Apoplasmic assimilates and grain growth of contrasting rice cultivars differing in grain dry mass and size

Apical dominance in assimilate filling impacts grain growth in basal spikelets of rice panicle. In this study, organic materials of the pericarp, apoplasmic space and endosperm of the apical and basal caryopses, and photosynthesis of the flag leaf were measured during early part of grain development in three types of rice cultivars with similar phenology, but difference in grain weight and size in the dry and wet seasons of 2006 and 2007, respectively. Photosynthetic activity of the flag leaf was consistently low in small-seeded cultivars. Rates of grain filling and cell division of endosperm and concentration of assimilates, starch, proteins and chlorophylls of the caryopsis were lower, but spikelet ethylene production and peroxidase activity were higher in a small-seeded cultivar compared to a big-seeded cultivar. Similar disparities in grain filling and other attributes were noticed for the inferior basal spikelets of the panicle compared to the superior apical spikelets, except the assimilate concentration of the pericarp and endosperm. Temporal fluctuation in assimilate concentration of the organs were similar between the cultivars. Concentration of apoplasmic assimilates mostly exhibited negative correlation with that of pericarp and endosperm. Compared to the apical spikelets, correlation was more negative for the basal spikelets. Conversely, correlation was positive between the concentration of apoplasmic assimilates and endosperm cell number and grain weight of the cultivars. Ethylene released from the spikelets at anthesis affected growth and cell division rates of endosperm and enhanced protein and chlorophyll degradation and peroxidase activity of the caryopsis. It was concluded that variation in spikelet ethylene production may be responsible for differences in size or weight of grains among rice cultivars and spikelets at different locations of the panicle. The concentration of apoplasmic assimilates could be an indicator for grain filling capacity, and ethylene regulated the concentration by affecting pericarp activity for assimilate unloading.     

大麦胚乳细胞增殖动态及其与粒重的关系

大麦籽粒胚乳细胞数在花后17 d左右就基本决定,增殖动态可用Richards曲线方程拟合,决定系数0.9900以上,达极显著水平.籽粒胚乳细胞数目、单个细胞重量与粒重均存在极显著正相关,r值分别为0.9019**和0.9409**.籽粒胚乳细胞数对粒重影响最大,单个胚乳细胞重次之,胚乳细胞数的多少是决定粒重的主要原因.     

Cell Production and DNA Accumulation in the Wheat Endosperm, and their Association with Grain Weight

Nuclear DNA content was measured in developing endosperm cellsof two wheat varieties, Chinese Spring and Spica. 3C, 6C, 12Cand 24C nuclei were detected, indicating that some form of endoreduplicationand/or endopolyploidization was occurring. The total amountof DNA in the endosperm continued to increase until 24 dayspost anthesis. This accumulation of DNA resulted both from productionof new nuclei and also from increases in the DNA content ofexisting nuclei. Estimates of endosperm cell numbers were made from the totalDNA content per endosperm and the mean DNA content per endospermnucleus for a range of genotypes differing in mature grain weight.Endosperm DNA content and cell number were both positively associatedwith mature grain weight among the genotypes examined. However,not all of the variation in grain weight could be attributedto variation in cell number because of differences in mean dryweight per endosperm cell. The large-grained variety, Spica, had a greater mean weightper endosperm cell than Chinese Spring and this difference aroseafter cell production in the endosperm had ceased. Triticum aestivum, grain weight, cell size, cell number, DNA content     

Effects of irradiance and water supply on grain development in wheat

The effects of two levels of irradiance and of water supply on grain development in two cultivars of winter wheat were studied by imposing treatments during the phases of cell production and cell expansion in the endosperm. The storage capacity of the grain was determined by cell number in the endosperm, which was reduced by both treatments during the cell production phase. Changes in cell number caused by the treatments were due to changes in the rate of production of cells; the duration of the cell production phase was constant. Weight per grain at maturity was proportional, both within and between cultivars, to the number of endosperm cells, except when treatments altered the supply of assimilate to the grain during the cell expansion phase. Reducing irradiance and water during the cell expansion phase decreased the rate of accumulation of dry matter in the grain and reduced weight per grain at maturity, with no effect on the duration of grain filling. Shrivelled grain resulted from a failure of the endosperm cells to fill completely, and was characterised by a reduction in the number of ‘B’-type starch granules.     

Grain growth and endosperm cell size under high night temperatures in rice (Oryza sativa L.)

BACKGROUND AND AIMS: High night temperatures are more harmful to grain weight in rice than high day temperatures. Grain growth rate and growth duration were investigated to determine which was the cause of the decrease in final grain weight under high night temperatures. Endosperm cell number and cell sizes were also examined to determine which might cause the decrease in final grain weight. METHODS: Rice plants were grown outdoors in plastic pots and moved at heading time to three temperature-controlled glasshouses under high night temperature (HNT; 22/34 degrees C), high day temperature (HDT; 34/22 degrees C) and control conditions (CONT; 22/22 degrees C). Grains were sampled periodically, and the time-course of grain growth was divided into rate and duration by logistic regression analysis. Endosperm cell numbers and cell sizes were analysed by digitalized hand-tracing images of endosperm cross-sections. KEY RESULTS: The duration of grain growth was reduced by high temperature both day and night. However, the rate of grain growth was lower in HNT than in HDT. The number of cells in endosperm cross-sections in HNT was similar to that in HDT, and higher than that in CONT. The average cell area was smaller in HNT than in either CONT or HDT. The differences in average cell areas between HNT and HDT were greater at distances 60-80 % from the central point of endosperm towards the endosperm surface. CONCLUSIONS: The results show that HNT compared with HDT reduced the final grain weight by a reduction in grain growth rate in the early or middle stages of grain filling, and also reduced cell size midway between the central point and the surface of endosperm.     

影响K型杂交小麦粒重潜在因素的遗传分析

1996-1997年采用5×7不完全双列杂交,对影响K型杂交小麦粒重的潜在因素进行分析。结果表明,K型杂交小麦粒重与胚乳细胞数和鲜籽烂一最大体积呈极显正相关。鲜籽粒最大体积对粒重贡献较大;胚乳细胞数的杂种优势较弱,鲜籽粒最大体积的杂种优势十分明显;这两个性状均符合加性-显性遗传模型。以加快效应为主,存在明显的母体效应。     

Hormones in the grains in relation to sink strength and postanthesis development of spikelets in rice

Inferior spikelets usually exhibit a slower grain filling rate and lower grain weight than superior spikelets in a rice (Oryza sativa L.) panicle. This study investigated whether the variations in grain filling between the two kinds of spikelets were attributed to their sink strength and whether the sink strength was regulated by the hormonal levels in the grains. Using two field-grown rice genotypes, the division rate of endosperm cells, hormonal levels in the grains, and grain weight of both superior and inferior spikelets were determined during the grain filling period. The results showed that superior spikelets had dominance over inferior spikelets in endosperm cell division rate and cell number, grain filling and grain weight. Changes in zeatin (Z) and zeatin riboside (ZR) contents paralleled and were very significantly correlated with the cell division rate and cell number. Cell division rate and the content of indole-3-acetic acid (IAA) in the grains were also significantly correlated. Gibberellin (GAs; GA₁+ GA₄) content of the grains was high but ABA levels were low at the early grain filling stage. ABA increased substantially during the linear phase of grain growth and was very significantly correlated with grain dry weight during this period. Application of kinetin at 2 through 6 days post anthesis (DPA) significantly increased cell number, while spraying ABA at 11 through 15 DPA significantly increased the grain filling rate. The results suggest that differences in sink strength are responsible for variations in grain filling between superior and inferior spikelets. Both cytokinins and IAA in the grains may mediate cell division in rice endosperm at early grain filling stages, and therefore regulate the sink size of the grain, whereas ABA content correlates with sink activity during the linear period of grain growth.     

玉米籽粒胚乳细胞增殖及其与淀粉充实的关系

用纤维素酶解离胚乳、滤膜法统计玉米胚乳细胞的数目,进一步借助Logistic方程模拟胚乳细胞增殖动态的结果表明,整个灌浆期间胚乳细胞增殖呈现“慢-快-慢”的变化趋势。授粉15d后,不同类型胚乳的细胞数目依序为普通玉米〉糯玉米〉甜玉米〉爆裂玉米;胚乳细胞数目主要取决于细胞的增殖速率,并与淀粉充实和粒重关系密切。胚乳发育前期以胚乳细胞增殖为主,后期以淀粉积累为主。     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

The movement of water in the dorsal region of the developing rice caryopsis was studied using solutions of the heavy metals lanthanum and uranium. In the electron microscope electron-opaque deposits were confined to the cell walls of the pigment strand indicating that this is the main route for the water which enters and leaves the caryopsis during grain filling. The pathway of assimilates into the developing caryopsis was examined using isolated caryopses which had taken up solutions of fluorescent dyes and also by autoradiography of caryopses which had transported ¹⁴C-labelled assimilates in vivo. The results show that assimilates unloaded from the phloem move through the pigment strand and circumferentially via cells of the nucellus before entering the endosperm. A scheme is presented for the interrelations of water and assimilate transport during grain filling     

The Number and Sizes of Starch Granules in the Wheat Endosperm, and their Association with Grain Weight

The number and size of starch granules was measured in developingand mature endosperms of two wheat varieties, Chinese Springand Spica. Chromosomal effects on particular aspects of starchgranule size and number were detected in the analysis of disomicgenotypes derived from reciprocal monosomic hybrids betweenthe two parental varieties. Among these genotypes, a greaterweight per endosperm cell was associated with a greater volumeper A-type starch granule. It is suggested that the number percell and volume of these large starch granules are the majordeterminants of endosperm cell weight, and there exists separategenetic control of these parameters. It should therefore bepossible genetically to combine these attributes to achievedirected increases in mature grain weight. Triticum aestivum, grain weight, starch granules     

Changes in Cell Number and Cell Division Activity during Endosperm Development in Allohexaploid Wheat, Triticum aestivum L.

Nuclear or cell number, and the mitotic index, were recordedin endosperms of Triticum aestivum cv. Mardler to test if aparticular stage of endosperm development was critical in determiningthe final grain weight. The basal four florets of emasculatedspikelets (controls), and the third and fourth florets of spikeletswhere the two basal ovaries were removed (ovary-removed), weresampled at various times up to 360 h after hand-pollination.The coenocytic phase in endosperms ended about 84 h after pollinationregardless of both grain position and the treatment. The onsetof the cellular stage was characterized by the final large fluctuationsin the mitotic index reflecting the culmination of the synchronousnuclear division of the coenocytic stage. Thereafter, the mitoticindex fluctuated with smaller amplitudes and, by 216 h afterpollination, was < 1%. Neither floret position in the spikeletnor the treatment affected the pattern of alteration to themitotic index. However, ovary removal from first and secondflorets resulted in significantly heavier grains and higherendosperm cell number in the 3rd and 4th florets compared withthe controls. In all florets, mean endosperm cell number peakedat 280 h but decreased by 360 h after pollination. At this time,the mean cell numbers in endosperms of the 3rd and 4th floretsof ovary-removed spikelets were significantly higher than inthe corresponding endosperms in the controls. Thus, a key contributoryfactor in determining the final endosperm cell number may bethe number of cells which are lost during the late period ofthe cellular stage of endosperm development. Key words: Endosperm cell number, florets, grain weight, mitotic index, Triticum aestivum     

Effect of Post-anthesis Drought on Cell Division and Starch Accumulation in Developing Wheat Grains

Wheat plants (Triticum aestivum L., cv. Warigal) were subjectedto 20 d of water deficit during the period of endosperm celldivision. Drought accentuated the differences in final grainweight between spikelets and between grains within spikelets.The distal grains of top spikelets were most affected by drought.The maximum number of endosperm cells was, respectively, 30and 40 per cent lower in basal grains and distal grains of draughtedplants. In basal grains of middle spikelets, the number of largestarch granules per cell was unaffected but the number of smallstarch granules per cell was 45 per cent lower in grains ofdraughted plants. The initiation of small starch granules wasmore affected than cell division because severe water deficitoccurred earlier during the former process than the latter.Final dry weight appeared to correlate well with the maximumnumber of endosperm cells, but depended also on the number ofstarch granules per cell. Consequently, the amount of dry matterper cell was not constant in both treatments. The concentration of sucrose per endosperm cell was lower onlyin the droughted distal grains of top spikelets. The supplyof sucrose to endosperm cells did not regulate the initiationof small starch granules. Triticum aestivum L., wheat, drought, grain growth, cell division, starch     

Factors influencing seed dormancy and germination in sand,and seedling survival under desiccation,of Psammochloa villosa (Poaceae), inhabiting the moving sand dunes of Ordos,China

Psammochloa villosa (Trin.) Bor. (Poaceae), is distributed primarily in moving sand dunes of the Ordos Plateau, China. Freshly harvested caryopses (seeds) are in non-deep physiological dormancy (non-deep PD). Germination is slow and low and only over a narrow temperature range. A treatment of four weeks cold stratification at 3 to 5 °C in darkness was required to break non-deep PD, allowing germination to reach high percentages at higher temperatures and without light requirement. Rate and percentages of germination were increased by scarifying the caryopsis coat and by artificial removal of different proportions of the endosperm. However, seedling dry weight and increases in root and shoot lengths, were significantly influenced by the proportion of the endosperm that remained on the caryopses. Higher percentages of seedling emergence were obtained from the shallowly buried caryopses, ranging from 0.5–2.0 cm, and the depth of the caryopses in the sand affected the time of germination. The deeper the caryopses were buried, the more that remained ungerminated and in enforced dormancy. The caryopses germinated when the upper sand layer was removed and buried caryopses were at a suitable sand depth for germination, or when the sand was aerated. In natural habitats, germinated seedlings may be wholly exposed to the air by sand erosion and thus be exposed to drought stress. However, young seedlings have the ability to tolerate desiccation and to recover after rehydration. Root length at the `point of no return' is 4 mm.     

Effects of partial endosperm removal on embryo dormancy breaking and salt tolerance of <Emphasis Type="Italic">Hordeum spontaneum</Emphasis> seeds

Seed damage is a common phenomenon in nature and in agricultural production. In this experiment, partial endosperm removal from wild barley (Hordeum spontaneum) caryopses, sampled from three ecotypes originated from xeric environments in Israel, was conducted. The aim was to examine seed dormancy and germination states in damaged caryopses and salt tolerance of young seedlings derived from them. Six treatments were made: (1) control seeds with intact caryopses; (2–4) removal of 0.25, 0.5, and 0.75 of the length of intact caryopses; (5) transection at the points, at which the endosperm and embryo meet; and (6) slitting of endosperm opposite the embryo. A significant negative correlation was found between germination percentage (dormancy release) and the relative distance from the dissection point to embryo. Partial removal of the endosperm could accelerate dormancy release. Seedling salt tolerance was assessed by the ratio of root or coleoptile length in a seedling grown in 100 or 200 mM NaCl solution to that of a seedling grown in water. The seedling salt tolerance was positively correlated with the removed portion of the seed endosperm. For each level of endosperm removal, the salt tolerance to 200 mM NaCl of the seedlings derived from the Dead Sea ecotype was higher than those from both the Sede Boker and the Mehola ecotypes. The results suggest that partial damage to seed endosperms in natural conditions may play a role in increasing the phenotypic plasticity of germination and salt tolerance.     

Studies on the early development of zygotic and synergid embryo and endosperm in polyembryonic rice ApIII

A morphological, cytological and embryological investigation was conducted on the early development of embryo and endosperm in polyembryonic rice (Oryza sativa L.) ApIII. We found that the percentage of single-, twin- and triple-seedlings from mature caryopses was 82.4–85.6%, 11.3–14.6% and 2.0–3.8%, respectively. From twin-seedlings one of the plants and in the triple-seedlings usually two seedlings were very weak having a mortality of about 50–60% and over 95%, respectively, and most of the dead seedlings were haploid. In addition, among 674 young caryopses the percentage of single-, double- and triple-embryo sacs was 77.60%, 18.10% and 4.30%, respectively. The development of both embryo and endosperm was observed in the embryo sacs of 653 young caryopses, while embryo development occurred in absence of endosperm in the embryo sacs of 21 ones. Furthermore, after pollen tube penetration, frequently one, rarely two, additional embryos arise via apogamy from the typical specialized synergid of normal egg apparatus with a collapsed cap-neck region that had a damaged filiform apparatus at the micropylar end. Finally, the formation of a strikingly degenerated endosperm cell region is a specialized functional structure that serves for the active transport of metabolites from endosperm to the developing embryo. Taken together, these results suggest that synergid apogamy seems to be the unique reason of polyembryony, poor growth and high mortality of many seedlings may be due to haploidy, and high genetic stability of polyembryony occurs in rice ApIII.     

Sucrose transport into barley seeds: molecular characterization of two transporters and implications for seed development and starch accumulation

In order to understand sucrose transport in developing seeds of cereals at the molecular level, we cloned from a caryopses library two cDNAs encoding sucrose transporters, designated HvSUT1 and HvSUT2. Sucrose uptake activity was confirmed by heterologous expression in yeast. Both transporter genes are expressed in maternal as well as filial tissues. In a series of in situ hybridizations we analysed the cell type-specific expression in developing seeds. HvSUT1 is preferentially expressed in caryopses in the cells of the nucellar projection and the endospermal transfer layer, which represent the sites of sucrose exchange between the maternal and the filial generation and are characterized by transfer cell formation. HvSUT2 is expressed in all sink and source tissues analysed and may have a general housekeeping role. The rapid induction of HvSUT1 gene expression in caryopses at approximately 5-6 days after fertilization coincides with increasing levels of sucrose as well as sucrose synthase mRNA and activity, and occurs immediately before the onset of rapid starch accumulation within the endosperm. Starch biosynthesis requires sucrose to be imported into the endosperm, as direct precursor for starch synthesis and to promote storage-associated processes. We discuss the possible role of HvSUT1 as a control element for the endospermal sucrose concentration.     

水稻淀粉胚乳程序性细胞死亡中的去核化

对水稻品种中籼8836淀粉胚乳细胞的去核化发育阶段的细胞超微结构变化和同期籽粒灌浆速率及相关酶活性的动态进行了观察和分析。开花受精后约在第3天胚乳完成细胞化,花后第5天少数淀粉胚乳细胞启动去核发育过程。核消亡是淀粉胚乳细胞程序性细胞死亡(PCD)的第一步。同一籽粒淀粉胚乳细胞的去核进程是不同步的。花后第13天所有淀粉胚乳细胞都已完成去核过程。在去核过程中,胚乳核的形态变化特征既有动植物PCD的共性又有其特殊性。伴随核降解过程,一部分线粒体解体,表明去核化与线粒体解体有一定联系。在去核化发育阶段,与PCD有关的酶类,如超氧化物歧化酶(SOD)、过氧化氢酶(CAT)活性非常高;与淀粉合成有关的酶类,如ADPG焦磷酸化酶、可溶性淀粉合成酶(SSS酶)、淀粉分支酶(或Q酶)也表现出很高的活性。去核化发育阶段籽粒灌浆速率最高,籽粒增重亦最快。淀粉胚乳细胞去核之后,细胞并未立即死亡,这些无核的细胞仍维持正常有序的代谢活动,继续进行淀粉和贮藏蛋白的合成与积累,但上述酶类的活性明显降低,灌浆速率也明显趋缓。淀粉胚乳细胞最终被贮藏物质充满时成为死细胞,完成其程序性死亡过程。Evan‘s blue染色鉴定表明淀粉胚乳细胞死亡不同步,细胞死亡在淀粉胚乳组织中是随机发生的。     

水稻淀粉胚乳程序性细胞死亡中的去核化

对水稻品种中籼8836淀粉胚乳细胞的去核化发育阶段的细胞超微结构变化和同期籽粒灌浆速率及相关酶活性的动态进行了观察和分析。开花受精后约在第3天胚乳完成细胞化,花后第5天少数淀粉胚乳细胞启动去核发育过程。核消亡是淀粉胚乳细胞程序性细胞死亡(PCD)的第一步。同一籽粒淀粉胚乳细胞的去核进程是不同步的。花后第13天所有淀粉胚乳细胞都已完成去核过程。在去核过程中,胚乳核的形态变化特征既有动植物PCD的共性又有其特殊性。伴随核降解过程,一部分线粒体解体,表明去核化与线粒体解体有一定联系。在去核化发育阶段,与PCD有关的酶类,如超氧化物歧化酶(SOD)过氧化氢酶(CAT)活性非常高;与淀粉合成有关的酶类,如ADPG焦磷酸化酶、可溶性淀粉合成酶(SSS酶)、淀粉分支酶(或Q酶)也表现出很高的活性。去核化发育阶段籽粒灌浆速率最高,籽粒增重亦最快。淀粉胚乳细胞去核之后,细胞并未立即死亡,这些无核的细胞仍维持正常有序的代谢活动,继续进行淀粉和贮藏蛋白的合成与积累,但上述酶类的活性明显降低,灌浆速率也明显趋缓。淀粉胚乳细胞最终被贮藏物质充满时成为死细胞,完成其程序性死亡过程。Evan’s blue染色鉴定表明淀粉胚乳细胞死亡不同步,细胞死亡在淀粉胚乳组织中是随机发生的。