A Raman-scattering study on the net orientation of biomacromolecules in the outer epidermal walls of mature wheat stems (Triticum aestivum)

BACKGROUND AND AIMS: Raman spectroscopy can be used to examine the orientation of biomacromolecules using relatively thick samples of material, whereas more traditional means of analysing molecular structure require prior isolation of the components, which often destroys morphological features. In this study, Raman spectroscopy was used to examine the outer epidermal cell walls of wheat stems. METHODS: Polarized Raman spectra from the epidermal cell walls of wheat stem were obtained using near-infrared-Fourier transform Raman scattering. By comparing spectra taken with Raman light polarized perpendicular or parallel to the longitudinal axis of the cell, the orientation of macromolecules in the cell wall was investigated. KEY RESULTS: The net orientation of macromolecules varies in the epidermal cell walls of the different components of wheat stem. The net orientation of cellulose is parallel to the longitudinal axis of the cells, whereas the xylan and the phenylpropane units of lignin tend to lie perpendicular to the longitudinal axis of the cells, i.e. perpendicular to the net orientation of cellulose in the epidermal cell walls. CONCLUSIONS: The results imply that cellulose, lignin and xylan form a relatively ordered network that defines the mechanical and structural properties of the cell wall. Such results are likely to have a significant impact on the formulation of definitive models for the static and growing cell wall.     

不同小麦品种茎秆特性及其与抗倒性的关系

以黄淮麦区优良品种矮抗58、周麦18、豫麦49、百农418为研究对象,采用田间试验与实验室分析相结合的方法,对不同小麦品种在不同生育时期的抗倒伏性状进行研究.结果表明: 茎秆机械强度在开花期至花后20 d处于较高水平,在花后30 d明显下降;倒伏指数在开花期最小,花后30 d最大,其余两个时期处于中间水平.相关分析表明,开花期机械强度与重心高度呈显著负相关,与纤维素、木质素含量呈显著正相关,倒伏指数与节长、株高、重心高度呈显著正相关,与纤维素、木质素含量呈显著负相关;花后10 d和花后20 d机械强度与节长、株高、重心高度呈显著负相关,与茎粗、纤维素、半纤维素、木质素含量呈显著正相关,倒伏指数这段时期正好与之相反;花后30 d机械强度与株高、重心高度呈显著负相关,倒伏指数与株高、重心高度呈显著正相关,与木质素含量呈显著负相关.因此,明确各个生育时期与抗倒性相关的茎秆特性,可为黄淮麦区高产抗倒性品种的选育提供依据.     

水稻茎秆形态结构特征和化学成分与抗倒伏关系综述

水稻茎秆形态结构特征和化学成分与其抗倒伏性能相关。本文对水稻茎秆高度、茎秆基部第1、2节间长度、茎秆粗细、茎壁厚度、厚壁组织(机械组织)数量和强度、维管束数量、细胞壁纤维素和木质素含量、细胞中碳水化合物积累的数量、硅与钾的含量以及茎秆抗倒伏相关的QTL等与水稻的抗倒伏性的相互关系进行了综述,为水稻抗倒伏优良品种性状的选育提供参考。     

Nitrogen fertilizer application affects lodging resistance by altering secondary cell wall synthesis in japonica rice (<Emphasis Type="Italic">Oryza sativa</Emphasis>)

Stem mechanical strength is an important agricultural quantitative trait that is closely related to lodging resistance in rice, which is known to be reduced by fertilizer with higher levels of nitrogen. To understand the mechanism that regulates stem mechanical strength in response to nitrogen, we analysed stem morphology, anatomy, mechanical properties, cell wall components, and expression of cell wall-related genes, in two varieties of japonica rice, namely, Wuyunjing23 (lodging-resistant variety) and W3668 (lodging-susceptible variety). The results showed that higher nitrogen fertilizer increased the lodging index in both varieties due to a reduction in breaking strength and bending stress, and these changes were larger in W3668. Cellulose content decreased slightly under higher nitrogen fertilizer, whereas lignin content reduced remarkably. Histochemical staining revealed that high nitrogen application decreased lignin deposition in the secondary cell wall of the sclerenchyma cells and vascular bundle cells compared with the low nitrogen treatments, while it did not alter the pattern of cellulose deposition in these cells in both Wuyunjing23 and W3668. In addition, the expression of the genes involved in lignin biosynthesis, OsPAL, OsCoMT, Os4CL3, OsCCR, OsCAD2, OsCAD7, OsCesA4, and OsCesA7, were also down-regulated under higher nitrogen conditions at the early stage of culm growth. These results suggest that the genes involved in lignin biosynthesis are down-regulated by higher nitrogen fertilizer, which causes lignin deficiency in the secondary cell walls and the weakening of mechanical tissue structure. Subsequently, this results in these internodes with reduced mechanical strength and poor lodging resistance.     

套作大豆苗期茎秆纤维素合成代谢与抗倒性的关系

为从茎秆强度的角度研究套作大豆苗期对荫蔽胁迫的响应及耐荫抗倒机制,采用耐荫性不同的3个大豆材料,在玉米大豆套作和单作两种种植模式下,对茎秆的纤维素、可溶性糖、蔗糖、淀粉含量及蔗糖代谢中关键酶活性以及茎秆抗折力、抗倒伏指数等进行测定,研究它们与套作大豆苗期倒伏的关系.套作大豆苗期倒伏严重,茎秆抗折力、抗倒伏指数、纤维素、可溶性糖、蔗糖、淀粉含量和相关酶活性均显著低于单作.不同大豆材料受套作荫蔽影响程度不同,强耐荫性大豆南豆12茎秆抗折力降低幅度最小,在套作环境下其茎秆抗折力、抗倒伏指数大,纤维素、可溶性糖、蔗糖、淀粉含量高,酶活性强.相关分析表明： 套作大豆苗期茎秆糖含量均与抗折力呈极显著正相关,与倒伏率呈极显著负相关;蔗糖含量与蔗糖磷酸合酶(SPS)、蔗糖合酶(SS)、中性转化酶(NI)活性呈极显著正相关,与酸性转化酶(AI)活性相关性不显著;纤维素含量与SPS、SS呈极显著正相关,与NI呈显著正相关,与AI相关性不显著.套作环境下,强耐荫性大豆苗期茎秆中较高的SPS、SS活性是其维持高蔗糖和纤维素含量的酶学基础,而高纤维素含量有利于提高茎秆强度,进而增强其抗倒伏能力.本研究应用玉米大豆套作种植系统,从苗期抗倒角度,探明了光环境对不同基因型大豆茎秆纤维素代谢的影响机制,为下一步筛选耐荫抗倒大豆品种提供了理论依据.     

Characterization of a cinnamoyl-CoA reductase that is associated with stem development in wheat

Cinnamoyl-CoA reductase (CCR) is responsible for the CoA ester to aldehyde conversion in monolignol biosynthesis, which diverts phenylpropanoid-derived metabolites into the biosynthesis of lignin. To gain a better understanding of lignin biosynthesis and its biological function, a cDNA encoding CCR was identified from wheat (Triticum aestivum L.), and designated as Ta-CCR1. Phylogenetic analysis indicated that Ta-CCR1 grouped together with other monocot CCR sequences while it diverged from Ta-CCR2. DNA gel-blot and mapping analyses demonstrated that Ta-CCR1 is present as a single copy gene in the wheat genome. Recombinant Ta-CCR1 protein converted feruloyl CoA, 5-OH-feruloyl CoA, sinapoyl CoA, and caffeoyl CoA, but feruloyl-CoA was the best substrate, suggesting the preferential biosynthesis of G-type lignin. RNA gel-blot analysis indicated that Ta-CCR1 was highly expressed in stem, with lower expression in leaves, and undetectable expression in roots. CCR enzyme activity was increased progressively along with the lignin biosynthesis and stem maturity. During stem development, Ta-CCR1 mRNA levels remained high at elongation, heading, and milky stages in the wheat H4564 cultivar, while they declined dramatically at the heading and milky stages in stems of the C6001 cultivar. Ta-CCR1 mRNA expression paralleled extractable CCR enzyme activity in these two cultivars. Furthermore, high Ta-CCR1 mRNA levels and high CCR enzyme activity in wheat stem were correlated with a higher Klason lignin content and greater stem mechanical strength in the H4564 cultivar. This suggests that Ta-CCR1 and its related CCR enzyme may be involved in the regulation of lignin biosynthesis during stem maturity and then contributes to stem strength support in wheat.     

Collapsed xylem phenotype of Arabidopsis identifies mutants deficient in cellulose deposition in the secondary cell wall.

Recessive mutations at three loci cause the collapse of mature xylem cells in inflorescence stems of Arabidopsis. These irregular xylem (irx) mutations were identified by screening plants from a mutagenized population by microscopic examination of stem sections. The xylem cell defect was associated with an up to eightfold reduction in the total amount of cellulose in mature inflorescence stems. The amounts of cell wall-associated phenolics and polysaccharides were unaffected by the mutations. Examination of the cell walls by using electron microscopy demonstrated that the decreases in cellulose content of irx lines resulted in an alteration of the spatial organization of cell wall material. This suggests that a normal pattern of cellulose deposition may be required for assembly of lignin or polysaccharides. The reduced cellulose content of the stems also resulted in a decrease in stiffness of the stem material. This is consistent with the irregular xylem phenotype and suggests that the walls of irx plants are not resistant to compressive forces. Because lignin was implicated previously as a major factor in resistance to compressive forces, these results suggest either that cellulose has a direct role in providing resistance to compressive forces or that it is required for the development of normal lignin structure. The irx plants had a slight reduction in growth rate and stature but were otherwise normal in appearance. The mutations should be useful in facilitating the identification of factors that control the synthesis and deposition of cellulose and other cell wall components.     

Ectopic lignification in primary cellulose-deficient cell walls of maize cell suspension cultures

Maize(Zea mays L.) suspension-cultured cells with up to 70% less cellulose were obtained by stepwise habituation to dichlobenil(DCB), a cellulose biosynthesis inhibitor. Cellulose de ficiency was accompanied by marked changes in cell wall matrix polysaccharides and phenolics as revealed by Fourier transform infrared(FTIR) spectroscopy.Cell wall compositional analysis indicated that the cellulosede ficient cell walls showed an enhancement of highly branched and cross-linked arabinoxylans, as well as an increased content in ferulic acid, diferulates and p-coumaric acid, and the presence of a polymer that stained positive for phloroglucinol. In accordance with this, cellulose-de ficient cell walls showed a fivefold increase in Klason-type lignin.Thioacidolysis/GC-MS analysis of cellulose-de ficient cell walls indicated the presence of a lignin-like polymer with a Syringyl/Guaiacyl ratio of 1.45, which differed from the sensu stricto stress-related lignin that arose in response to shortterm DCB-treatments. Gene expression analysis of these cells indicated an overexpression of genes specific for the biosynthesis of monolignol units of lignin. A study of stress signaling pathways revealed an overexpression of some of the jasmonate signaling pathway genes, which might trigger ectopic lignification in response to cell wall integrity disruptions. In summary, the structural plasticity of primary cell walls is proven, since a lignification process is possible in response to cellulose impoverishment.     

Solid-stemmed spring wheat cultivars give better androgenic response than hollow-stemmed cultivars in anther culture

Solid-stemmed spring wheat cultivars (Triticum aestivum L.) are resistant to the stem sawfly (Cephus cinctus Nort.) and lodging. Anthers of 24 spring wheat cultivars with varying content of pith in the stem were used in the experiment. All were classified into three groups: solid, medium–solid and hollow stems. There was considerable influence of the cultivar on callus formation and green plant regeneration. The highest efficiency of green plant regeneration (24%) was observed for the solid-stemmed AC Abbey cultivar. There was no regeneration from the explants of four cultivars: CLTR 7027, Alentejano, Marquis and Bombona. Principal component analysis showed no differences between the cases under observation (callus induction and green plant regeneration) in their response to pre-treatment temperatures (4 and 8°C). The examination of the effects of various auxin types in the induction medium on callus formation and green plant regeneration revealed that the strongest stimulation of these processes was observed in the C17 medium with 2,4-D and dicamba. The efficiency of callus formation and green plant regeneration was greater in solid-stemmed cultivars than in hollow-stemmed cultivars.     

FTIR imaging of wheat endosperm cell walls in situ reveals compositional and architectural heterogeneity related to grain hardness.

Endosperm cell walls of cultivars of wheat (Triticum aestivum L.) selected for their endosperm texture (two soft and two hard) were analysed in situ by Fourier transform infrared (FTIR) microspectroscopy. FTIR imaging coupled with statistical analysis was used to map the compositional and structural heterogeneity within transverse sections from which cell contents had been removed by sonication. In the majority of grains analysed, two distinct populations of endosperm cells could be identified by spectral features that were related to cell morphology and age, regardless of cultivar. The main cell-wall component responsible for these differences was the polysaccharide arabinoxylan. In a few samples, this heterogeneity was absent, for reasons that are not understood, but this was not correlated to endosperm texture or growth conditions. Within the same population of endosperm cells, cell walls of hard endosperm could be distinguished from those of soft endosperm by their spectral features. Compared to hard cultivars, the peripheral endosperm of soft cultivars was characterised by a higher amount of polymer, whose spectral feature was similar to water-extractable arabinoxylan. In contrast, no specific compound has been identified in the central endosperm: structural differences within the polysaccharides probably contribute to the distinction between hard and soft cultivars. In developing grain, a clear difference in the composition of the endosperm cell walls of hard and soft wheat cultivars was observed as early as 15 days after anthesis.     

Biosynthesis and incorporation of side-chain-truncated lignin monomers to reduce lignin polymerization and enhance saccharification

Lignocellulosic biomass is utilized as a renewable feedstock in various agro‐industrial activities. Lignin is an aromatic, hydrophobic and mildly branched polymer integrally associated with polysaccharides within the biomass, which negatively affects their extraction and hydrolysis during industrial processing. Engineering the monomer composition of lignins offers an attractive option towards new lignins with reduced recalcitrance. The presented work describes a new strategy developed in Arabidopsis for the overproduction of rare lignin monomers to reduce lignin polymerization degree (DP). Biosynthesis of these ‘DP reducers’ is achieved by expressing a bacterial hydroxycinnamoyl‐CoA hydratase‐lyase (HCHL) in lignifying tissues of Arabidopsis inflorescence stems. HCHL cleaves the propanoid side‐chain of hydroxycinnamoyl‐CoA lignin precursors to produce the corresponding hydroxybenzaldehydes so that plant stems expressing HCHL accumulate in their cell wall higher amounts of hydroxybenzaldehyde and hydroxybenzoate derivatives. Engineered plants with intermediate HCHL activity levels show no reduction in total lignin, sugar content or biomass yield compared with wild‐type plants. However, cell wall characterization of extract‐free stems by thioacidolysis and by 2D‐NMR revealed an increased amount of unusual C₆C₁ lignin monomers most likely linked with lignin as end‐groups. Moreover the analysis of lignin isolated from these plants using size‐exclusion chromatography revealed a reduced molecular weight. Furthermore, these engineered lines show saccharification improvement of pretreated stem cell walls. Therefore, we conclude that enhancing the biosynthesis and incorporation of C₆C₁ monomers (‘DP reducers’) into lignin polymers represents a promising strategy to reduce lignin DP and to decrease cell wall recalcitrance to enzymatic hydrolysis.     

Double benefits of mechanical wounding in enhancing chilling tolerance and lodging resistance in wheat plants

• Chilling and lodging are major threats to wheat production. However, strategies that can be used to effectively mitigate the adverse effects of these threats are still far from clear. Mechanical wounding is a traditional agronomic measure, whereas information about the role it plays in wheat chilling and lodging is scant.
• The aim of the present study was to investigate mechanisms underlying the protective roles of mechanical wounding in alleviating damage from chilling at jointing stage and enhancing lodging resistance after anthesis of winter wheat (Triticum aestivum L.).
• Our data show that net photosynthesis rate, maximum photochemical efficiency of photosystem II, activity of the antioxidant enzymes and osmolytes were significantly increased in the latest fully expanded leaves of wounded plants under chilling. Wounding also reduced hydrogen peroxide accumulation, electrolyte leakage and water loss in wounded plants. Moreover, mechanical wounding significantly reduced the length but increased the diameter and wall thickness of the basal second internode of the main stem. Quantitative and histochemical analysis further indicated that wounding increased lignin accumulation and activity of enzymes involved in lignin synthesis, which resulted in increased mechanical strength and the lodging resistance index in the main stem.
• We conclude from our data that mechanical wounding confers both cold tolerance by alleviating the damage caused by chilling at jointing stage and lodging resistance after anthesis of wheat plants.

     

Characterising the morphological characters and carbohydrate metabolism of oat culms and their association with lodging resistance

• Lodging resistance can be improved by enhancing the mechanical strength of culms, and culm carbohydrates could improve this mechanical strength. Culm carbohydrates can regulate development of the culm and affect its toughness.
• The present study determines the relationship between lodging and carbohydrate content in oat culms. Field experiments were conducted in alpine regions in 2017 and 2018 using three oat varieties with different lodging resistance. Lodging‐related morphological characteristics were directly determined and culm carbohydrate content and enzyme activity related to cellulose synthesis and sucrose metabolism were evaluated with ultraviolet spectrophotometry.
• Results showed that the lower the gravity height or the lower ratio of gravity height to plant height, the stronger the lodging resistance of the varieties. Higher culm nonstructural (NSC) and structural (SC) carbohydrate content contributed to the ability of culms to resist lodging, especially the content of cellulose and sucrose. PCA showed that sucrose metabolism and SC content were closely related to lodging resistance. Correlation analysis showed that the lodging index (LI) was significantly negatively correlated with NSC. Sucrose content was highly and significantly positively correlated with NSC. Additionally, the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS) were highly and significantly positively correlated with sucrose and cellulose content.
• The relationship between field characters and oat lodging, as well as the regulatory mechanism of carbohydrate content on lodging resistance of the culm are discussed.

     

Arabidopsis fragile fiber8, which encodes a putative glucuronyltransferase, is essential for normal secondary wall synthesis

Secondary walls in vessels and fibers of dicotyledonous plants are mainly composed of cellulose, xylan, and lignin. Although genes involved in biosynthesis of cellulose and lignin have been intensively studied, little is known about genes participating in xylan synthesis. We found that Arabidopsis thaliana fragile fiber8 (fra8) is defective in xylan synthesis. The fra8 mutation caused a dramatic reduction in fiber wall thickness and a decrease in stem strength. FRA8 was found to encode a member of glycosyltransferase family 47 and exhibits high sequence similarity to tobacco (Nicotiana plumbaginifolia) pectin glucuronyltransferase. FRA8 is expressed specifically in developing vessels and fiber cells, and FRA8 is targeted to Golgi. Comparative analyses of cell wall polysaccharide fractions from fra8 and wild-type stems showed that the xylan and cellulose contents are drastically reduced in fra8, whereas xyloglucan and pectin are elevated. Further structural analysis of cell walls revealed that although wild-type xylans contain both glucuronic acid and 4-O-methylglucuronic acid residues, xylans from fra8 retain only 4-O-methylglucuronic acid, indicating that the fra8 mutation results in a specific defect in the addition of glucuronic acid residues onto xylans. These findings suggest that FRA8 is a glucuronyltransferase involved in the biosynthesis of glucuronoxylan during secondary wall formation.     

玉米茎秆细胞壁和组织构建对抗压强度的影响

耐密抗倒伏玉米品种是玉米育种的重要方向,探究影响玉米茎秆抗压强度的机制是培育玉米新品种的重要途径。本实验采用组织化学、显微观察的方法研究了10个玉米品种茎秆的形态结构、解剖特征和细胞壁的化学组成,并分析了这些变量之间的相关性,结果表明:茎的皮层/半径、厚壁组织比例、机械组织比例和纤维素含量、木质素含量与抗压强度呈极显著正相关关系;薄壁组织比例、茎长/茎粗、维管束个数与抗压强度呈极显著负相关关系。利用共线性诊断和逐步线性回归分析发现,影响茎秆抗压强度的主要因素为皮层/半径、机械组织比例、维管束个数、纤维素含量和木质素含量。利用通径分析进一步定量研究了这5个变量与抗压强度之间的直接作用和间接作用,明确了决定玉米茎秆抗压强度的主要因素为纤维素含量、木质素含量和单位面积维管束个数。本实验还建立了玉米茎微观结构与细胞壁化学构成的数学模型,为进一步揭示玉米茎微观力学形成机理提供了思路,进而为耐密抗倒伏玉米育种提供了研究方向。     

甘蓝型油菜抗茎倒材料的筛选与候选基因挖掘

该研究采用茎秆抗折力指标评价了200份甘蓝型油菜种质资源抗茎倒伏能力,据此筛选出极端抗茎倒材料和不抗茎倒材料各1份,随后测定了2份材料成熟期茎秆的理化组分含量,并对2份极端抗茎倒差异材料蕾苔期、盛花期茎秆进行转录组测序分析,为甘蓝型油菜抗茎倒伏的遗传改良奠定基础。结果表明:(1)200份甘蓝型油菜种质资源的茎秆抗折力和茎粗均为正态分布,均属于数量遗传性状,并依据茎秆抗折力和农艺性状筛选出生育期相近,株型相似,茎秆粗度差异不显著,但茎秆抗折力差异显著的极端不抗茎倒材料GY172和抗茎倒材料GY199。(2)GY199的韧皮部比GY172更加致密,而GY172成熟期茎秆中的半纤维素、木质素、中性洗涤纤维及总可溶性糖含量均显著高于GY199,而其纤维素含量极显著低于GY199,即成熟期茎秆纤维素含量与这2个材料的茎秆抗折力呈正相关。(3)蕾苔期、盛花期茎秆转录组测序发现,碳代谢、碳固定、磷酸戊糖途径、氨基酸的生物合成、糖酵解/糖异生等途径的14个基因(BnaA10G0056100ZS、BnaC08G0455100ZS、BnaA08G0262400ZS、BnaC08G0239700ZS、BnaA07G0362300ZS、BnaC02G0081300ZS、BnaC04G0273000ZS等)以及纤维素合成相关的9个基因(BnaA05G0152200ZS、BnaA01G0411100ZS、BnaA03G0018900ZS、BnaA03G0037800ZS等)在抗茎倒材料GY199中显著上调表达,这些基因可能参与调控了茎秆强度性状,可作为油菜抗茎倒候选基因。     

小麦抗倒性研究进展

倒伏是严重影响小麦子粒产量和品质的一个重要因素。本文系统阐述了小麦茎秆形态和结构特性、茎秆化学成分与抗倒伏关系以及抗倒性的遗传和分子标记等方面的最新研究进展。株高、基部节间长度与抗倒性呈负相关;而基部节间粗度、秆壁厚、单位长度干重与抗倒性呈正相关。茎秆机械组织细胞层数、厚度,维管束数目、面积以及髓腔大小与抗倒性密切相关。茎秆化学成分中纤维素、木质素以及碳水化合物含量和硅、钾元素含量与抗倒性呈正相关。小麦抗倒性呈数量性状遗传特征,除受多对主基因控制外,可能还受微效修饰基因作用。采用分子标记技术已将抗倒性以及与抗倒性相关的茎秆形态性状进行了QTL定位。     

Improvement of lodging resistance with QTLs for stem diameter in rice (Oryza sativa L.)

Varietal differences among ten rice cultivars showed that stem diameter is a key factor in lodging resistance (measured in terms of pushing resistance). Two near-isogenic lines (NILs) were selected from a series of chromosome segment substitution lines developed between cultivars Nipponbar and Kasalath, one containing a single stem diameter QTL (sdm8; NIL114), and another with four stem diameter QTLs (sdm1, sdm7, sdm8, sdm12; NIL28). Compared with the Nipponbare control, stem diameters were larger in NIL114 and NIL28 by about 7 and 39%, respectively. Pushing resistance in NIL28 was significantly greater than in Nipponbare, but NIL114 was similar to Nipponbare. The two NILs had greater weight of lower stem and culm wall thickness than Nipponbare. NIL28 had higher plant height, which is a negative effect on lodging resistance, than Nipponbare. The non-structural carbohydrate contents of NIL stems were higher than that of Nipponbare, whereas the silicon contents were lower in the NILs, and cellulose contents were lower only in NIL28. The basal internodes of the two NILs were significantly stiffer than those of Nipponbare. These results suggest that increasing stem diameter in rice breeding programs would improve lodging resistance, although the combination of multiple QTLs would be necessary to produce thicker stems with higher pushing resistance, whereas the higher plant height could also result from the combination of multiple QTLs.     

The effect of nitrogen and growth regulators on stem and root characteristics associated with lodging in two cultivars of winter wheat

The effects of nitrogen and plant growth regulators (stem shorteners)on root and shoot characteristics associated with lodging resistancewere investigated in two winter wheat (Triticum aestivum L.)cultivars of contrasting lodging resistance: the susceptibleGalahad and the resistant Hereward. The morphology and mechanicalstrength of the stems and anchorage systems grown at two levelsof nitrogen and with or without growth regulators were measuredand related to the incidence of lodging recorded in a fieldtrial. In both cultivars high levels of nitrogen increased theheight of the stem, thereby increasing the ‘self-weight’moment transmitted into the ground and weakened both the stemsand the anchorage coronal roots. As a result, the anchoragestrength was also reduced, plants failing in the root systemin simulated lodging tests. Growth regulators, in contrast,had little effect on the bending strength of the shoots androot systems, but reduced plant height so that the over turningmoments generated by the weight of the shoot were less. Therewere also differences between cultivars: Galahad plants hadweaker anchorage due to the smaller number and lower strengthof the coronal roots. The morphological and mechanical measureswere used to calculate a safety factor against both stem androot lodging. Five factors were found to influence the safetyfactors, these were: cultivar type, the type of lodging, therate of nitrogen and growth regulator application, and time,being lowest in Galahad plants at high levels of nitrogen andwithout growth regulators and at grain filling when the earswere heaviest. This was consistent with the observed patternof lodging: root lodging occurred at grain filling and onlyin Galahad which had been treated with high nitrogen rates,most strongly in plants without growth regulators. Key words: Lodging, safety factors, anchorage, ‘self-weight’ moment     

Lignification in relation to the biennial growth habit in brassicas

The forage brassicas are a useful model system for the study of wood formation because the thickened cell walls of their vascular tissue can vary widely in lignin content. Solid-state 13C NMR spectroscopy was used to quantify lignin, and determine features of its structure, in the vascular cell walls of forage rape (Brassica napus L.), and Thousandhead and marrowstem cultivars of kale (Brassica oleracea L. var. acephala). During the first season of vegetative growth, lignin levels in these cell walls remained low in the upper part of the stems despite the physical resemblance of this tissue to wood. The extended flowering stems produced in the following year were thinner and their vascular tissue contained much more strongly lignified cell walls. The structure of the lignin was typical of angiosperm wood. It showed only small variations in syringyl/guaiacyl ratio, but this ratio increased with lignin content and thus with the proportion of the lignin that was associated with secondary cell-wall layers.