 enhanced the uptake of essential nutrients (N, P, and K) under both well-watered and drought conditions. Overall, Faisalabad-08 demonstrated greater drought tolerance than Galaxy-13, as reflected in its superior morphological, physiological, and biochemical performance. The study concludes that rice straw vermicompost, particularly at 6 t ha− 1 (RVC-2), is effective in mitigating the adverse effects of drought on wheat growth and physiology.

ABSTRACT Chloroplasts, mitochondria, and peroxisomes are consistently positioned in close proximity with physical contact in mesophyll cells, as shown by two-dimensional transmission electron microscopy observations. However, the three-dimensional arrangement of these organelles has not yet been investigated in rice. In this study, we investigated the positions and proximity of the chloroplasts, mitochondria, and peroxisomes in rice mesophyll cells three-dimensionally. Chloroplasts aligned along the cell walls, and mitochondria and peroxisomes were located on the intracellular side of the cell rather than in the peripheral chloroplasts. In total, 345 mitochondria were included in nine replicated cells, and 89.4% of the mitochondria formed binary complexes with peroxisomes. Binary complexes were located between the chloroplasts, forming clusters in the three organelles. Among the 134 chloroplasts and 210 peroxisomes detected in the nine mesophyll cells, only one chloroplast and one peroxisome were not included in the clusters. The clustering of chloroplasts, mitochondria, and peroxisomes at a high frequency has not been observed in other plants; therefore, this arrangement is considered to be unique to rice mesophyll cells, likely due to the smaller and denser cells than those of other plants. In the clusters, the three organelles were in contact with each other and in close membrane proximity. Considering the high correlation between the total volumes of mitochondria and peroxisomes and the area of membrane proximity among the three organelles, it is suggested that the proximity areas in rice mesophyll cells are determined by the size of the binary complexes within the clusters.

ABSTRACT Rice (Oryza sativa L.) serves as the staple food for over half of the global population. To address the pressing need for increased rice production, hybrid rice technology leverages the concept of hybrid vigor, or heterosis, achieving a 15–20% yield advantage over traditional varieties. This study focuses on developing hybrid varieties through the cytoplasmic genetic male sterility (CGMS) mechanism. It explores the genetic inheritance of fertility restoration in two distinct CMS lines, IR58025A (WA) and RTN13A (Gambiaca), using 165 diverse male lines. The study successfully generated F1 hybrids and evaluated them for pollen and spikelet fertility, identifying NVSR 2965 and NVSR 3280 as effective restorers, with fertility rates surpassing 90%. Analysis of the F2 generations from these crosses revealed that fertility restoration is controlled by two dominant genes, Rf3 and Rf4, which interact epistatically. The cross IR58025A × NVSR 2965 exhibited a 12:3:1 segregation pattern, suggesting simple dominance with a masking effect, while RTN13A × NVSR 3280 showed a 15:1 ratio, indicating duplicate dominance. These findings enhance our understanding of the genetic mechanisms underlying fertility restoration and provide valuable insights for applying marker-assisted selection and breeding strategies to develop high-yield hybrid rice varieties.

ABSTRACT Rice fields are among the leading global agricultural sources of anthropogenic methane emissions, with the rice plant tissue playing a central role in the generation of methane (CH4) and the emission process. Due to the variability of methane emissions trends across studies, contrasting rice plant traits and mechanisms have often been reported to affect methane emissions. Here, we review the reported traits and mechanisms together with genes that have been identified as being associated with those traits, as a resource for future research using genome editing to pinpoint the factors affecting methane emissions in rice. Methane production in flooded soils involves several interacting agents, with the rice plant supplying root exudates to methanogenic archaea and facilitating methane transport from soil to the atmosphere through aerenchyma. Conversely, the same aerenchyma system transports atmospheric oxygen to the roots; this oxygenation of the rhizosphere supports methanotrophs which consume methane, thus mitigating emissions. Varietal differences in traits such as barriers to radial oxygen loss, root exudates, root architecture, and growth duration, have also been reported to influence methane production and emissions. Previous studies were typically conducted on lines with large differences in the genetic backgrounds; comparisons of methane emissions between lines with very similar backgrounds, for example as developed through genome editing, might help better define the effects of these traits. The genes highlighted in this review are linked to the traits and mechanisms potentially affecting methane emissions in rice and could be targeted for genome editing and subsequent comparison of methane emissions.

ABSTRACT Producing cash and green manure crops during the same growing season is a promising strategy for sustainable crop production. The present study evaluated the relay intercropping of a legume green manure crop (Vicia villosa Roth, hairy vetch) and root crops (radish and carrot). The hairy vetch was interseeded approximately 40 d after root crop seeding. Competitive effects were not observed in the production of either root crop, even when relay intercropped with hairy vetch. The biomass production of hairy vetch ranged from 376–990 kg DW 10a−1, and the biomass and nutrient (nitrogen, phosphorus, and potassium) accumulation of hairy vetch relay intercropped with carrot were significantly higher than those of radish. The fertilizer effects of hairy vetch incorporation obtained by relay intercropping were compared with those of conventional chemical fertilizer input in subsequent crop production (vegetable soybean and sweet corn). The yield of vegetable soybean was maintained at the same level as that of chemical fertilizer, even when cultivated with hairy vetch alone. In the sweet corn, although hairy vetch alone could not provide sufficient nutrients and growth was inferior compared to chemical fertilizers, the incorporation of hairy vetch relay intercropped with carrot showed relatively higher fertilizer effects and significant difference in ear weight was not observed with chemical fertilizer. Our results show the possibility of reducing chemical fertilizer input in succeeding crop production by hairy vetch relay intercropped with root crops.

ABSTRACT Sugarcane (Saccharum spp. hybrids) is a globally important crop for food and bioenergy, but its production is increasingly threatened by drought driven by climate change. Drought causes complex interactions between root function and leaf photosynthesis, but their underlying mechanisms remain largely unstudied in sugarcane. To address this knowledge gap, we grew up to eight sugarcane cultivars in one field and two pot experiments inside a rain-out shelter. The relative growth rate of these eight cultivars under both well-watered and water-limited conditions was largely explained by net assimilation rate, which depended on bleeding sap rate caused by root pressure. Among the cultivars, Harunoogi – a first-generation backcross cultivar between a wild relative (S. spontaneum) and a commercial cultivar – exhibited improved drought avoidance through adequate root water supply, whereas the commercial cultivar NiTn18 showed the opposite. Throughout the drought and recovery, Harunoogi exhibited significantly higher leaf area, stomatal conductance, chlorophyll content, and maximum amplitude of the reaction center chlorophyll of PSI than cultivar NiTn18, which resulted in a higher net CO₂ assimilation rate and biomass. These results highlight the potential of improving both root function and photosynthesis as an effective approach to developing drought-tolerant sugarcane in the field.

ABSTRACT Grain filling of rice is affected mainly by the source – sink ratio, but the ratio does not explain some cultivar differences. The high-yielding cultivar Momiroman generally has poorer grain filling than the high-yielding Takanari at a similar source – sink ratio. Previous analysis using an F3 population derived from Momiroman × Takanari identified a quantitative trait locus (QTL) for grain filling percentage (GFP) on chromosome 10. The objective here was to validate the effect of the QTL and to clarify whether it is associated with source – sink ratio, using two near-isogenic lines (NILs) with Takanari segments on the long arm of chromosome 10 at regions dubbed A and B (namely NIL10A and NIL10B) and a NIL with a Momiroman segments covering both of the regions (NIL10M). In field experiments using these three NILs in 2023 and 2024, we determined dry matter content, N content, grain yield, and yield components. NIL10A and NIL10B had higher GFP than NIL10M, verifying QTLs for grain filling in regions A (qGF 10A ) and B (qGF 10B ). A higher GFP in NIL10A and NIL10B than in NIL10M at the same source – sink ratio suggests that these QTLs are associated with grain-filling ability. A QTL for the ability of N accumulation in grain or N remobilization from vegetative parts appears to be unrelated to qGF10 B but to be located in region A. The association between qGF10 and N partitioning should be further explored.