Effects of Nitrogen Fertilizer on the Distribution of Photosynthate during Grain Growth of Spring Wheat

The distribution of photosynthate labelled with ¹⁴C was studiedin spring wheat grown with different amounts of nitrogen fertilizerin the three years 1972–4, after exposing the flag leafor the leaf below the flag leaf to ¹⁴CO₂ at 6–10 or 19–26days after anthesis. The movement of ¹⁴C to ears was unaffectedby nitrogen fertilizer except after early exposure in 1973,when nitrogen increased the retention of ¹⁴C in stems at maturity The concentration of sugar in the top part of the shoot at theend of the day was unaffected by nitrogen in 1973, but at 22days after anthesis in 1974 the concentration of sucrose inthe glumes and rachis, and in the flag leaf lamina was increasedby nitrogen. Loss of sugar by translocation and respirationduring the night may explain why this increase in concentrationwas not reflected in the ¹⁴C distribution 24 h after supplying¹⁴C. The proportion of the total ¹⁴C content of the shoot that wasin the ear at maturity ranged from 68 to 95 per cent dependingon when and to which leaf the ¹⁴CO₂ was supplied. Less than5 per cent remained in the leaf exposed to ¹⁴CO₂. The proportionof the final ear weight contributed by the leaf below the flagleaf was about half that contributed by the flag leaf. In 1974 about 24 per cent of the ¹⁴C absorbed by the flag leaf,and 56 per cent of that absorbed by the second leaf, was lostby maturity, presumably by respiration. Most loss occurred inthe first 24 h.     

Pathway of Photosynthate Transfer in the Developing Seed of Vicia faba L. Transfer in Relation to Seed Anatomy

The seed coat vascular system of the developing seed of Viciafaba consists of a chalazal and two lateral veins. The veinsare embedded in parenchymatous tissue which lies beneath thehypodermis and is divided into chlorenchyma, ground parenchymaand thin-walled parenchyma. The thin-walled parenchyma cellsand, in old seed coats, the vascular parenchyma of the veinsundergo additional secondary wall development to form transfercells. Thus, transfer cells line the entire inner surface ofthe seed coat. Initial distribution of ¹⁴C-photosynthates andsodium fluorescein within the seed coat was in the vascularsystem. Subsequent transfer towards the embryo was either radiallythrough vascular parenchyma and thin-walled parenchyma to thin-walledparenchyma/transfer cells, or by lateral spread within the groundand thin-walled parenchyma/transfer cells of the non-vascularregion of the seed coat prior to radial transfer. One-thirdof the ¹⁴C-photosynthate delivered to the enclosed embryo wasestimated to be transferred via the non-vascular region of theseed coat. The cotyledons consist of a single-layered epidermisenclosing storage parenchyma in which a differentiating reticulatevascular system is embedded. Epidermal cells juxtaposed to theseed coat develop wall ingrowths characteristic of transfercells. Initial distribution of ¹⁴C-photosynthate within thecotyledons reflected the unequal delivery to the seed apoplastfrom the vascular and non-vascular regions of the seed coat.Subsequent even distribution of photosynthate within the cotyledonspossibly occurred by transfer within their vascular system. Key words: Cellular pathway, photosynthate transfer, seed anatomy, transfer cell     

Limitations to the Accumulation of Starch in the Developing Wheat Grain

The postulate that photosynthetic capacity limits the rate ofaccumulation of starch and dry matter in the developing wheatgrain has been tested by defoliating plants growing in the field,and by culturing detached ears on solutions of sucrose. In mostcases complete defoliation did not reduce the amounts of starchor dry matter produced 4 to 10 days later, or affect the amountsof sucrose in the grain. Increasing the potential supply ofsucrose above normal levels by culturing did not lead to increasesin either the deposition of starch or the quantity of sucrosein the grain. Removing leaves caused small depressions in thegain in fresh weight of the grain, and where defoliation didreduce the accumulation of starch there was no comparable effecton the amount of sucrose in the grain. Accordingly, it is concludedthat the postulate stated above is invalid. The case in support of the concept that it is the mechanismregulating the passage of sucrose into the grain which imposesa limitation on the accumulation of starch is presented.     

Pathway of Photosynthate Transfer in the Developing Seed of Vicia faba L: A Structural Assessment of the Role of Transfer Cells in Unloading from the Seed Coat

Photosynthate movement within the coat of the developing seedof Vicia faba occurs radially inward from the restricted vascularsystem and laterally through the non-vascularized region ofthe seed coat prior to exchange to the seed apoplast. Thin-walledparenchyma/transfer cells line the entire inner surface of theseed coat and thus are located at the terminus of the photosynthatetransfer pathway. The principal cellular route of transfer withinthe seed coat and the role of the thin-walled parenchyma/transfercells in membrane exchange to the seed apoplast has been investigated.Sucrose fluxes, computed from estimates of the plasma membranesurface areas of the cell types of the pathway, the plasmodesmatalcross-sectional areas interconnecting contiguous cells and theobserved rate of sucrose delivery to the embryo indicate thatsieve element unloading and subsequent transfer to the thin-walledparenchyma/transfer cells is through the symplast. For the cellsof the ground tissue, plasmodesmatal density is consistentlyhigher on their anticlinal walls. This observation supportsthe reported pattern of lateral transfer through these tissuesin the non-vascular regions of the seed coat. Wall ingrowthsare initiated sequentially in the thin-walled parenchyma cellsto maintain 1–3 rows of thin-walled parenchyma/transfercells. The development of these wall ingrowths results in a58% increase in the plasma membrane surface area of these cellsand provides them with the capacity to act as the principalcellular site for membrane exchange of sucrose to the seed apoplast.This cellular route of symplastic transfer from the sieve elementsto the ground tissues where membrane exchange to the seed apoplastoccurs is consistent with that reported for Phaseolus vulgaris Key words: Cellular pathway, photosynthate transfer, transfer cell, Vicia seed coat     

Factors Limiting the Supply of Sucrose to the Developing Wheat Grain

The level of sucrose in the flag leaf blade of wheat plantsgrowing in the field fluctuates in a conspicuous diurnal pattern,although in the endosperm the concentration of sucrose remainsunaltered throughout the night and the day. The effects of defoliationon the quantity of sucrose in the grain are relatively small,and appear to depend on the length of the period between treatmentand observation. In ears cultured on solutions of sucrose, the sugar accumulatesto higher concentrations than normal in all parts of the earexcept the grain where it remains at the concentration foundin intact plants. It is concluded that the magnitude of photosynthesis is adequateto maintain at a maximum the flow of sucrose into the grain,and that the flow of sucrose is limited by the capacity of theprocess(es) transporting the sugar on the final stages of itspassage into the grain.     

Embryoless Wheat Grain: A Natural System for the Study of Gibberellin-induced Enzyme Formation

Yorkstar wheat, grown in New York State, has a high percentage (10-11) of grains without embryos. The embryoless grains have viable aleurone layers and show no sign of injury. These grains are able to support α-amylase synthesis only in the presence of gibberellin A₃ (GA₃). In the absence of GA₃ some protein synthesis occurs in embryoless grains during the early hours of soaking, indicating that such activity occurs prior to and independent of GA₃ induction of α-amylase. The level of β-amylase on a dry weight basis is the same in embryoless and normal grains and decreases with time of soaking. In the presence of GA₃, β-amylase decreases at a slower rate. Isoenzymes of α-amylase from GA₃-treated embryoless and normal grains show quantitative as well as qualitative differences. Cycloheximide (60 μg/ml) completely inhibits the synthesis of α-amylase by embryoless grains. Of the RNA synthesis inhibitors, actinomycin D (60 μg/ml) was ineffective while 6-methylpurine (60 μg/ml) gave 65% inhibition without decreasing the number of isoenzymes.     

A Model for the Translocation of Photosynthate in the Soybean

The transport of photosynthetically incorporated carbon downthe soybean stem is discussed in terms of flow from a reservoirin the upper part of the plant. A comparison of this model withthe experimental data from the preceding paper leads to an estimateof translocation velocity of about 60 cm. per hr. and to leakageof material from the sieve tubes of about o.8 per cent. percm. of stem traversed. The effects of velocity distributionsand delays in the translocation system are considered.     

A Genetic Framework for Grain Size and Shape Variation in Wheat

Grain morphology in wheat (Triticum aestivum) has been selected and manipulated even in very early agrarian societies and remains a major breeding target. We undertook a large-scale quantitative analysis to determine the genetic basis of the phenotypic diversity in wheat grain morphology. A high-throughput method was used to capture grain size and shape variation in multiple mapping populations, elite varieties, and a broad collection of ancestral wheat species. This analysis reveals that grain size and shape are largely independent traits in both primitive wheat and in modern varieties. This phenotypic structure was retained across the mapping populations studied, suggesting that these traits are under the control of a limited number of discrete genetic components. We identified the underlying genes as quantitative trait loci that are distinct for grain size and shape and are largely shared between the different mapping populations. Moreover, our results show a significant reduction of phenotypic variation in grain shape in the modern germplasm pool compared with the ancestral wheat species, probably as a result of a relatively recent bottleneck. Therefore, this study provides the genetic underpinnings of an emerging phenotypic model where wheat domestication has transformed a long thin primitive grain to a wider and shorter modern grain.     

用基因枪法将抗除草剂基因导入小麦栽培品种的研究

利用基因枪法将抗除草剂bar基因导入西南地区的 3个小麦栽培品种 ,共获得 7个转基因植株 ,转化频率在 0 .45 %～ 1 .2 %之间 ,转化周期缩短至 3个月左右。对抗性植株进行PCR和PCR_Southern杂交检测 ,初步确定bar基因已导入小麦基因组。做转基因植株叶片对除草剂PPT的抗性试验 ,有 4株呈抗性 ,3株呈部分抗性 ,表明bar基因已在小麦植株中得到表达。     

用基因枪法将抗除草剂基因导入小麦栽培品种的研究

利用基因枪法将抗除草剂bar基因导入西南地区的3个小麦栽培品种,共获得7个转基因植株,转化频率在0.45%～1.2%之间,转化周期缩短至3个月左右。对抗性植株进行PCR和PCR_Southern 杂交检测,初步确定bar基因已导入小麦基因组。做转基因植株叶片对除草剂PPT的抗性试验,有4株呈抗性,3株呈部分抗性,表明bar基因已在小麦植株中得到表达。     

小麦腺苷二磷酸葡萄糖焦磷酸化酶同工酶类型及时空表达分析

以9个小麦品种为材料,采用非变性聚丙烯酰胺凝胶电泳(Native-PAGE)和SDS聚丙烯酰胺凝胶电泳(SDS-PAGE)鉴定ADP-葡萄糖焦磷酸化酶(AGP)同工酶类型、表达数量及亚基组成,分析AGP同工酶空间分布特点和器官表达特异性。结果表明:(1)小麦植株中共表达6种AGP同工酶,即AGPp、AGPs、AGPe1、AGPe2、AGPe3和AGPe4。(2)AGPp分布在种皮中,AGPs分布在种皮与叶片中,而AGPe1、AGPe2、AGPe3和AGPe4专一在胚乳中表达;在小麦籽粒灌浆过程中,AGPe1首先表达,AGPe2和AGPe3紧随其后,AGPe4最后表达。(3)AGP各同工酶均由大、小2个亚基组成,小亚基分子量为50kD左右,大亚基分子量在51~54kD之间。(4)AGP同工酶空间分布具有器官特异性,并在籽粒发育进程中顺序表达;AGPe3、AGPe1和AGPe2是占主导地位的AGP同工酶,且可能是决定AGP总酶活性的主效应酶,在灌浆后期籽粒淀粉合成中起关键作用。     

小麦籽粒品质性状的QTL分析

利用小麦中国春(母本)和兰考大粒(父本)F2群体构建了169个标记的分子遗传图谱,将F2∶3家系分别种植于3个环境中,利用基于完备区间混合模型的单环境作图模型和多环境作图模型对小麦籽粒容重、硬度、蛋白含量和结合水含量性状进行了QTL分析。结果显示:(1)两种作图模型下,检测到容重的6个共同QTL(QTW-6B-6、QTW-7B-6、QTW-7B-9、QTW-5D-2、QTW-6D-1、QTW-6D-4),单环境模型下遗传贡献率为1.99%~6.57%,多环境模型下遗传贡献率为3.66%~20.07%,其中QM TW-7B-9、QM TW-6D-1和QM TW-6D-4在多环境模型中表现为主效QTL。(2)检测到硬度的3个共同QTL(QHD-4A-5、QHD-7A-1和QHD-7B-9),单环境模型下的遗传贡献率为6.00%~6.95%,多环境模型中遗传贡献率为5.43%~9.64%。(3)检测到蛋白含量1个共同QTL(QPR-6D-1),单环境模型下的遗传贡献率为5.39%,多环境模型中遗传贡献率为10.06%,表现为主效QTL。(4)检测到籽粒结合水含量1个共同QTL(QMO-1B-4),单环境模型下的遗传贡献率为39.20%,多环境模型下的遗传贡献率为75.01%,均表现为主效QTL。(5)1B染色体上存在同时控制籽粒容重、硬度、蛋白和结合水含量的QTL,说明1B染色体对小麦品质的影响可能很大。研究表明,小麦容重、硬度、蛋白含量、结合水含量的遗传主要受加性效应控制。该研究初步定位的一些重要QTL可为进一步精细定位、基因挖掘和育种早代品质性状分子标记辅助选择提供依据。     

Starch Biosynthesis in Developing Wheat Grain : Evidence against the Direct Involvement of Triose Phosphates in the Metabolic Pathway

We have used ¹³C-labeled sugars and nuclear magnetic resonance (NMR) spectrometry to study the metabolic pathway of starch biosynthesis in developing wheat grain (Triticum aestivum cv Mardler). Our aim was to examine the extent of redistribution of ¹³C between carbons atoms 1 and 6 of [1-¹³C] or [6-¹³C]glucose (or fructose) incorporated into starch, and hence provide evidence for or against the involvement of triose phosphates in the metabolic pathway. Starch synthesis in the endosperm tissue was studied in two experimental systems. First, the ¹³C sugars were supplied to isolated endosperm tissue incubated in vitro, and second the ¹³C sugars were supplied in vivo to the intact plant. The ¹³C starch produced by the endosperm tissue of the grain was isolated and enzymically degraded to glucose using amyloglucosidase, and the distribution of ¹³C in all glucosyl carbons was quantified by ¹³C-NMR spectrometry. In all of the experiments, irrespective of the incubation time or incubation conditions, there was a similar pattern of partial (between 15 and 20%) redistribution of label between carbons 1 and 6 of glucose recovered from starch. There was no detectable increase over background ¹³C incidence in carbons 2 to 5. Within each experiment, the same pattern of partial redistribution of label was found in the glucosyl and fructosyl moieties of sucrose extracted from the tissue. Since it is unlikely that sucrose is present in the amyloplast, we suggest that the observed redistribution of label occurred in the cytosolic compartment of the endosperm cells and that both sucrose and starch are synthesized from a common pool of intermediates, such as hexose phosphate. We suggest that redistribution of label occurs via a cytosolic pathway cycle involving conversion of hexose phosphate to triose phosphate, interconversion of triose phosphate by triose phosphate isomerase, and resynthesis of hexose phosphate in the cytosol. A further round of triose phosphate interconversion in the amyloplast could not be detected. These data seriously weaken the argument for the selective uptake of triose phosphates by the amyloplast as part of the pathway of starch biosynthesis from sucrose in plant storage tissues. Instead, we suggest that a hexose phosphate such as glucose 1-phosphate, glucose 6-phosphate, or fructose 6-phosphate is the most likely candidate for entry into the amyloplast. A pathway of starch biosynthesis is presented, which is consistent with our data and with the current information on the intracellular distribution of enzymes in plant storage tissues.     

Manipulation of Grain Dormancy in Wheat

To help in understanding the mechanisms of dormancy in wheat(Triticum aestivum L.), brief drying of intact ears has beenexamined as a technique for rapid imposition of dormancy. Beforenatural grain desiccation at 40 d post anthesis (dpa), wholeplants were moved for 15 to 20 d to ‘wet’, humidconditions (90–100% RH) or to ‘dry’ conditions(35–40% RH). The duration of subsequent dormancy was atleast doubled by the ‘dry’ treatment, however, onlyif the grain was allowed to dry before 50 dpa. In the ‘wet’ears there was a low level (up to 22%) in-ear germination by55 dpa and the remaining, non-sprouted grain no longer becamedormant on drying. Brief (4 d) drying episodes could also preventsprouting and induce some dormancy, but only over a 4 to 10d window of sensitivity before 50 dpa. Grain drying when properlytimed not only arrests development but, perhaps through damagingeffects of drying, causes dormancy. Dormancy imposition wasnot related to embryo abscisic acid (ABA) or sucrose content. Key words: Abscisic acid, desiccation, dormancy, in-ear sprouting, wheat     

A Novel STS Marker for Polyphenol Oxidase Activity in Bread Wheat

The enzyme activity of polyphenol oxidase (PPO) in grain has been related to undersirable brown discoloration of bread wheat (Triticum aestivum L.) based end-products, particularly for Asian noodles. Breeding wheat cultivars with low PPO activity is the best approach to reduce the undesirable darkening. Molecular markers could greatly improve selection efficiency in breeding programs. Based on the sequences of PPO genes (GenBank Accession Numbers AY596268, AY596269 and AY596270) conditioning PPO activity during kernel development, 28 pairs of primers were designed using the software ‘DNAMAN’. One of the markers from AY596268, designated as PPO18, can amplify a 685-bp and an 876-bp fragment in the cultivars with high and low PPO activity, respectively. The difference of 191-bp size was detected in the intron region of the PPO gene. The STS marker PPO18 was mapped to chromosome 2AL using a DH population derived from a cross Zhongyou 9507× CA9632, a set of nulli-tetrasomic lines and ditelosomic line 2AS of Chinese Spring. QTL analysis indicated that the PPO gene co-segregated with the STS marker PPO18 and is closely linked to Xgwm312 and Xgwm294 on chromosome 2AL, explaining 28–43% of phenotypic variance for PPO activity across three environments. A total of 233 Chinese wheat cultivars and advanced lines were used to validate the correlation between the polymorphic fragments of PPO18 and grain PPO activity. The results showed that PPO18 is a co-dominant, efficient and reliable molecular marker for PPO activity and can be used in wheat breeding programs targeted for noodle quality improvement.     

The Pathway of Radial Transfer of Photosynthate in Decapitated Stems of Phaseolus vulgaris L.

[¹⁴C]Sucrose was found to be the predominant component of the¹⁴C-photosynthates that accumulated in the free space of decapitatedstems of P. vulgaris plants. The ¹⁴C-photosynthates appearedto occupy the entire free-space volume of the stems at totalsugar concentrations in the range of 3–12 mM. The free-spacesugar levels were found to rapidly decline once photosynthatetransfer to the stems was halted. Moreover, it was found thatestimates of the rate of in vitro sucrose uptake by the stemscould account fully for the decline in free-space sugar levels.Overall, the evidence indicated that at least part of the radialpathway of photosynthate transfer in bean stems involved thestem apoplast. It is tentatively proposed that, based on celland tissue distribution of ¹⁴C-photosynthates, the apoplasticpathway extends from the membrane boundary of the sieve element/companion-cellcomplex to all other cells of the stem. Apoplast, Phaseolus vulgaris L., bean, phloem unloading, photosynthates, symplast     

Photosynthesis in the Pericarp of Developing Wheat Grains

Oxygen exchange in grains of wheat was measured in both lightand dark over the period of grain development. Between 10 dand 30 d after anthesis, the rate of photosynthesis exceededthe rate of respiration. Peak photosynthetic activity was observedat 20 d after anthesis, coinciding with maximum chlorophyllcontent in the pericarp green layer. Removal of the pericarptransparent layer increased rates of oxygen exchange in boththe light and the dark. Attempts to inhibit photosynthesis withDCMU were only successful with the pericarp transparent layerremoved. Key words: Wheat, pericarp, photosynthesis