Characterization of mean stem density, fi bre length and lignin from two Acacia species and their hybrid

Asif Javed Muhammad?Seong Siang Ong?Wickneswari Ratnam

Introduction

Fast-growing species such asAcacia mangium,A.auriculiformis,andA.mangium×A.auriculiformishybrids are suitable for the pulp and paper industry.Acaciaspecies are grown extensively in South-East Asia with over 10 million hectares planted.In Indonesia,A.mangiumhas become the dominating plantation pulpwood species during the last 10 years as an excellent source of fi bre for papermaking(Hillman 2002;Malinen et al.2006).In addition,plantations of fast-growing species likeA.mangiumandA.auriculiformishave been promoted by the Vietnamese Government to ensure adequate supplies for pulp consumption in the country(Hai 2009).The natural hybrid ofA.mangiumandA.auriculiformishas an increased growing demand,especially in the pulp and paper industry due to its superior growth rate,better adaptation to a wide range of soils and better pulp yield than both parent species(Yamada et al.1990).

AlthoughAcaciaspecies are important to pulp and paper industries,little is known about the wood properties of this genus.There is little information to justify the wood quality of all the different genotypes within these species.It is well documented that natural variation in the quantity and quality of lignin has not been studied extensively even within a model species.Furthermore,wood properties may vary greatly due to factors such as age,environment,changes in climate and fertilization(Fagerstedt et al.1998).This scarcity of information raises concerns about wood properties from intensively managed forests,as similar volumes of wood may have very different values due to the presence of signi fi cant portions of juvenile wood which may result in low wood quality(Zobel and van Buijtenen 1989).

One of the most important characteristics necessary for effective wood utilization in the pulp and paper industry is the quantity and composition of lignin of the stem wood.Approximately two-thirds of all virgin wood pulp(recycled fi bers excluded)is produced by the kraft process that uses NaOH and Na2S to extract≥90% of the lignin.The rate of this process is believed to be enhanced by a high S:G rate(Kondo et al.1987;Tsutsumi et al.1995).Therefore wood with low amounts of lignin and high syringyl to guaiacyl ratios is often associated with improved pulping ef fi ciencies(Huntley et al.2003).This is because lignin is removed from wood chips during the pulping process,usually by chemical pulping and bleaching process,which are expensive,energy-extensive and environmentally unfriendly(Boerjan et al.2003).A high percentage of lignin in wood requires higher chemical consumption during pulping and this gives lower pulp yields(Pinto et al.2004).The objective of this study is to determine the variation and association of lignin contents,mean stem densities,and fi bre lengths ofA.mangium,A.auriculiformisand their hybrid.Correlations between phenotypic traits are explored in order to understand their effects on wood development,which could be used for selection and improvement of desired traits.The data generated are useful for selection breeding and improved utilization of these fast-growing plantation species for the pulp and paper industries.

Materials and methods

Sampling

Wood samples from 10 year-old trees were collected from Plot W,Experimental Field,UKM,Bangi,Malaysia.Three trees varying in diameter at breast height of each species and hybrid were selected and felled.Five disks measuring 3-cm in thickness were taken from the trunk at 4 m height.The sapwood was sampled from the whole disc between the transition and the inner bark tissues zones.Heartwood was sampled from 1 to 3 cm from the transition zone(Fig.1).Sapwood and heartwood tissues were separated from the desired region with a chisel and hammer,and the wood chips air-dried for 1 week or oven-dried for 2 days at 40°C before chemical analysis.

Fig.1 Wood disc(Dbh=54 cm)of A.mangium showing the sapwood and heartwood zone with bark removed.Locations where sapwood chips were sampled are in white circles while heartwood wood chips were sampled are in black circles

Lignin analysis

Lignin content of sapwood and heartwood tissues were determined using procedures described in TAPPIT 222-om-88(Schoening and Johansson 1965).Klason lignin was determined from the extractive free wood.One g of wood meal was placed in a 100 mL beaker,with 15 mL of 72%H2SO4added.The mixture was subjected to occasional stirring for 2 h at room temperature.The subsequent solution was transferred to a 1 L Erlenmeyer fl ask,topped up with 575 mL of deionized water and re fl uxed for 4 h.The solution was fi ltered using crucible no.4 and the acid insoluble lignin was determined gravimetrically.

Core sampling for mean stem density

Core samples were collected without removing the bark.Fresh sample mass was measured with an electronic balance immediately after collection,followed by oven-drying at 75°C to a constant weight and reweighing to determine the dry mass.Based on the fresh volume of the collected samples,measured from the length of the sample(5 cm)and the diameter of the increment borer(0.25 cm)and their dry mass,the mean stem density of each sample(in terms of dry mass per fresh volume)was calculated(Guendehou et al.2012).

Fibre length analysis

Fibre length of sapwood and heartwood was determined based on TAPPI T 232-om-85.A 10-mm thick and 5-mm wide radial stick of sapwood and heartwood was sawn from each disk.Samples were softened by gentle boiling in water till they sank,and were then macerated by dipping in boiling 5%glacial acetic acid at 100°C.2 mL of glacial acetic acid and 5 g sodium chloride were added hourly into the boiling solution until the fi bre was softened.The fi bres were stained with safranin and a drop of the macerated sample was placed on a slide and oven-dried for 1 h.Fifty wood fi bres each of sapwood and heartwood were measured for length using a pro fi le projector(Nikon V-12,Japan)under 50×magni fi cation.

Data analysis

A one-way ANOVA using the MINITAB software was used to test whether signi fi cant differences(p≤0.05)exist within and betweenA.mangium,A.auriculiformisand their hybrid for Klason lignin,mean stem density and fi bre length.If the differences were signi fi cant,Duncan’s multiple range test(DMRT)was used to compare the mean values.Association between the three traits was analyzed by Pearson’s correlations.

Results and discussion

A comparison of the extractives of sapwood and heartwood clearly demonstrates a higher percentage present in the heartwood(Table 1).The highest extractives were present inA.mangiumin both sapwood and heartwood followed by the hybrid.However,high extractive values are not correlated with high lignin content.The averageFvalues,mean signi fi cance,standard deviation and mean ranking for lignin,mean stem density and fi bre length in sapwood and heartwood traits are presented in Table 2.Overall,lignin content,mean stem density and fi bre length differed signi fi cantly among different individuals of the two species and their hybrid.High mean lignin content was observed forA.mangium(36.25%)followed byA.auriculiformis(33.91%)and interspeci fi c hybrid(31.99%)respectively(Table 3).Lignin content ranged from 18.16 to 46.58%withinA.mangium,22.54 to 42.09%withinA.auriculiformisand 21.43 to 38.45%for the interspeci fi c hybrids(Table 2).However,overall mean values of lignin content were lower inA.auriculiformis(33.90±9.33)compared toA.mangium(36.24±13.71).High standard deviation values for both species clearly indicate a wide range of diversity present within and between different individuals of the two species.For high quality pulp and yield,low lignin content requires low processing.Wood is composed of cellulose,hemicelluloses,lignin and extractives formed into a cellular structure,with cellulose accounting as the major component(Yang et al.2003).Wood with low amounts of lignin and high syringyl to guaiacyl ratio that affects the lignin extraction during kraft process which uses NaOH and Na2S has improved pulping ef fi ciency and paper quality(Kondo et al.1987;Tsutsumi et al.1995;Huntley et al.2003).Lignin content inA.mangiumwas higher than inA.auriculiformisand the hybrid.High lignin content inA.mangiumimplies lower pulp yields,as largeamounts of chemicals have to be used during the pulping process(Pinto et al.2004).Lower lignin content in theAcaciahybrid suggests better pulp yields than from parent species and its importance for the pulp and paper industry(Yamada et al.1990).Sapwood tissues generally gave better yields as they require less bleaching compared to heartwood tissues which contain high extractives,indirectly increasing chemical consumption.Our results indicate that the amount of extractives in sapwood was lower than that in heartwood tissues which can result in higher pulp yields.Our fi ndings of lignin content ofA.mangiumbetween xylem stress compression wood and xylem stress tension wood(unpublished)support fi ndings of Yamashita et al.(2007).Cellulose content in wood forming on the upper side of branches is higher compared to the lower sides of branches(Qiu et al.2008).Wood forming in vertical stems has cellulose and Klason lignin contents that are intermediate between upper and lower branch wood(Qiu et al.2008).Hence our sampling strategy de fi nitely provides accurate justi fi cation on the lignin amount in the two species and their hybrid.

Table 1 Klason lignin and extractives in sapwood and heartwood tissues in A.mangium(Am),A.auriculiformis(Aa)and Acacia hybrids(Ah)of 10-year-old trees

Table 2 F-statistics and signi fi cant mean values for lignin content,mean stem density and fi bre length in Acacia mangium,Acacia auriculiformis and interspeci fi c hybrid

Table 3 Overall mean values of lignin,mean stem density and mean fi bre length in Acacia mangium,Acacia auriculiformis and interspeci fi c hybrids

Mean stem densities of both sapwood and heartwood were signi fi cantly different(Table 2).High mean values were recorded forA.auriculiformis(699.84 g/cm3)followed by the hybrid(633.55%g/cm3)andA.mangium(584.0 g/cm3)(Table 3).Generally,heartwood showed highermean stem densitiescompared to sapwood(Table 2).Mean stem density was higher than 0.50 g/cm3indicating thatAcacia’s are hardwood species with hybrids of intermediate wood densities.Besides lignin content and composition,mean stem density is by far the most important parameter for measuring wood quality.Mean stem density is closely related to pulp yields and pulping properties(Malan et al.1994;Wimmer et al.2002).Wood with densities of 470–550 kg/m3is well suited for pulp production(Hai 2009),while higher density wood is suitable for construction(Barnett and Jeronimidis 1990).The mean stem density ofA.auriculiformiswas higher than forA.mangiumand their hybrid.These fi ndings are consistent with empirical studies conducted in Vietnam where the mean stem density ofA.auriculiformiswas higher thanA.mangiumat the same age(Kha 2001).Furthermore,A.auriculiformiswood is regarded by saw millers to be denser and harder than theAcaciahybrid andA.mangium(Hai 2009).The moderate mean stem density and lignin content of the hybrid suggest that this genotype is wellsuited for the pulp and paper industry.Wood basic density was typically lower in fast-growing trees than in slowgrowing trees(Quang et al.2010).This study reveals that wood densities and lignin contents of the investigated species were inversely correlated.Strong negative correlations were observed in all the genotypes.Growth differences may also be due to differences in stand density due to their capacity to capture more sunlight,moisture and nutrients to accelerate tree growth(Jiang et al.1994).Hu et al.(1999)demonstrated that lignin reduction in transgenic aspen was accompanied by higher cellulose content together with substantially enhanced growth rates.A signi fi cantly negative correlation was reported between lignin content and diameter growth in a hybrid backcross population ofEucalyptus(Kirsten et al.2004)and inPopulus(Kim et al.2009).Mean stem densities for both sapwood and heartwood were signi fi cantly different(Table 2).High mean values forA.auriculiformis(699.84 g/cm3)were followed by the hybrid(633.55%g/cm3)andA.mangium(584.0 g/cm3)(Table 3).Generally,heartwood showed highermean stem densitiescompared to sapwood(Table 2).Mean stem density was higher than 0.50 g/cm3indicating thatAcaciaare hardwood species with the hybrid of intermediate wood density.

Fibre length was signi fi cantly different within and between species(Table 2).Interestingly,the hybrid of the two species showed signi fi cantly longer fi bres than both parents(Table 2).Among the parental species,A.auriculiformis(0.945±0.03)had signi fi cantly longer fi bre lengths compared toA.mangium(0.848±0.02),both in the sapwood and heartwood.Linear comparison analysis between different traits,i.e.,for lignin content,mean stem density and fi bre length for both sapwood and heartwood is summarized in Table 4.A positive and signi fi cantly high correlation was observed between sapwood and heartwood for all three traits.Linear relationship revealed that both sapwood and heartwood shared similar traits for lignin,mean stem density and fi bre length.However,correlation comparison among the three traits showed different levels of relationship.Generally,correlations between all traits,lignin content,mean stem density,and fi bre lengths for sapwood and heartwood were high and positively correlated,revealing that the transition wood was not different for these components(Table 4).Correlations between lignin in sapwood and heartwood were negative with mean stem density of the sapwood(-0.699,-0.719),and similarly with mean stem density in the heartwood(-0.202,-0.231)respectively(Table 4).Small but positive correlations were observed between mean stem densities of both sapwood and heartwood and fi bre length sapwood and heartwood(Table 4).The fi bre length of theAcaciahybrid was superior to that of the parental species.Sapwood tissues generally have longer fi bre lengths compared to heartwood tissues.Fibre length is positively correlated with diameter at breast height in poplars(Jiang et al.1994).Fibre length is an important trait because it has major effects on both yield and quality of pulp and wood products(Kim et al.2009;Macdonald and Hubert 2002).Wood with longer fi bre is associated with better tearing,burst and tensile strength in paper for greater pulp yields(Kim et al.2009;Macdonald and Hubert 2002).

Fast-growing species have a signi fi cant impact on the quality of wood produced.The present study demonstrates that lignin content and mean stem density of these species vary greatly within different regions of the trunk.Hence,more replicates on mean stem densities and lignin content measurements were conducted to obtain a convergence value to represent the wood characteristics investigated.Fast-growing species have a high proportion of low density juvenile wood(Maeglin 1987).An imbalance in the proportion of low density juvenile wood in fast-growing species contributes to the imbalance in the lignin deposition across various sections of the trunk(Maeglin 1987).A.mangiumis generally regarded as the species that gives lower pulp yields thanA.auriculiformisand their hybrid.Ho et al.(1999)reported a loss of volume of about 7.5% of the total log volume due to heart rot.Higher amount of lignin content and wood defects inA.mangiumwarrant timely genetic improvement in order to improve the wood quality of this fast-growing species.The presence of low density juvenile wood is an undesirable property for both wood strength and pulp yield(Hai 2009).Imbalance in the proportion of low density juvenile wood raises concerns on the wood utility in timber from intensively managed forests compared to slow-growing species and mature natural stands(Zobel and van Buijtenen 1989).From an anatomical point of view,wood properties of a species greatly affect the pulp yield.The presence of large heart rot inA.mangiumis an indicative of defective wood(Fig.1).Higher amounts of lignin and wood defects suggest thatgenetic improvement is needed to improve the wood quality of this fast-growing species.The presence of low density juvenile wood is an undesirable property for both wood strength and pulp yield(Hai 2009).Variation in wood properties(mean stem density,lignin and fi bre length)from pith to sapwood tissues in theseAcaciaspecies suggests more studies are needed in order to improve wood quality.It is interesting that lignin content is signi fi cantly and negatively correlated with mean stem density.Moreover,mean stem density is relatively easier to measure than lignin content,therefore selection for high mean stem density will be more ef fi cient to indirectly select for low lignin content.Interspeci fi c hybrids show better wood quality than parents and thus have a great potential for pulp production.BreedingAcaciahybrids with better tree form,disease resistance and low lignin content can help increase yield and quality of pulp(Asif et al.2016).The utilization of DNA markers can expedite the process of selecting superior hybrid genotypes from a breeding population(Asif et al.2015,2017).

Table 4 Pearson correlation coef fi cients between lignin,mean stem density and fi bre length traits

AcknowledgementsWe wish to thank Dr.Koh Mok Poh,Puan Salamah Selamat and Puan Zaiton Saad for access to the Wood Chemistry Laboratory,Forest Research Institute Malaysia(FRIM)to conduct lignin analysis.

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