| This paper was conducted to study the growth and development characteristics ofsuperior and inferior wheat caryopsis and response to microorganism-sourced ABA. Thegrain-filling processes of grains at different spikelet and grain positions were matched byRichards equation. The endosperm development was studied by semithin section andmaceration, including endosperm genesis, cell differentiation, cell proliferation and cell size.The number, volume and surface area distribution of starch granules in mature caryopsis, aswell as the changes of starch granule number during endosperm development, were studiedusing wheat cultivar Jimai20, Shannong20and Zimai1grown in the field. The main resultsare as follows:(1) Weight of grains at different spikelet and grain positions were different. Superiorgrains (the first and the second grains) entered filling stage earlier than inferior grains (thethird and the fourth grains), and had faster grouting rates, therefore, they had heavier grainweight. Grains at middle and upper spikelets started filling earlier. Grains from lowerspikelets had faster grouting rates in late filling stage. Final grain weight had a trend ofmiddle> lower> upper at different spikelet positions. The variation trend of grain weight,grain length, grain width, hormone content and grain-filling rate are all fit quadratic curve,and the middle spikelets have significant advantage. The upper and top spikelets have greatweight potential.(2) The ovary wall formed large numbers of starch granules before the start ofendosperm development. Number and size of these starch granules were different at differentposition in ovary wall. Starch granule size and number increased gradually from surface tomiddle of the ovary wall. Starch granules in the cells near the integument became small insize and more in amount, and distributed close to cell wall. These starch granules diapered inthe process of endosperm development. These starch granules were likely to be the temporarystore form of starch, and could break down into soluble sugar to transport into endospermtissue. These starch granules might be important to the endosperm tissue before the abdominalvascular bundle was fully developed. Temporary starch granules in ovary wall cell of inferior grains were less than in superior grains, and were slowly to degrade. As a result, endospermtissue of inferior grains could not get enough nutrients at free nuclear stage and early cellularperiod. Accumulation of starch granules in ovary wall may be one of the sink limiting factorsto inferior grains.(3) The results showed that the development of wheat grain endosperm cells includedendosperm cell proliferating and filling. For inferior grains, delayed starting time, lowproliferation rate and short proliferation time, led to less endosperm cell number. Slowendosperm cell proliferation rate and less cell number became one of the limiting factors ofgrain weight. Microorganism-sourced ABA could change grain endogenous hormone levels.At7DAA, CTK content was increased in inferior grains after treated at5-7DAA bymicroorganism-sourced ABA, which of IAA decreased. The produced more s, and got grainweight increased. Microorganism-sourced ABA can also decrease endosperm cell number ofsuperior grains.(4) The development of abdominal vascular bundle in inferior grains was about2-3dayslater than in superior grains. Inferior grains had about32-36sieve tubes in fully developedvascular bundle, while the superior grains had35-40. This might influence the assimilatetranslocation ability in inferior grains. Developments of aleurone transfer cells, nucellarprojection transfer cells and starchy endosperm transfer cells were all delayed, and functionalperiod of these cells was short in inferior grains. Functions of transfer cells become limitingfactors in the pathways of assimilation enter into endosperm cells after unloaded from phloem.Development degree of abdominal vascular bundle and quantity and quality of transfer cellsformed the limiting factors to flow between the endosperm sink and the source in inferiorgrains.(5) The development of starch granules were closely related with grain and spikeletpositions. At the same spikelet position, starch grains formed4-5dearlier in superior grainsthan in inferior grains. At the same grain position, starch granules developed first in the grainsof the middle spikelets, and then in upper and lower spikelets. The amount of starch granuleswas highest in the grains in middle spikelets all the time. The lower spikelet grains had theleast starch granules at the beginning of grain filling, however, the number of starch granulesincreased gradually and surpassed the upper spikelet grains as caryopsis matured. Correlationcoefficients between grain weight and starch granule amount became more significant withgrain filling. Correlation coefficients between grain weight and large starch granule amountwere higher than0.96**, and had no difference among different spikelet positions.Correlation coefficients between grain weight and small starch granule amount were higher than0.93**, and had a trend of upper> lower> middle in different spikelet positions. Thetrend of size distribution was BL-type> BS-type> A-type. There were more BS-type granulesin superior grains than in inferior grains. The number of BS-type granules increased with thespikelet position rising, whereas the number of BL-type granules showed a reverse trend. Thesize distribution of starch granules had similar changes to volume and surface area. Theformation and size distribution of starch granules were affected significantly by grain positionand spikelet position, and inferior grains had the potential of increasing grain weight throughincreasing the quantity of starch granules.(6) Under microorganism-sourced ABA spraying treatment, the senescence of flag leaveswas retard and functional period was prolonged, and the supply of photosynthate increased. Inother words，microorganism-sourced ABA increased the photosynthesis source. Endospermcells number was significantly increased in inferior grains, which magnified the sink capacity.Therefore, the grain weight of inferior grains was significantly increased. Grain weight of thefourth grains increased most significantly. Among different spikelet positions, grain weight ofupper and top spikelets increased most significantly, showing upper and top spikelets havegreat potentialities in weight.(7) Half of the time from flowering to maturity can define as "half-maturity stage",which is the turning point in grain filling. At the half-maturity stage of superior grains,grain-filling rate, caryopsis length, endosperm cell number, and hormone content all reachedmaximum. The division of endosperm cells (including aleurone cells) ceased, the cell volumeincreased rapidly, and accumulation of starch became rapid. Starch granule in the aleuronecells disappeared. Aleurone cells formed massive proteins, lipids and mineral. The smallamyloplasts began to form. The abdominal vascular bundle was fully developed. The starch inovary wall cells exhausted. Endosperm transfer cells developed. Photosynthetic ability of flagleaves decreased. Affected by internal and external factors, the actual "half-maturity stage" ofinferior grains came earlier than the theoretical time, and the filling process could not finishproperly. |
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