Effects of Drought on Partitioning of Nitrogen in Two Wheat Varieties Differing in Drought-tolerance

A model was constructed to describe the translocation and partitioningof nitrogen on the seventh day after anthesis for well-wateredand droughted plants of two wheat varieties (Triticum aestivumL. cv. Warigal and Condor). The glasshouse-grown plants weredetillered so that a simplified model could be derived for themain stem. A 9-d drought treatment was imposed just after anthesisand this coincided with the period of endosperm cell divisionin the grains. Warigal, which had a higher grain yield thanCondor under drought, absorbed up to 15-times more nitrogenand translocated 1.5-fold more nitrogen to the shoot via thexylem. In both varieties, nitrogen redistributed from vegetativeorgans accounted for more than 60 per cent in control and 70per cent in droughted plants of the nitrogen needed for eargrowth. The net loss of nitrogen increased by 4-3 per cent inthe leaves, but decreased by 60 per cent in the stem under drought.Stem and roots appeared to play an important role in the nitrogeneconomy of droughted plants: less nitrogen was translocateddirectly to the grains from the senescing leaves and 40–60per cent more nitrogen was translocated to the roots. Nearlyall the nitrogen reaching the roots in the phloem was reloadedinto the xylem stream and translocated back to the shoot. Thetransfer of nitrogen through the stem was reduced under droughtand this resulted in a constant C:N ratio of the grains whichmay be important in the regulation of endosperm cell division. Triticum aestivum L., wheat, drought, nitrogen, senescence, translocation     

Regulation of Grain Weight by Supply of Assimilates and Starch Granule Development in Three Winter Wheat Varieties

On removing the top half of the ear (halving) on several datesafter anthesis, dry weight per grain increased in three winterwheat (Triticum aestivum L.) varieties, in two pot experiments;the increase was greater with early than with late halving.The variety Splendeur had a lower dry weight and water percentagein grains than either Hobbit or Maris Huntsman. The ratio ofthe green area integrated over the post-anthesis period to thenumber of grains per ear (green area per grain) was highestin Splendeur and lowest in Hobbit in the first experiment; inthe second, Splendeur gave a lower ratio than the other twovarieties, which showed similar values. The green area per grainwas greater the earlier the ear was halved. The number of A-typestarch granules per endosperm, but not the volume per A granule(modal volume) and the modal volume of B starch granules, butnot their number, increased to a greater extent with early thanwith late halving. In Splendeur the grains had fewer A starchgranules, although these were of greater modal volume than inHobbit and Maris Huntsman and a number of B starch granulessimilar to the other two varieties, but of smaller modal volume.Maris Huntsman had more A granules than Hobbit, but with smallermodal volume. Dry weight per grain increased linearly with thenumber of A starch granules per endosperm, which in turn increasedasymptotically with green area per grain. The regressions forthe three varieties differed significantly. The influence ofthe supply of assimilates and the capacity for starch granuleformation in the regulation of grain weight is discussed. Key words: Grain weight, starch granules, assimilate supply, variety, wheat     

Differences of Starch Granule Distribution in Grains from Different Spikelet Positions in Winter Wheat

Wheat starch development is a complex process and is markedly difference by changes in spikelet spatial position. The present study deals with endosperm starch granule distribution and spatial position during filling development. The study was conducted with pure starch isolated from wheat (Triticum aestivum L.), Jimai20 and Shannong1391, at 7–35 days after anthesis (DAA). The results showed that grain number, spikelet weight and grain weight per spikelet in different spatial position showed parabolic changes. Upper spikelets had highest starch and amylose content followed by basal spikelets, then middle spikelets. The paper also suggested the volume percents of B-type and A-type granule in grain of middle spikelets were remarkably higher and lower than those of basal and upper spikelets, respectively. However, no significant difference occurred in the number percents of the two type granule. The ratio of amylase to amylopectin was positively correlated with the volume proportion of 22.8–42.8 µm, but was negatively related to the volume proportion of <9.9 µm. The results indicated that the formation and distribution of starch granules were affected significantly by spikelet position, and grains at upper and basal spikelet had the potential of increasing grain weight through increasing the volume of B-type 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     

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     

The Developing Endosperm of Wheat -- A Stereological Analysis

A stereological analysis of light and electron micrographs ofwheat endosperm during grain formation and development providesinformation on a range of parameters of cell structure. Thecell volume increases approximately ten-fold; mitochondrialnumber per cell increases but the individual mitochondrial volumedecreases, the overall volume fraction of the cell occupiedby mitochondria remaining fairly constant. Amyloplast divisionstops before cell division, resulting in the distribution ofpreviously-formed plastids; there are differences in starchgranule growth rate in different cell layers of the endosperm.The rough endoplasmic reticulum increases four-fold in areaper cell and its surface-to-volume ratio increases just priorto protein deposition. Triticum aestivum, L., wheat, endosperm, seed development, starch, stereology     

Effect of Drought on Metabolism and Partitioning of Carbon in Two Wheat Varieties Differing in Drought-tolerance

A model was constructed to describe the partitioning of carbonon the third and seventh day from anthesis for well-wateredand droughted plants of two wheat varieties (Triticum aestivumL. cv. Warigal and Condor). The glasshouse-grown plants weredetillered so that a simplified model could be derived for themain stem. The 9-d drought treatment, imposed just after anthesisduring the period of cell division in the grains, reduced grainyield by 18 per cent in Warigal and 30 per cent in Condor. Netcarbon fixation was up to 60 per cent higher in Warigal thanCondor towards the end of the drought period and this correlatedwith better osmotic adjustment in the flag leaf. Carbon partitioningbetween plant organs responded to water deficit more rapidlythan net carbon fixation. On day 3, carbon allocation to theroots of droughted plants was maintained in Condor and increasedby 14 per cent in Warigal, whereas carbon allocation to theear decreased in both varieties. However the roots did not competewell with the ear when the water deficit became more severe.Warigal accumulated 3 times more stem reserves than Condor underdrought. In the roots, the pattern of carbon allocation betweenrespiration and carbon accumulation changed soon after impositionof drought. Although total root respiration decreased underdrought it became more energy efficient, particularly for Warigal,as less respiration took place via the alternative pathway.On day 3, the larger carbon allocation to the roots and thelower root respiration accounted for the 4-times larger sugaraccumulation in droughted roots of Warigal compared with thoseof Condor. Osmotic adjustment in mature leaves and roots maybe of importance for the maintenance of vital processes andfor recovery after drought. Triticum aestivum L., wheat, drought, carbon, partitioning     

Embryo and endosperm development in wheat (Triticum aestivum L.) kernels subjected to drought stress

The aim of the present work was to reveal the histological alterations triggered in developing wheat kernels by soil drought stress during early seed development resulting in yield losses at harvest. For this purpose, observations were made on the effect of drought stress, applied in a controlled environment from the 5th to the 9th day after pollination, on the kernel morphology, starch content and grain yield of the drought-sensitive Cappelle Desprez and drought-tolerant Plainsman V winter wheat (Triticum aestivum L.) varieties. As a consequence of water withdrawal, there was a decrease in the size of the embryos and the number of A-type starch granules deposited in the endosperm, while the development of aleurone cells and the degradation of the cell layers surrounding the ovule were significantly accelerated in both genotypes. In addition, the number of B-type starch granules per cell was significantly reduced. Drought stress affected the rate of grain filling shortened the grain-filling and ripening period and severely reduced the yield. With respect to the recovery of vegetative tissues, seed set and yield, the drought-tolerant Plainsman V responded significantly better to drought stress than Cappelle Desprez. The reduction in the size of the mature embryos was significantly greater in the sensitive genotype. Compared to Plainsman V, the endosperm cells of Cappelle Desprez accumulated significantly fewer B-type starch granules. In stressed kernels of the tolerant genotype, the accumulation of protein bodies occurred significantly earlier than in the sensitive variety.     

Seed Development in a Shrunken Endosperm Barley Mutant

Seg8 is one of eight barley (Hordeum vulgare L.) mutants whoseendosperm development is affected by the maternal plant genotype.This study was initiated to determine the nature and onset ofabnormal development to provide a basis for further studiesaimed at understanding the mechanism of genetic control. Seeddevelopment and synthesis and accumulation of reserve substanceswere compared between seg8 and its normal counterpart, cv. Klages.Light microscopic examination showed that the mutant phenotypeappeared as early as 4 d after anthesis (DAA), and seg8 graindry weight was significantly lower than cv. Klages by 8 DAA.Grain cell number was significantly lower in seg8 by 8 DAA,indicating an early termination of cell division. The mutanthad a lower starch concentration and higher sucrose concentration,also evident at 8 DAA. Rates of [¹⁴C]sucrose incorporation intostarch in excised half seeds were similar in both genotypesat 2 and 4 DAA, but at 8 and 12 DAA seg8 had a lower rate. Totalprotein concentration was not significantly different betweenthe two genotypes throughout endosperm development. These resultsindicate that the mutation affects cell division and starchaccumulation prior to 8 DAA. It is not known if the reductionin starch biosynthesis and accumulation results from a reducedcapacity for starch or a defect in starch biosynthesis. Hordeum vulgare L., barley, shrunken endosperm mutant, endosperm development, starch, protein, endosperm cell number     

Interactions Among Number of Spikelets, Number of Grains and Grain Weight in the Spikes of Wheat (Triticum aestivum L.)

In a field experiment, comprising four spring wheat cultivars,the frequency and final weight of the grains developing fromeach individual floret were determined in intact spikes andin spikes of which up to nine spikelets had been removed. Theextent of damage caused by the cutting procedure was estimated. Characteristic distributions of the frequencies and weightsof the individual grains were found for each cultivar. Removalof spikelets resulted, in most cases, in a small increase inthe number of grains and in a considerable increase in the weightof the grains of the remaining spikelets. These increases compensatedonly partially, and differently in the different cultivars,for the loss of the removed spikelets. Defoliation at the timeof earing caused a subsequent reduction in grain yield of intactspikes but no reduction in the yield of spikes from which ninespikelets had been removed. The removal of the upper floretsin each spikelet resulted in a certain increase in the weightof the two basal grains. It is concluded that an increase in the number of spikeletsper spike may reduce grain weight but will nevertheless contributeto yield. The number of grains per spikelet is cultivar dependentbut not causally associated with grain weight. Grain set indistal florets is expected to add rather small grains to thespike's yield. Under conditions of limited supplies it may causea reduction in the weight of the basal grains. Any increasein grain weight is anticipated to contribute to grain yieldand is not liable to affect spikelets per spike or grains perspikelet. Wheat cultivars, Triticum aestivum, growth of inflorescence, grain yield, spikelet number     

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.     

Sucrose synthase activity in developing wheat endosperms differing in maximum weight

Past research on kernel growth in wheat (Triticum aestivum) has shown that the kernel itself largely regulates the influx of sucrose for consequent starch synthesis in the endosperm of the grain. The first step in the conversion of sucrose to starch is catalyzed by sucrose synthase (EC 2.4.13). Sucrose synthase activity was assayed in developing endosperms from kernels differing in growth rate and in maximum dry weight accumulation. From 10 to 22 days after anthesis, sucrose synthase activity per wheat endosperm remained constant with respect to time in all grains. However, kernels which had higher rates of kernel growth and which achieved greatest maximum weight had consistently and significantly higher sucrose synthase activities at any point in time than did kernels with slower rates of dry matter accumulation and lower maximum weight. In addition, larger kernels had a significantly greater amount of water in which this activity could be expressed. Although the results do not implicate sucrose synthase as the “rate limiting” enzyme in wheat kernel growth, they do emphasize the importance of sucrose synthase activity in larger or more rapidly growing kernels, as compared to smaller slower growing kernels.     

The Structure of Mature Rye Endosperm

Endosperm tissue of mature kernels of rye (Secale cereale L.)cv. Dominant was examined by light and transmission electronmicroscopy. It was found that storage protein in sub-aleuronecells occupies up to 35 per cent of the cell volume and formsa continuous matrix in which starch grains and cytoplasmic remnantsare embedded. In the prismatic endosperm, the storage proteinis present as a fine network interspersed between the numeroustype A and B starch grains. Protein bodies are not found inthe prismatic endosperm; only a few, less than 1 µm indiameter, are observed in pockets of disorganized cytoplasmin the sub-aleurone tissue. Thick cell walls and intercellularmaterial may contribute to the high pentosan content of ryeendosperm. Secale cereale L., rye, endosperm, protein matrix, ultrastructure     

Transport of Recently Assimilated15N Nitrogen to Individual Spikelets in Spring Wheat Grown in Culture Solution

Two cultivars of spring wheat ( Triticum aestivum L.), Sport(high protein) and WL4 (low protein), were grown to maturityin culture solution. Nitrogen in the form of nitrate was addedin daily doses at stepwise-decreasing relative rates to ensurenormal development, and both cultivars received the same totalamount of N during development. At weekly intervals from anthesisto maturity the daily nitrate dose was, for selected groupsof plants, labelled with¹⁵N. After the labelling period theselected plants were harvested and analysed. The cultivar WL4produced more biomass than Sport, as well as more spikeletsand more grains per ear, with a higher mean grain weight, suchthat grain yield of WL4 was 57% greater than Sport. The earsof both cultivars were heterogenous: mean grain weight was highestin middle spikelets, which also contained more grains; the Ncontent followed the pattern of dry weight with more N in themiddle spikelets; but the N concentration was practically thesame in all spikelets (2.15% of d. wt in WL4 and 3.33% in Sport).The distribution of¹⁵N showed that the main stem ear maturedmuch earlier than tiller ears. The results of this nitrogen-labellingexperiment show that, late in development, substantial amountsof recently-absorbed N were immediately assimilated and transportedto the ears. Transport of¹⁵N decreased earlier to the top spikeletsthan to the bottom spikelets. As both cultivars were grown underidentical conditions and both received the same amount of Nit was concluded that the difference in grain N concentrationwas not caused by differences in the capacity of N assimilationand translocation but rather by different rates of accumulationof non-nitrogenous dry matter in the grains. Ear; grain; nitrate; nitrogen transport; Triticum aestivum L.; yield     

Relationship Between Photosynthate Supply and Endosperm Development in Maize

Maize (Zea mays L., cultivar Pioneer 3925) plants were givenshaded, thinned and control light treatments during 10 d or20 d periods surrounding pollination. Glucose, sucrose, starch,and dry matter (DM) contents were measured at intervals in compositesamples of pericarp/nucellus (PN), and in endosperms taken fromdeveloping kernels. Total kernel DM per ear at maturity washigher in the thinned treatment than control and shaded treatmentsdue to higher kernel set in apical regions of ears. In PNs at11 d after pollination (DAP), DM and sucrose contents were slightlygreater in thinned than control and shaded plants. Glucose contentswere substantially greater than controls in PNs of thinned plantsand were less than controls in shaded plants. In endospermsfrom apical kernels at 8 to 12 DAP (during cell division), DM,glucose and sucrose contents were substantially less in shadedthan control and thinned plants. Sucrose contents were greaterin endosperms of thinned than control plants. Sugar contentsin endosperms from basal kernels were nearly the same in thethree light treatments. At 12 DAP, apical and basal endospermsin shaded plants had fewer nuclei than those of the other lighttreatments. The light treatments appeared to effect apical kernelgrowth by influencing the extent of cell division. Zea mays L, maize, light treatment, endosperm, cell division, glucose, sucrose, starch     

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.     

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     

Influence of Vernalization and Photoperiod on Supernumerary Spikelet Expression in Wheat

Two tetraploid (Triticum turgidum L.emend gr. turgidum and gr.durum) and five hexaploid wheats (Triticum x aestivum L. emendgr. aestivum) with reported tendencies for ‘branched heads’(supernurnerary spikelets) exhibited variation in its expressionunder different vernalization photoperiod and temperature regimes. Two main types of supernumerary spikelets were identified, multiplesessile spikelets (MSS) with two or more complete spikeletsat a rachis node and indeterminate rachilla spikelets (IRS)with two to 13 spikelets on an extended rachilla. The degree of supernumerary spikelet expression in wheats withvernalization response differed from those without. Short photoperiods(9–14 h) both outdoors and in a glasshouse environment,were more conducive to supernumerary spikelet expression than24 h photoperiod in both environments. The 24 h photoperiodglasshouse environment (higher mean temperatures) was leastconducive to its expression except in lines with a strong vernalizationresponse. The high stability of supernumerary spikelet expression in certaingenotypes in the different environments indicated the feasibilityof incorporating this character in breeding and selecting commercialwheats to increase grain number per head. Triticum, wheat, ear-branching, supernumerary spikelets, vernalization, photoperiod, temperature     

Floret Initiation and Development in Spring Wheat (Triticum aestivum L.)

The number and developmental stages of florets were determinedin each spikelet of the spike in the main shoots of spring wheat.Samples were taken frequently from plants grown in a phytotronand in a nitrogen application field-test. Ten stages of development,from floret initiation until anthesis, were recognized and described. Inter-spikelet variation in the total number of initiated floretswas rather small. However, the number of florets at advancedstages of development, as well as the number of grains, washighest in the central spikelets in which florets initiatedfirst. Floret initiation did not proceed beyond spike emergence,whereafter the distal florets and the spikelet apex degenerated.Grain-set was restricted to florets which had developed at leastto the stage of visible anther lobes at spike emergence. Thenumber of these florets was increased significantly by nitrogenapplication. Wheat, Triticum aestivum L., spikelet, floret, grain set, nitrogen