Rice Spikelet Research Articles

The SEPALLATA-Like Gene OsMADS34 Is Required for Rice Inflorescence and Spikelet Development

Grass plants develop distinct inflorescences and spikelets that determine grain yields. However, the mechanisms underlying the specification of inflorescences and spikelets in grasses remain largely unknown. Here, we report the biological role of one SEPALLATA (SEP)-like gene, OsMADS34, in controlling the development of inflorescences and spikelets in rice (Oryza sativa). OsMADS34 encodes a MADS box protein containing a short carboxyl terminus without transcriptional activation activity in yeast cells. We demonstrate the ubiquitous expression of OsMADS34 in roots, leaves, and primordia of inflorescence and spikelet organs. Compared with the wild type, osmads34 mutants developed altered inflorescence morphology, with an increased number of primary branches and a decreased number of secondary branches. In addition, osmads34 mutants displayed a decreased spikelet number and altered spikelet morphology, with lemma/leaf-like elongated sterile lemmas. Moreover, analysis of the double mutant osmads34 osmads1 suggests that OsMADS34 specifies the identities of floral organs, including the lemma/palea, lodicules, stamens, and carpel, in combination with another rice SEP-like gene, OsMADS1. Collectively, our study suggests that the origin and diversification of OsMADS34 and OsMADS1 contribute to the origin of distinct grass inflorescences and spikelets.

ELE restrains empty glumes from developing into lemmas

Although there is evident homology among reproductive organs when comparing Poaceae (grass) and eudicots, the identity of grass specific organs, such as lodicules, palea, lemma, and glumes has been the subject of a vast and largely inconclusive discussion. Here we provide some direct evidence to support the idea that the empty glumes of rice (Oryza sativa) are counterparts of lemmas. We show that the development of empty glumes is regulated by ELE (elongated empty glume), which belongs to a plant specific novel gene family. Mutations at the ELE locus cause elongated empty glumes, which mimic the lemmas and have the epidermal morphology of lemmas with four or five vascular bundles. As a nuclear-localized gene, ELE is specifically expressed at the empty glumes of immature spikelets, and its ectopic expression causes many floral development defects, including lemma-like palea, extra palea-like structures, elongated lodicules, extra stamens and stigmas. Our result suggests that empty glumes are lemmas of the sterile florets located at the lateral side of the rice spikelet, and ELE acts as a regulator restraining its growth to maintain its small size in wild-type plants.

Receptor-like kinase OsSIK1 improves drought and salt stress tolerance in rice (Oryza sativa) plants

Receptor-like kinases (RLKs) play essential roles in plant growth, development and responses to environmental stresses. A putative RLK gene, OsSIK1, with extracellular leucine-rich repeats was cloned and characterized in rice (Oryza sativa). OsSIK1 exhibits kinase activity in the presence of Mn(2+), and the OsSIK1 kinase domain has the ability to autophosphorylate and phosphorylate myelin basic protein (MBP). OsSIK1 promoter-GUS analysis revealed that OsSIK1 is expressed mainly in the stem and spikelet in rice. The expression of OsSIK1 is mainly induced by salt, drought and H(2)O(2) treatments. Transgenic rice plants with overexpression of OsSIK1 show higher tolerance to salt and drought stresses than control plants. On the contrary, the knock-out mutants sik1-1 and sik1-2, as well as RNA interference (RNAi) plants, are sensitive to drought and salt stresses. The activities of peroxidase, superoxide dismutase and catalase are enhanced significantly in OsSIK1-overexpressing plants. Also, the accumulation of H(2)O(2) in leaves of OsSIK1-overexpressing plants is much less than that of the mutants, RNAi plants and control plants, as measured by 3,3'-diamino benzidine (DAB) staining. We also show that OsSIK1 affects stomatal density in the abaxial and adaxial leaf epidermis of rice. These results indicate that OsSIK1 plays important roles in salt and drought stress tolerance in rice, through the activation of the antioxidative system.

Convenient Estimation of Unfertilized Grains in Rice

Failure of fertilization in rice is a critical yield-determining factor in plants subjected to temperature or water stress at the early-reproductive stage and in high-yield cultivars bearing heavy spikelets. Although it is important to identify quickly the unfertilized spikelets for research and selection of stress-resistant or high-ripening cultivars from bulksamples, the identification takes time because unfertilized spikelets are usually determined by visual and manual procedures. Our objective was to develop a convenient method to identify unfertilized spikelets in rice. Takanari spikelets at maturity grown in the paddy field were separated into floating and sinking spikelets by different specific gravity solutions of ethanol/water mixture. The unfertilized spikelets were identified by checking the grains inside the spikelets by light penetration and examining the spikelets manually. The percentage of floating spikelets decreased with the increase in ethanol concentration, and that of floating spikelets approximately coincided with the percentage of unfertilized spikelets when the specific gravity was below 0.90×10>3 kg m-3, corresponding to over 70% -ethanol. In a practical range of temperature the specific gravity scarcely changed. In an 80%-ethanol solution, the percentages of floating spikelets in Takanari grown under different nitrogen applications and in rice cultivars having different spikelet size approximately coincided with percentages of unfertilized spikelets, though the percentages of floating spikelets was 5 to 7% higher than the unfertilized spikelets. The use of 70%-ethanol solution increased the difference in some rice cultivars. We concluded that the gravitation method would be convenient for identification of unfertilized spikelets in bulk samples of rice.

Post-Anthesis Changes in Concentrations of Polyamines in Superior and Inferior Spikelets and Their Relation with Grain Filling of Super Rice

Success in super rice breeding has been considered a great progress in rice production in China. However, many un-filled inferior spikelets limit the realization of great yield. As polyamines have been frequently described as endogenous plant growth regulators or intracellular messengers mediating physiological processes, this study investigated whether and how poly-amines are involved in the regulating post-anthesis development of rice spikelets. Four super rice cultivars, Liangyoupeijiu and II you 084 (indica hybrids), Huaidao 9 and Wujing 15 (japonica), and two high-yielding check cultivars, Shanyou 63 (indica hybrid) and Yangfujing 8 (japonica), were field grown. Concentrations of putrescine (Put), spermidine (Spd) and spermine (Spm) in both inferior and superior spikelets during the grain filling period and grain filling rate were determined. The relationship between polyamine concentrations and grain filling rate were analyzed, and chemical regulators were applied to verify the roles of poly-amines in grain filling. The results showed that the maximum grain filling rate, mean grain filling rate, and brown rice weight for superior spikelets showed small difference between the super rice and check rice, but those of inferior spikelets were significantly lower for super rice than for check rice cultivars. Changes in concentrations of polyamines in grains exhibited single peak during the grain filling period. The peak and mean concentrations of Put were greater in inferior than in superior spikelets. The peak and mean concentrations of Spd and Spm were greater in the superior than in the inferior, and greater in the check rice than in the su-per rice. The mean grain filling rate and brown rice weight were negatively and very significantly correlated with the Put concen-trations, and positively and very significantly correlated with Spd and Spm concentrations, Spd/Put, and Spm/Put. Application of Spd or Spm to panicle at the early grain filling stage significantly enhanced activities of sucrose synthetase, adenine di-phosphoglucose pyrophosphorylase, and soluble starch synthetase in inferior spikelets, and significantly increased grain filling rate, seed setting rate and grain weight of inferior spikelets. Application of Put or methylglyoxal-bis (guanylhydrazone) (MGBG, an inhibitor of S-adenosyl-L-methionine decarboxylase) exhibited an opposite effect. The results suggest that polyamines play a role in regulating grain filling. Low concentrations of Spd and Spm and low Spd/Put and Spm/Put may be an important physiological reason for the small grain filling rate and low grain weight of inferior spikelets in super rice.

Post-Anthesis Changes in Concentration of Polyamine in Superior and Inferior Spikelets and Their Relationship with Grain Filling of Super Rice

To reveal the physiological role of polyamines and the mechanism involved in grain-filling of rice ( Oryza sativa L.) superior and inferior spikelets, the relationship between grain-filling rate and polyamine concentrations was analyzed and verified using 4 super rice cultivars and 2 high-yielding cultivars (controls). Compared with the controls, super rice had similar maximum grain-filling rate, mean grain-filling rate, and brown rice weight in superior spikelets and significantly lower values in inferior spikelets. The concentrations of putrescine (Put), spermidine (Spd), and spermine (Spm) varied in single-peak curves in both inferior and superior spikelets during grain-filling period. The peak and mean concentrations of Put were greater in inferior spikelets than in superior spikelets. However, the peak and mean concentrations of Spd and Spm were greater in superior spikelets than in inferior spikelets. The control cultivars had higher concentrations of polyamines than super rice. The mean grain-filling rate and brown rice weight were negatively correlated with Put concentration, and positively correlated with Spd and Spm concentrations and the ratios of Spd/Put and Spm/Put. Spraying of Spd or Spm to panicle at early grain-filling stage significantly enhanced activities of sucrose synthetase, adenine diphosphoglucose pyrophosphorylase, and soluble starch synthetase in inferior spikelets, resulting in improvement of grain-filling rate, seed-setting rate, and grain weight. Application of Put or methylglyoxal-bis (guanylhydrazone) (MGBG, an inhibitor of S-adenosyl- L-methioninedecarboxylase) exhibited an opposite effect. These results indicated a regulative function of polyamines during grain filling. The low grain-filling rate and grain weight in inferior spikelets of super rice are probably in relation to the low concentrations of Spd and Spm and the small ratios of Spd/Put and Spm/Put in inferior spikelets.

The homeotic gene long sterile lemma ( G1 ) specifies sterile lemma identity in the rice spikelet

The mechanism of floral organ specification is principally conserved in angiosperms, as demonstrated by the ABC model. By contrast, mechanisms that regulate the development of organs or structures specific to a group of species remain unclear. Grasses have unique inflorescence units, comprising spikelets and florets. In the genus Oryza (rice), the single spikelet consists of a fertile floret subtended by a lemma and a palea, two sterile lemmas, and rudimentary glumes. Each sterile lemma is a tiny glume-like organ with a smooth surface. Here, we have examined a long sterile lemma1 (g1) mutant, in which the sterile lemma is enlarged like the lemma. Detailed phenotypic analysis reveals that the large sterile lemma in the g1 mutant appears to be caused by homeotic transformation of the sterile lemma into a lemma, suggesting that G1 is involved in the repression of lemma identity to specify the sterile lemma. Gene isolation reveals that G1 is a member of a plant-specific gene family that encodes proteins with a previously uncharacterized domain, named here ALOG (Arabidopsis LSH1 and Oryza G1). G1 mRNA is expressed in sterile lemma primordia throughout their development, and G1 protein is localized in the nucleus. A trans-activation assay using the yeast GAL4 system suggests that G1 is involved in transcriptional regulation. Repression of lemma identity by G1 is consistent with a hypothesis proposed to explain the morphological evolution of rice spikelets. We also show that a wild rice species, Oryza grandiglumis, that forms large sterile lemmas has serious mutations in the G1 gene.

Gene mapping and expression analysis of a novel mutantreproduce organs absent (roa) in rice

Mutant plays an important role in function analysis in plant. A rice flower mutantreproduce organs absent (roa), showing a stable inheritance during several years of study, was identified in rice (Oryza sativa L.ssp. Indica) cultivar Jinhui10 treated with EMS and used in this study. This mutant showed following: elongated palea and pedicle; absence of inner three whorls of floral organs; multi-whorls glume like organs inside the lemma/palea; spikelet meristem like organ upon the pedicle. These phenotypes suggested thatROA is a key gene in rice spikelet development. Genetic analysis confirmed that the mutant traits were controlled by a single recessive nuclear gene. By gene mapping,ROA was restricted between two SSR markers RM221 and RM1342 on the chromosome 2. It concluded thatROA was a novel gene involving in flower development in rice. Besides, the mutation ofROA influenced the transcription level of floral homeotic genes; the expression of floral homeotic genes decreased in roa panicle compared with wild-type, and it suggested thatROA affected flower development by influencing the expression of floral homeotic genes.

Methyl jasmonate triggers loss of grain yield under drought stress

Drought is one of the major constraints to rice production worldwide. The development of rice panicle and spikelet meristem is repressed under the drought conditions, resulting in a reduction in the numbers of panicles and spikelets. In our recent report, we demonstrated that methyl jasmonate (MeJA) plays an important role in drought-induced loss of grain yield. Transgenic overexpression of the Arabidopsis gene jasmonic acid carboxyl methyltransferase (AtJMT) in rice resulted in a large reduction in grain yield through increased MeJA and ABA levels in young panicles. Exposure of nontransgenic plants to drought conditions also increased MeJA and ABA levels in young panicles and significantly reduced grain yield. In both cases, the reduction in grain yield was due to lower numbers of spikelets and lower filling rates than were observed for nontransgenic (NT) controls. The ABA increase in AtJMT transgenic panicles grown in non-drought conditions suggests that MeJA, rather than drought stress, induces ABA biosynthesis under drought conditions. These results led us postulate that plants produce MeJA during drought stress, which in turn stimulates the production of ABA, together leading to a loss of grain yield.

The Domestication Process and Domestication Rate in Rice: Spikelet Bases from the Lower Yangtze

The process of rice domestication occurred in the Lower Yangtze region of Zhejiang, China, between 6900 and 6600 years ago. Archaeobotanical evidence from the site of Tianluoshan shows that the proportion of nonshattering domesticated rice (Oryza sativa) spikelet bases increased over this period from 27% to 39%. Over the same period, rice remains increased from 8% to 24% of all plant remains, which suggests an increased consumption relative to wild gathered foods. In addition, an assemblage of annual grasses, sedges, and other herbaceous plants indicates the presence of arable weeds, typical of cultivated rice, that also increased over this period.

Involvement of polyamines in the post-anthesis development of inferior and superior spikelets in rice

Early-flowered superior spikelets usually exhibit a faster grain filling rate and heavier grain weight than late-flowered inferior spikelets in rice (Oryza sativa L.). But the intrinsic factors responsible for the variations between the two types of spikelets are unclear. This study investigated whether and how polyamines (PAs) are involved in regulating post-anthesis development of rice spikelets. Six rice genotypes differing in grain filling rate were field grown, and PA levels and activities of the enzymes involved in PA biosynthesis were measured in both superior and inferior spikelets. The results showed that superior spikelets exhibited higher levels of free spermidine (Spd) and free spermine (Spm) and higher activities of arginine decarboxylase (ADC, EC 4.1.1.19), S-adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) and Spd synthase (EC 2.5.1.16) than inferior spikelets at the early endosperm cell division and grain filling stage. The maximum concentrations of free Spd and free Spm and the maximum activities of ADC, SAMDC and Spd synthase were significantly correlated with the maximum cell division and grain filling rates, maximum cell number and grain weight. Application of Spd and Spm to panicles resulted in significantly higher rates of endosperm cell division and grain filling in inferior spikelets along with the activities of sucrose synthase (EC 2.4.1.13), ADP glucose pyrophosphorylase (EC 2.7.7.27) and soluble starch synthase (EC 2.4.1.21), suggesting that these PAs are involved in the sucrose-starch metabolic pathway. The results indicate that the poor development of inferior spikelets is attributed, at least partly, to the low PA level and its low biosynthetic activity.

Hormones in Rice Spikelets in Responses to Water Stress During Meiosis

Meiosis stage is the most stress-sensitive period of reproduction in rice ( Oryza sativa L.). However, the underlying biochemical mechanism is poorly understood. This study was carried out to investigate whether and how plant hormones are involved in the regulation of spikelet sterility when exposed to water stress during meiosis. Two rice cultivars, Han A-3 (HA-3, drought-resistant) and Wuyujing 7 (WY-7, drought-susceptible), were used and subjected to well-watered (WW) and water-stressed (WS) treatments during meiosis (15 to 2 d before heading). Leaf water potentials of both cultivars markedly decreased during the period as a result of WS treatment, but panicle water potentials remained constant. The percentage of sterile spikelets in WS plants was increased by 48.5% to 50.9% for WY-7, whereas only by 12.6% to 12.8% for HA-3. The concentrations of zeatin + zeatin riboside (Z+ZR), indole-3-acetic acid and gibberellins (GA 1+GA 4) in spikelets showed no significant differences either between the WW and WS treatments or between the 2 cultivars. Abscisic acid (ABA), ethylene, and 1-aminocylopropane-1-carboxylic acid were all enhanced in spikelets following water stress, with further enhancement for ethylene than for ABA in WY-7, whereas elevated ABA balanced ethylene production in HA-3. The spikelet sterility was significantly reduced when amino-ethoxyvinylglycine (an inhibitor of ethylene synthesis) or ABA was applied to the panicles of WS plants at the early meiosis stage. Application of ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of ABA synthesis) had the opposite effect. These results indicate that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of water stress on spikelet fertility. A higher ratio of ABA to ethylene would be a physiological trait of rice adaptation to water stress.

Abscisic Acid and Ethylene Interact in Rice Spikelets in Response to Water Stress During Meiosis

This study investigated whether and how the interaction between abscisic acid (ABA) and ethylene is involved in the regulation of rice (Oryza sativa L.) spikelet sterility when subjected to water stress during meiosis. Two rice cultivars, HA-3 (drought-resistant) and WY-7 (drought-susceptible), were used and subjected to well-watered and water-stressed (WS) treatments during meiosis (15–2 days before heading). Leaf water potentials of both cultivars markedly decreased during the day as a result of the WS treatment, but panicle water potentials remained constant. The percentage of sterile spikelets in WS plants was increased by 49.7% for WJ-7 but only 12.7% for HA-3. ABA, ethylene, and 1-aminocyclopropane-1-carboxylic acid were all enhanced in spikelets by the water stress, but ethylene was enhanced more than ABA in WY-7 when compared with that in HA-3. Spikelet sterility was significantly reduced when ABA or amino-ethoxyvinylglycine, an inhibitor of ethylene synthesis, was applied to the panicles of WS plants at the early meiosis stage. Application of ethephon, an ethylene-releasing agent, or fluridone, an inhibitor of ABA synthesis, had the opposite effect, and sterility was increased. The results suggest that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of water stress on spikelet fertility. A higher ratio of ABA to ethylene would be a physiologic trait of rice adaptation to water stress.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Impact of elevated CO 2 and temperature on rice yield and methods of adaptation as evaluated by crop simulation studies

Impact of elevated CO 2 and temperature on rice yield in eastern India was simulated by using the ORYZA1 and the INFOCROP rice models. The crop and weather data from 10 different sites, viz., Bhubaneswar, Chinsurah, Cuttack, Faizabad, Jabalpur, Jorhat, Kalyani, Pusa, Raipur and Ranchi, which differed significantly in their geographical and climatological factors, were used in these two models. For every 1 °C increase in temperature, ORYZA1 and INFOCROP rice models predicted average yield changes of −7.20 and −6.66%, respectively, at the current level of CO 2 (380 ppm). But increases in the CO 2 concentration up to 700 ppm led to the average yield increases of about 30.73% by ORYZA1 and 56.37% by INFOCROP rice. When temperature was increased by about +4 °C above the ambient level, the differences in the responses by the two models became remarkably small. For the GDFL, GISS and UKMO scenarios, ORYZA1 predicted the yield changes of −7.63, −9.38 and −15.86%, respectively, while INFOCROP predicted changes of −9.02, −11.30 and −21.35%. There were considerable differences in the yield predictions for individual sites, with declining trend for Cuttack and Bhubaneswar but an increasing trend for Jorhat. These differences in yield predictions were mainly attributed to the sterility of rice spikelets at higher temperatures. Results suggest that the limitations on rice yield imposed by high CO 2 and temperature can be mitigated, at least in part, by altering the sowing time and the selection of genotypes that possess higher fertility of spikelets at high temperatures.

Effect of Silicon Deficiency from Panicle Stage on Formation of Cell Fraction Containing Nucleus of Rice Spikelets

Effect of Silicon Deficiency from Panicle Stage on Formation of Cell Fraction Containing Nucleus of Rice Spikelets

Amino acid substitutions in GyrA of Burkholderia glumae are implicated in not only oxolinic acid resistance but also fitness on rice plants.

Oxolinic acid (OA) resistance in field isolates of Burkholderia glumae, a causal agent of bacterial grain rot, is dependent on an amino acid substitution at position 83 in GyrA (GyrA83). In the present study, among spontaneous in vitro mutants from the OA-sensitive B. glumae strain Pg-10, we selected OA-resistant mutants that emerged at a rate of 5.7 x 10(-10). Nucleotide sequence analysis of the quinolone resistance-determining region in GyrA showed that Gly81Cys, Gly81Asp, Asp82Gly, Ser83Arg, Asp87Gly, and Asp87Asn are observed in these OA-resistant mutants. The introduction of each amino acid substitution into Pg-10 resulted in OA resistance, similar to what was observed for mutants with the responsible amino acid substitution. In vitro growth of recombinants with Asp82Gly was delayed significantly compared to that of Pg-10; however, that of the other recombinants did not differ significantly. The inoculation of each recombinant into rice spikelets did not result in disease. In inoculated rice spikelets, recombinants with Ser83Arg grew less than Pg-10 during flowering, and growth of the other recombinants was reduced significantly. On the other hand, the reduced growth of recombinants with Ser83Arg in spikelets was compensated for under OA treatment, resulting in disease. These results suggest that amino acid substitutions in GyrA of B. glumae are implicated in not only OA resistance but also fitness on rice plants. Therefore, GyrA83 substitution is thought to be responsible for OA resistance in B. glumae field isolates.

Hormonal changes are related to the poor grain filling in the inferior spikelets of rice cultivated under non-flooded and mulched condition

Non-flooded plastic mulching cultivation (PM) of rice can save much irrigation but usually exhibits a poor grain filling and low grain weight when compared to traditional flooding cultivation (TF). This study measured the variations of plant hormones during grain filling and investigated whether they were related to the grain-filling problem under mulching in a field experiment. Hormonal levels in the grains and the grain development of both superior (early flowered) and inferior spikelets (later flowered) were monitored continuously during the grain filling period. The contents of indole-3-acetic acid (IAA), zeatin riboside (ZR) and abscisic acid (ABA) were higher in superior grains than in inferior ones at early grain-filling stage. For individual spikelets, the peaks of IAA and ZR contents appeared just before the peak grain-filling rate but the peak ABA content matched it. The earlier flowered superior spikelets did not show much difference in grain filling characteristics and hormonal changes under the two cultivation treatments. Distinctively with the inferior spikelets in a panicle, PM led to lower grain weight, higher grain-filling rate at early stage and shorter active grain-filling period than the TF. PM also resulted in less IAA and ZR but more ABA contents in the grains than TF at early and middle grain filling stages in these inferior spikelets. Peaks of these hormone contents in the PM inferior grains usually appeared earlier and also disappeared more quickly than those in TF inferior grains. When exogenous IAA was applied to plants at the initial grain-filling stage, IAA and ZR were increased and ABA was reduced in the inferior grains. The treatment prolonged the active grain-filing period and reduced the peak grain-filling rate of inferior grains, similarly as the case with TF. As a result, IAA spraying enhanced the weight of PM inferior grains but reduced that of TF ones. Exogenous ABA spraying led to the opposite effects to those of IAA. The results suggest that the shortened grain-filling period in the inferior grains under PM is related to the increased ABA and reduced IAA and ZR in the grains. Regulation of the ratio of ABA to IAA in grains could potentially increase the weight of inferior grains under the water-saving cultivation with film mulching.

A model explaining genotypic and environmental variation of rice spikelet number per unit area measured by cross-locational experiments in Asia

Rice ( Oryza sativa L.) yield is closely related to the number of spikelets per unit area which varies with the genotype and environment. The objectives were to identify plant factors associated with genotypic and environmental variations in spikelet number and to develop a model that may explain the variations based on plant factors. Two-year cross-location experiments were conducted with nine widely different rice genotypes at eight locations in Asia, which covered a wide climate range from the cool temperate zone in northern Japan to the tropics in Thailand. We determined the spikelet density, crop biomass and plant nitrogen (N) content at different development stages. The spikelet density of the genotypes grown at the eight locations ranged from 5130 to 71,480 m −2. Spikelet density was related ( R 2 = 0.58) to N accumulation in plants up to the late spikelet differentiation stage, i.e., 2 weeks before heading. The spikelet production per unit crop-N content, however, was under the additional influence of crop growth rate (CGR) during 2-week period preceding heading. On the basis of these results and previous studies, we modeled the spikelet number per unit area as a function of plant N accumulation up to 2 weeks before heading and CGR during the 2-week period preceding heading. Approximately half of the data sets were used for parameter estimation by a SIMPLEX method and the rest for model validation. The observed genotypic and environmental variation in spikelet number was well explained ( R 2 = 0.85) by the model with only one genotype-dependent parameter to represent the number of juvenile spikelets produced per unit plant N. The other three parameters were genotype-independent. Quite a large genotypic difference was recognized in the spikelet number produced per unit plant N; the efficiency was higher in improved indica genotypes, lower in unimproved indica and javanica genotypes, and intermediate in japonica genotypes. These results suggested that the primary plant factors for increased spikelet number per unit area in rice are: (1) larger accumulation of plant N up to 2 weeks before heading to increase spikelet differentiation, (2) higher CGR during 2-week period preceding heading to suppress degeneration of juvenile spikelets and (3) high spikelet production efficiency per unit N of genotypes.

Post-anthesis development of inferior and superior spikelets in rice in relation to abscisic acid and ethylene

The purpose of this study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the post-anthesis development of spikelets in rice (Oryza sativa L.). Two rice genotypes were field-grown, and the changes of ABA, ethylene, and 1-aminocylopropane-1-carboxylic acid (ACC) levels in spikelets during grain filling and their relationships with endosperm-division and grain-filling rates were investigated. The results showed that earlier-flowering superior spikelets exerted dominance over later-flowering inferior spikelets in endosperm cell-division and grain-filling rates. The two genotypes behaved the same. Later-flowering spikelets had higher levels of ethylene and ACC than earlier-flowering spikelets. The ethylene evolution rate was significantly and negatively correlated with the cell division and grain filling rates. By contrast to ethylene, later-flowering spikelets contained a lower ABA content/concentration and showed a low content ratio of ABA to ACC than earlier-flowering ones. The cell-division and grain-filling rates were significantly and positively correlated with both ABA contents and the ratio of ABA to ACC. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at an early grain-filling stage significantly increased endosperm cell division rate and cell number, grain-filling rate, and grain weight of inferior spikelets. Application of ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of carotenoid synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate endosperm cell-division and grain-filling in rice. A higher ratio of ABA to ethylene in rice spikelets is required to maintain a faster grain-filling rate.