Secondary Rachis Branches Research Articles

Contribution of genes related to grain number (Gn1a and WFP) introgressed into NERICA 1 to grain yield under tropical highland conditions in central Kenya

ABSTRACT In Kenya and many other African countries, improving rice productivity is vital for future food security. To improve rice productivity in Kenya, near-isogenic lines (NIL) were developed by introducing Gn1a and WFP, genes that function to increase grain number per panicle by increasing secondary and primary rachis-branches, into NERICA 1, a registered variety in Kenya. The aim of this study was to determine whether the introduction of these genes has the potential to improve rice productivity under tropical highland conditions in central Kenya. Field experiments were conducted in 2016 and 2017 at a lowland rice field in Mwea, Kenya, under different nitrogen fertilization conditions using three lines of NERICA 1 introgressed with Gn1a and/or WFP (NIL-Gn1a, NIL-WFP, and NIL-Gn1a+WFP) and their recurrent parents, NERICA 1. Two years of field experiments revealed that the introgression lines enhanced grain yield by increasing the number of primary and secondary rachis-branches and the number of grains per panicle. Gn1a and WFP showed additive effects and the introgression of both genes alleviated the negative effects on yield components observed in the introgressed lines, resulting in higher grain yield. Sufficient nitrogen fertilization was required to increase yield in the single-gene introgression lines. However, the line with both Gn1a and WFP achieved reasonable yields even under low fertilizer conditions in Mwea, Kenya. As these introgression lines have similar growing characteristics to their parent variety, they are expected to contribute to increased rice production in Kenya and similar environments in sub-Saharan Africa.

Proteomic analysis reveals a role of ADP-glucose pyrophosphorylase in the asynchronous filling of rice superior and inferior spikelets

The poor grain filling of inferior spikelets (IS) situated on the lower secondary rachis branch leads to a remarkable decrease in rice yield and quality. The AGPase small subunit 2 (AGPS2) encodes a small subunit of adenosine diphosphate-glucose pyrophosphorylase (AGPase) enzyme, which plays an important role in sucrose-starch conversion and starch biosynthesis in the grain filling of rice. In the present study, qPCR analysis showed low expression abundance of AGPS2 in IS, compared to the superior spikelets (SS), which was consistent with the lower grain weight of IS. To evaluate the molecular mechanism of AGPS2, we first identified the AGPS2 interaction network through Co-immunoprecipitation (Co-IP). In total, 29 proteins of AGPS2 interaction network were characterized by LC–MS/MS. Bioinformatics analysis revealed that, the characterized proteins in the interaction network are likely to be related to starch synthesis, sugar conversion, energy pathway, and folding/modification, and most of them were involved in the grain filling of rice. The sequent Co-IP analysis showed that AGPS2 can bind to starch branching enzyme (SBE), pullulanase (PUL) and starch debranching enzyme (DBE) and assemble into starch synthesizing protein complex (SSPC). In addition, the 14-3-3 protein GF14e was also found to interact with AGPS2. Further analysis by qPCR showed that the expression of GF14e was much higher on IS than on SS. The qPCR results also showed that the expression of GF14e was relatively stable in SS, but changed significantly in IS under alternate wetting and moderate soil drying (WMD), which is consistent with the AGPS2 expression pattern. Our present work provides direct molecular evidence for the different expression patterns of AGPS2 in SS and IS, which could be greatly helpful for the molecular amelioration of the poor grain filling of IS in rice.

OsTPR boosts the superior grains through increase in upper secondary rachis branches without incurring a grain quality penalty.

SummaryTo address the future food security in Asia, we need to improve the genetic gain of grain yield while ensuring the consumer acceptance. This study aimed to identify novel genes influencing the number of upper secondary rachis branches (USRB) to elevate superior grains without compromising grain quality by studying the genetic variance of 310 diverse O. sativa var. indica panel using single‐ and multi‐locus genome‐wide association studies (GWAS), gene set analyses and gene regulatory network analysis. GWAS of USRB identified 230 significant (q‐value < 0.05) SNPs from chromosomes 1 and 2. GWAS targets narrowed down using gene set analyses identified large effect association on an important locus LOC_Os02g50790/LOC_Os02g50799 encoding a nuclear‐pore anchor protein (OsTPR). The superior haplotype derived from non‐synonymous SNPs identified in OsTPR was specifically associated with increase in USRB with superior grains being low chalk. Through haplotype mining, we further demonstrated the synergy of offering added yield advantage due to superior allele of OsTPR in elite materials with low glycaemic index (GI) property. We further validated the importance of OsTPR using recombinant inbred lines (RILs) population by introgressing a superior allele of OsTPR into elite materials resulted in raise in productivity in high amylose background. This confirmed a critical role for OsTPR in influencing yield while maintaining grain and nutritional quality.

Molecular and Genetic Aspects of Grain Number Determination in Rice (Oryza sativa L.).

Rice grain yield is a complex trait determined by three components: panicle number, grain number per panicle (GNPP) and grain weight. GNPP is the major contributor to grain yield and is crucial for its improvement. GNPP is determined by a series of physiological and biochemical steps, including inflorescence development, formation of rachis branches such as primary rachis branches and secondary rachis branches, and spikelet specialisation (lateral and terminal spikelets). The molecular genetic basis of GNPP determination is complex, and it is regulated by numerous interlinked genes. In this review, panicle development and the determination of GNPP is described briefly, and GNPP-related genes that influence its determination are categorised according to their regulatory mechanisms. We introduce genes related to rachis branch development and their regulation of GNPP, genes related to phase transition (from rachis branch meristem to spikelet meristem) and their regulation of GNPP, and genes related to spikelet specialisation and their regulation of GNPP. In addition, we describe other GNPP-related genes and their regulation of GNPP. Research on GNPP determination suggests that it is possible to cultivate rice varieties with higher grain yield by modifying GNPP-related genes.

Sink production and grain-filling ability of a new high-yielding rice variety, Kitagenki

• Kitagenki is a promising high-yielding rice variety adapted to a cool climate in the Hokkaido region, Japan. • Kitagenki showed the maximum grain yield of more than 1000 g m −2 . • Grain yield of Kitagenki was stably superior to the other three varieties. • Kitagenki had higher sink capacity than Tachijoubu and Nanatsuboshi. • Kitagenki had higher grain-filling ability than Kita-aoba, due to higher source ability per spikelet and spikelet fertility. In recent years, several new high-yielding rice varieties (HYV) have been rapidly developed in Japan for multiple uses, including animal feed. Kitagenki (KG) is a new japonica-type HYV growing in the Hokkaido region, the most northern parts of Japan with a cool climate, and it is expected to replace the previously developed HYV: Kita-aoba (KA) and Tachijoubu (TA). However, the physiological traits of KG remain unknown. In this study, a five years field experiment was conducted to elucidate grain yield and physiological traits of KG by comparing with KA and TA as well as with Nanatsuboshi (NA) as a reference. KG showed higher maximum grain yield of 1082 g m −2 , and its grain yield was stably superior to those of KA, TA, and NA, throughout five years. The high grain yield of KG was attributed to its higher sink capacity (1080 g m −2 ) than those of TA (967 g m −2 ) and NA (819 g m −2 ); and higher sink-filling percentage (86.0 %) than that of KA (80.8 %). Compared with TA, KG had a higher harvest index (HI), shorter growth duration (GD), and a lower shoot dry matter at maturity (SDM). Compared with KA, KG had higher grain-filling ability on the secondary rachis branch (SRB), which was due to both higher source ability per spikelet and higher spikelet fertility. These physiological traits contributed significantly to increasing grain-filling ability. In particular, the source ability per spikelet was significantly different between KG and KA during the early grain-filling stage (0–20 days after full-heading [DAH]). As available carbohydrate sources, dry matter production (ΔW) during the grain-filling stage directly contributed to increasing filled grain percentage, whereas stem nonstructural carbohydrate (NSC) translocation played a supportive role in compensating for the shortage of carbohydrate especially when the solar radiation was low. Further increase in grain yield is expected by increasing the source ability in KA, whereas increasing both sink capacity and source ability would be useful in KG.

Development and Characterization of Rice Lines with Clustered Spikelets and Dense Panicles

Rice panicle architecture is an important factor affecting yield potential. Korean rice cultivars have a narrow genetic background for panicle architecture. To enhance the yield potential of Korean rice cultivars, we developed and characterized rice lines with new panicle architecture. Rice with improved panicle architecture has clustered spikelets and dense panicles (CD type). CD rice was derived from a cross between “Binhae Col.#1” carrying dense panicles, and “ARC10319” that has the clustered spikelets gene (Cl). CD rice lines had short and semi-erect panicles with two to five high density spikelets clustered at the tips of primary and secondary rachis branches. CD rice lines had dramatically increased numbers of spikelets; almost twice as many as those of Korean rice cultivars. The increase in spikelet number was mainly caused by the increased spikelets and branches on secondary rachises compared to those on primary rachises. The increase in spikelet number was expected to enhance the yield of CD rice by expanding sink capacity. However, the yield of selected lines; CD9, CD27, CD34, and CD39, did not reach the level of the Korean high-yielding cultivars “Boramchan” and “Hanareum2,” due to the reduction in panicle number and grain weight, and poor ripening. Although no substantial yield increase was observed in CD rice, the panicle architecture of CD rice, clustered spikelets, and dense panicles could be new genetic resources as breeding material for diversifying panicle architecture and enhancing yield potential.

Comparison of yield traits in rice among three mechanized planting methods in a rice-wheat rotation system

Understanding the differences in yield traits of rice among pothole seedling of mechanical transplanting (PSMT), carpet seedling of mechanical transplanting (CSMT) and mechanical direct seeding (MDS) is of great importance not only for rice scientists but also for rice farmers to develop a high-yield production system under mechanical conditions in a rice-wheat rotation system. However, such traits are yet to be studied among rice varieties of japonica-indica hybrid rice (JIHR), japonica conventional rice (JCR) and indica hybrid rice (IHR). Field experiments were conducted in 2014 and 2015, where six cultivars of the three rice types JIHR, JCR and IHR were grown individually with PSMT, CSMT and MDS methods, under respective managements for each method to achieve the maximum attainable yield. Results showed that (i) the PSMT significantly increased grain yield of JIHR by 22.0 and 7.1%, of JCR by 15.6 and 3.7% and of IHR by 22.5 and 7.4%, compared to MDS and CSMT on average across the two years, respectively. The highest yield was produced by the combination of JIHR and PSMT; (ii) high yield under PSMT was mainly attributed to large sink capacity and high-efficient dry matter accumulation. With sufficient panicles per hectare, the increase of spikelet number per panicle, especially the increase in spikelet number of the secondary rachis-branches was determined to be the optimal approach for developing a large sink capacity for rice under PSMT. The optimal tillers development, large leaf area index at heading stage, and high leaf area duration, crop growth rate and net assimilation rate during grain-filling phase could be the cause of sufficient dry matter accumulation for rice under PSMT; (iii) moreover, the PSMT favored plant growth as well as enriched the stems plus sheaths during grain-filling phase, as compared with CSMT and MDS. These results suggest that PSMT may be an alternative approach to increasing grain yield in a rice-wheat rotation system in the lower reaches of the Yangtze River in China.

Effects of elevated CO 2 on rice grain yield and yield components: Is non-flooded plastic film mulching better than traditional flooding?

The atmospheric carbon dioxide (CO2) concentration has been increasing rapidly since the Industrial Revolution. The responses to elevated CO2 of rice (Oryza sativa L.) growth and yield have been widely reported, but the majority of these studies investigated rice grown under traditional flooding at two contrasting CO2 levels. The effects of a range of CO2 concentrations (CO2 gradient) on the yield and its components of rice grown under non-flooded vs. flooded conditions remain unclear. Using a CO2 Gradient Tunnel (CGT), we investigated the effects of elevated CO2 (450, 500, 550 and 600 μmol mol−1) on rice yield and yield components under two cultivation practices, viz. traditional flooding (TF) and non-flooded plastic film mulching (PM). Elevated CO2 increased rice yield by 25% under the TF treatment at 450–500 μmol mol−1, but had no effect or decreased the rice yield under the PM treatment. The number of panicles per square meter was decreased by 4–26% under progressive elevation of CO2 concentration, regardless of cultivation practice. Elevated CO2 increased the spikelet number per panicle and filled spikelet percentage under the TF treatment, but had no effect on these parameters under the PM treatment. Specifically, elevated CO2 decreased the number of degenerated spikelets on secondary rachis branches of rice grown under the TF treatment by 75%, but increased that of filled spikelets by 43%. This was the major reason for the CO2-induced increase in rice yield under the TF treatment. The 1000-grain weight and Harvest Index (HI) under the two cultivation practices was increased only when CO2 concentration was elevated to 550–600 μmol mol−1. The CO2 × cultivation interaction was detected for grain yield. When CO2 concentration was increased to 600 μmol mol−1, the rice yield of the PM treatment was 2% higher than the TF treatment. This study demonstrated that improved management practices are needed to maximize the benefits of non-flooded plastic film mulching cultivation in a CO2-rich world. Our results provide major implications for water management of rice production systems and global food security under future higher CO2, and potentially drier, environments.

Growth and yield responses of upland NERICAs to variable water management under field conditions

This study aimed to investigate the possible causes for inconsistent performances of upland New Rice for Africa (NERICA) varieties in uplands and lowlands, while identifying important determinants in grain yield under deficient soil moisture. We compared the growth and yield of NERICA 1 and NERICA 5 to those of Yumenohatamochi, a Japanese upland variety, and Hinohikari, a Japanese lowland variety, subjected to different water management regimes (continually flooded, supplementary irrigation, and non-irrigation). Under conditions of deficient soil moisture, panicle number per square meter, spikelet number per panicle, and 1000-grain weight of NERICAs decreased, whereas the panicle number of the Japanese varieties experienced little change. In contrast, the grain filling ratio was unaffected by water management, irrespective of variety. The primary source of yield reduction under low soil water conditions was a decrease in spikelet number per panicle, and water stress intensity was the primary factor for the degree of this reduction. Variation in the abortion of secondary rachis-branches caused differences between NERICAs in their spikelet number response to soil moisture deficiency. The inconsistency in NERICA performance across uplands vs. lowlands can be partially attributed to variation in yield response to low soil water conditions. Moreover, water stress intensity and the presence of a water gradient along the vertical soil profile may combine to affect the fluctuation in NERICA performance under upland conditions.

Varietal differences in sink production and grain-filling ability in recently developed high-yielding rice (Oryza sativa L.) varieties in Japan

In Japan, the rice production oriented toward use for flour and animal feed to substitute for imported wheat flour and maize has been promoting, and many high-yielding varieties (HYVs) for the above purpose have been developed and released recently. In order to establish cultivation methods for newly released high-yielding varieties (HYVs) and to develop breeding targets for further increases in yield, evaluation of their yield potential and growth characteristics is essential. The objective of this study was to evaluate yield potential and the constraints of the recently developed HYVs in Japan. Field trials were conducted for two years using five Japanese varieties including reference variety Nipponbare (NIP); japonica-dominant HYVs Bekoaoba (BEKO) and Momiroman (MOMI); indica-dominant HYVs Takanari (TAKA) and Hokuriku 193 (H193). We compared the sink capacity (total number of spikelet per unit area×a thousand grain (brown rice) weight) and grain-filling ability in HYVs by the analysis of the characteristics related to them. The hulled grain yields of the japonica-dominant HYVs were 9–23% greater, and those of indica-dominant HYVs were 24–34% greater than that of reference variety NIP, respectively. The main factor for the high yield was remarkably large sink capacity in HYVs due to the high sink production efficiency per unit of absorbed nitrogen (SPE). Although all HYVs showed high SPE, the yields showed varietal differences, in that the yields of japonica-dominant HYVs were lower than those of indica-dominant HYVs. The yield constraint was the grain-filling ability, which indicated larger difference especially at high sink capacity. In the present study, three factors were suggested to be involved with grain-filling. First one is dry matter production during the ripening period, in that the CGRs of the indica-dominant varieties were greater during this period, especially the CGR of H193, which achieved the highest yield and had the highest CGR. Second one is the unfertilized spikelets due to the large number of spikelets per panicle. In MOMI, the relatively high percentage of unfertilized spikelets on secondary rachis-branches on the lower part of the panicle can explain the lower grain-filling. On the other hand, the grain-filling ability of another japonica-dominant variety BEKO was superior to that of MOMI. The better grain-filling was attributed to the lower spikelet number and lower ratio of spikelets on secondary rachis-branches, which may be advantageous for a variety with large grain size. Another factor is the mobilization of the non-structural carbohydrate (NSC). The net decreased amount of NSC (difference of the maximum and minimum accumulated NSC in the post-heading) of H193 was largest among used varieties. However, in the case of TAKA which indicated high grain-filling ability as well as H193, the net amount of translocated NSC was similar to those of japonica-dominant HYVs. On the other hand, both of the indica-dominant HYVs showed an earlier decrease of NSC and a larger increase of panicle weight at earlier ripening period. From these results, the rapid translocation of NSC during the ripening period was assumed to contribute to the high grain-filling ability in indica-dominant HYVs. These results indicate that the improvement of grain-filling by the stimulation of source activity and translocation of NSC is important for japonica-dominant varieties, and the improvement of sink production efficiency would result in the increase of the yield potential in the indica-dominant varieties.

A near isogenic line of rice carrying chromosome segments containing OsSPS1 of Kasalath in the genetic background of Koshihikari produces an increased spikelet number per panicle

The aim of this study was to analyze the yield characteristics of a near isogenic line (NIL) that carried chromosome segments containing OsSPS1, a gene encoding sucrose phosphate synthase (SPS), of the indica cultivar ‘Kasalath’ in the genetic background of the japonica cultivar ‘Koshihikari’ (designated as NIL-SPS1). To determine the growth and yield characteristics of NIL-SPS1, we compared NIL-SPS1 with the parental cultivar, Koshihikari, from field trials conducted over the course of a three-year period. The SPS activity in the source leaves of NIL-SPS1 was higher than that of Koshihikari at the transplanting and panicle formation stage. However, at the heading stage, no significant differences were observed in the leaf SPS activities. An analysis of the yield components revealed that the spikelet number per panicle in NIL-SPS1 was 38–47% higher than that in Koshihikari, mainly due to the larger number of secondary rachis branches. In the substituted chromosome region of NIL-SPS1, no quantitative trait loci (QTLs) for the number of secondary rachis branches have been reported, so far. Collectively, these results suggest that the chromosome segments of Kasalath in NIL-SPS1 contain a new putative QTL for the number of secondary rachis branches. Analysis of the distribution of dry matter revealed that a higher source-leaf SPS activity in NIL-SPS1 at the panicle formation stage might promote the distribution of dry matter to panicles and increase the number of secondary rachis branches.

Effect of Different Nitrogen Doses on the Storage Proteins and Palatability of Rice Grains of Primary and Secondary Rachis Branches

The effects of applied nitrogen (N) doses on crude protein and amylose accumulation in rice grains on primary and secondary rachis branches and their palatability were examined. The crude protein content of rice grains on both primary (PRB) and secondary rachis branches (SRB) increased with the increase in N supply showing a strong negative correlation with palatability. The correlation coefficient between protein content of the grains on PRB and SRB was –0.92 and –0.98, respectively. Although the amylose content also decreased with increasing N supply, the change was not significant oneither PRB or SRB. However, it showed a good correlation with palatability; i.e., the correlation coefficient on PRB and SRB was 0.93 and 0.84, respectively. Analysis of protein by SDS-PAGE showed that the contents of protein bodies PB-I and PB-II, were correlated with palatability, but the latter had a higher correlation. The grains on PRB showed a higher palatability than those on SRB due to the difference in the accumulation pattern of proteins and amylose. These results suggest that when breeding for varieties with good palatability, the plants with more grains on PRB should be selected and by maintaining the nitrogen level in the field, we can control protein accumulation and thus can improve the palatability of rice crop.

Evaluation of yield performance in rice near-isogenic lines with increased spikelet number

Rice yield potential is determined by the balance between sink size and source capacity. To clarify the factors that limit yield in temperate japonica cultivars, we compared the yield performance of Sasanishiki, a temperate japonica cultivar, with those of three near-isogenic lines (NILs) of Sasanishiki with introgression of quantitative trait loci (QTL) derived from a high-yielding indica cultivar, Habataki: qSBN1, which increases the number of secondary rachis branches; qPBN6, which increases the number of primary rachis branches; and a pyramid line that combines these two QTLs. NIL ( SBN1), NIL ( PBN6), and NIL ( SBN1 + PBN6) produced 28–37%, 9–16%, and 62–65% more spikelets per panicle than Sasanishiki, respectively. However, the NILs with increased spikelet number per panicle did not increase grain yield significantly, because compensation is taken place among different yield components. The pyramid line nevertheless had 4–12% higher yield than Sasanishiki due to greater translocation of carbohydrates from stem to panicle. There was no difference in carbohydrate accumulation before heading or in biomass production among Sasanishiki and the three NILs. The results indicate that increasing sink size does not substantially improve yield in Sasanishiki, which lacks sufficient substrate supply to fully satisfy the increased sink demand that results from the spikelet-number QTLs.

Effects of Deep-Flooding Irrigation on Growth, Canopy Structure and Panicle Weight Yield Under Different Planting Patterns in Rice

For rice cultivation in Japan, deep-flooding irrigation is used as a growth control method. To clarify the effects of deep-flooding cultivation under the different planting-pattern, we arranged conventional (Con, 22 hills m-2), narrow (Nar, 33 hills m-2) and very narrow (broadcast direct-seeding Model:BDSM, 100 hills m-2) planting plots under shallow-flooding (SF: 5 cm) and the deep-flooding (DF: 27 cm) conditions from active to maximum tillering stage and evaluated the growth, panicle weight yield, panicle components and community structure. DF was effective in controlling the weak tiller over the whole planting plots arranged. The panicle weight yield (m-2) was high in DF and high yield was kept even in BDSM. The panicle weight yield (m-2) increasesin DF was based on the panicle weight (panicle-1) increased by increasing secondary rachis-branches and their grains (Con and Nar), or on the panicle numbers (m-2) increase (BDSM). Shoot nitrogen content (tiller-1), reported to have the correlation with panicle components, was high in the Con and Nar in DF, and this might contribute to the increase in secondary rachis-branch grains. In BDSM in DF, the marked decrease in the percentage of productive tillers and serious lodging observed in SF were improved. The lodging resistance value in DF was higher than that in SF over the whole planting plots. The light transmission in DF was superior to that in SF in spite of the large LAI. Thus, the growth improving effect of DF was obvious over the wide planting pattern range.

Preflowering Abortion Reduces Spikelet Number in Upland Rice (Oryza sativa L.) under Water Stress

Spikelet number per panicle is a major target trait for improving rice (Oryza sativa L.) yield in upland fields and is strongly affected by water stress. The rice panicle consists of rachis branches and spikelets, in which preflowering spikelet abortion often occurs. We investigated the effect of timing and intensity of water stress at the early reproductive stage on the morphology of rice panicles in field and pot experiments. In the field experiment, water stress caused high rates of preflowering spikelet abortion, and spikelet number was reduced by 48% on average. Pot experiments revealed the effects of water stress on panicle morphology were dependent on the developmental stage, and water stress at the meiosis stage (10–20 d before heading) induced the highest frequency of preflowering spikelet abortion. At the meiosis stage, a mild water stress (no visible effect and less than 20% reduction in biomass production) was sufficient to cause a considerable increase in secondary rachis branch abortion (70%) and resulted in a 40 to 45% reduction in spikelet number per panicle. Reduction in plant water status under drought conditions was related to preflowering spikelet abortion, suggesting that the drought avoidance mechanism is effective for maintaining spikelet number in upland rice.

大粒超多収品種「秋田６３号」における窒素生産効率と穂相の特徴

大粒超多収品種「秋田６３号」における窒素生産効率と穂相の特徴

Morphological and molecular characterization of a new frizzy panicle mutant, “fzp-9(t)”, in rice (Oryza sativa L.)

The spikelet identity gene "fzp" (frizzy panicle) is required for transformation of the floral meristems to inflorescent shoots. In fzp mutants, spikelets are replaced by branches and spikelet meristems produce massive numbers of branch meristems. We have isolated and characterized a new fzp mutant derived from anther culture lines in rice and designated as fzp-9(t). The fzp-9(t) mutant showed retarded growth habit and developed fewer tillers than those of the wild-type plant. The primary and secondary rachis branches of fzp-9(t) appeared to be normal, but higher-order branches formed continuous bract-like structures without developing spikelets. The genetic segregation of fzp-9(t) showed a good fit to the expected ratio of 3: 1. The sequence analysis of fzp-9(t) revealed that there is a single nucleotide base change upstream of the ERF (ethylene-responsive element-binding factor) domain compare to wild-type plant. The mutation point of fzp-9(t) (W66G) was one of the six amino acids of the ERF domain that contributed to GCC box-specific binding. The premature formation of a stop codon at the beginning of the ERF domain might cause a non-functional product.

Analysis of the Number of Spikelets per Panicle on the Main Stems, Primary and Secondary Tillers of Different Rice Genotypes Grown under the Conventional and Nitrogen-Free Basal Dressing Accompanied with Sparse Planting Density Practices

The number and development of spikelets on a panicle directly affect the grain yield. In this study, we examined the differences among various rice cultivars in the number of spikelets per panicle (NSP−1) on the main stems (MS), primary tillers (PT) and secondary tillers (ST), and in the numbers of spikelets on the primary and secondary rachis branches. The difference among cultivars in the responses of these characteristics to the practice of nitrogen-free basal dressing accompanied with sparse planting density (BNo) was also elucidated. The results showed that NSP−1 varied with the tiller position on a plant, and it was highest on MS followed by PT and ST in this order in all cultivars. NSP−1 on all MS, PT and ST also varied with the cultivar, and was larger in the cultivars of the panicle-weight type than in those of the panicle-number type. The difference between MS and ST in NSP−1 was larger in the cultivars of the panicle-weight type than in those of the panicle-number type.ǽ NSP−1 was larger in BNo than in the conventional cultivation (CONT) in most cultivars. The difference between BNo and CONT in NSP−1 varied with the tiller position on a plant and with the earliness of the cultivar, but did not clearly vary with the plant type of the cultivar. It was larger in the panicle on ST than on MS or PT, and was larger in the late-maturing cultivars than in the early- and medium-maturing ones. The larger NSP−1 on MS or PT compared with ST, in the cultivars of the panicle-weight type compared with those of the panicle-number type, and in BNo compared with CONT was mainly attributed to the increased number of spikelets on secondary rachis branches.

Cultivar differences in the number of differentiated spikelets and percentage of degenerated spikelets as determinants of the spikelet number per panicle in relation to dry matter production and nitrogen absorption

The plant type in high-yielding cultivars (HYC) bred in recent years has tended to change from a higher number of panicles with smaller stems and panicles (panicle number type) to a lower number of panicles with larger stems and panicles (panicle weight type or superpanicle weight type) with a concomitant increase of the contribution ratio of the spikelet number per panicle to the number of spikelets per square meter. Therefore, field experiments were conducted by using twenty-two rice cultivars with different plant types belonging to both japonica and indica ecospecies to analyze cultivar differences in the number of differentiated spikelets per panicle (NDS) and the percentage of degenerated spikelets (PDGS) which determines the number of spikelets per panicle, in relation to the amount of dry matter produced and nitrogen (N) absorbed until and / or during the panicle formation stage. The NDS, number of degenerated spikelets (NDGS) and surviving spike lets per panicle (NSS) were significantly higher in the indica cultivars than in the japonica ones mainly due to the spike lets on the secondary rachis branches. The NDS ranged from 83.9–149.6 and 133.1–229.9 in the japonica and indica cultivars, respectively. The PDGS ranged from 15.3 to 26.8% in the japonica cultivars and 7.2–38.1% in the indica ones with averages of 21.6 and 27.0%, respectively. Correlation showed that the higher NDS increased the number of both NSS and NDGS per panicle and that the latter one significantly increased the PDGS in both indica and japonica cultivar groups. The NSS was not closely related to the PDGS in both ecospecies. The amount of dry matter per productive stem (per stem) at YPS (young panicle stage: young panicle 1–2 mm long on superior stems) or heading and its increase during the period from YPS to heading were correlated with the NDS, and the amount of absorbed N per stem showed similar correlations with the NDS except at YPS. The amount of dry matter or absorbed N per stem at YPS or heading showed a similar range in both ecospecies, but for the same amount of dry matter, the NDS was remarkably higher in the indica than in the japonica cultivars, i.e., SPE-D or SPE-N (production efficiency of the differentiated spikelets per dry matter or N content) was higher in the indica than in the japonica cultivars. SPE-D or SPE-N showed a high and negative correlation with the amount of dry matter or absorbed N per stem at YPS or heading and their increase during the period from YPS to heading in both indica and japonica cultivar groups. These results imply that the increase in the amount of dry matter or absorbed N per stem at YPS and heading also increased the NDS with decreasing SPE-D or SPE-N. We concluded that the changes in the plant type from panicle number to panicle weight type may be brought about by the increase of NDS and NSS with a concomitant decrease in the spikelets production efficiency per amount of dry matter produced and N absorbed per stem irrespective of ecospecies.

Diversity of the Rachis-Branching System in a Panicle in Japónica Rice

Recently, rice varieties having large panicles with many spikelets are expected to produce high yield. However, the ripening, growth pattern, priority to photoassimilate partitioning and final grain weight in each spikelet vary with the position of the spikelet in a panicle. Therefore, not only the panicle size but also rachis-branching system in a panicle is an important factor determining the yield. In this report, we performed principal component analysis to characterize the rachis-branching system in 65 japónica varieties. In the principal component analysis, the proportions of variability explained were 49.6 and 22.2% for the first two attribute components. The first principal component was assumed as the factor of size in number, and the second principal component was assumed as the factor of shape of panicle. The distribution of the scores for each variety in the scatter diagram showed a large diversity in panicle characteristics in japónica varieties. A small number of varieties had scores distributed in the first quadrant of the scatter diagram. These varieties would be high-yielding because the number of spikelets is high in the panicle having a relatively large number of primary rachis-branches compared with the secondary rachis-branches. Some clusters in the scatter diagram were related with their origins.