Please find some links and PDFs ..

Lignin content and chemical characteristics in ... - SpringerLink

link.springer.com/article/10.1007/s11104-011-0725-y

Lignin content and chemical characteristics in maize and wheat vary between plant organs and growth stages: consequences for assessing lignin dynamics in soil

Differences in Anatomical Structure and Lignin Content of ...

link.springer.com/article/10.1023/B:BIOP...

Differences in Anatomical Structure and Lignin Content of Roots of pedunculate Oak and Wild Cherry-Tree Plantlets During Acclimation

The Compact Root Architecture1 Gene Regulates Lignification, Flavonoid Production, and Polar Auxin Transport in Medicago truncatula ( DOI: https://doi.org/10.1104/pp.110.156620)

The root system architecture is crucial to adapt plant growth to changing soil environmental conditions and consequently to maintain crop yield. In addition to root branching through lateral roots, legumes can develop another organ, the nitrogen-fixing nodule, upon a symbiotic bacterial interaction. A mutant, cra1, showing compact root architecture was identified in the model legume Medicago truncatula. cra1 roots were short and thick due to defects in cell elongation, whereas densities of lateral roots and symbiotic nodules were similar to the wild type. Grafting experiments showed that a lengthened life cycle in cra1 was due to the smaller root system and not to the pleiotropic shoot phenotypes observed in the mutant. Analysis of the cra1 transcriptome at a similar early developmental stage revealed few significant changes, mainly related to cell wall metabolism. The most down-regulated gene in the cra1 mutant encodes a Caffeic Acid O-Methyl Transferase, an enzyme involved in lignin biosynthesis; accordingly, whole lignin content was decreased in cra1 roots. This correlated with differential accumulation of specific flavonoids and decreased polar auxin transport in cra1 mutants. Exogenous application of the isoflavone formononetin to wild-type plants mimicked the cra1 root phenotype, whereas decreasing flavonoid content through silencing chalcone synthases restored the polar auxin transport capacity of the cra1 mutant. The CRA1 gene, therefore, may control legume root growth through the regulation of lignin and flavonoid profiles, leading to changes in polar auxin transport...

Plant lignin and nitrogen contents control carbon dioxide production and

nitrogen mineralization in soils incubated with Bt and non-Bt corn residues..PDF enclosed for  further reading...

Which crop , are you talking about..??.Lignin content will vary greatly according to age of the crop.Lignin content of perennial crop  roots will be much higher than cereals...

I am talking in specifically about orange root, but I didn't find. So I'm looking over other trees, like Pinus.

I need of lignin content in orange roots to understand the root's dynamics.

You may go for NREL Lap Protocols for determination

Please find some links and PDFs ..

Lignin content and chemical characteristics in ... - SpringerLink

link.springer.com/article/10.1007/s11104-011-0725-y

Lignin content and chemical characteristics in maize and wheat vary between plant organs and growth stages: consequences for assessing lignin dynamics in soil

Differences in Anatomical Structure and Lignin Content of ...

link.springer.com/article/10.1023/B:BIOP...

Differences in Anatomical Structure and Lignin Content of Roots of pedunculate Oak and Wild Cherry-Tree Plantlets During Acclimation

The Compact Root Architecture1 Gene Regulates Lignification, Flavonoid Production, and Polar Auxin Transport in Medicago truncatula ( DOI: https://doi.org/10.1104/pp.110.156620)

The root system architecture is crucial to adapt plant growth to changing soil environmental conditions and consequently to maintain crop yield. In addition to root branching through lateral roots, legumes can develop another organ, the nitrogen-fixing nodule, upon a symbiotic bacterial interaction. A mutant, cra1, showing compact root architecture was identified in the model legume Medicago truncatula. cra1 roots were short and thick due to defects in cell elongation, whereas densities of lateral roots and symbiotic nodules were similar to the wild type. Grafting experiments showed that a lengthened life cycle in cra1 was due to the smaller root system and not to the pleiotropic shoot phenotypes observed in the mutant. Analysis of the cra1 transcriptome at a similar early developmental stage revealed few significant changes, mainly related to cell wall metabolism. The most down-regulated gene in the cra1 mutant encodes a Caffeic Acid O-Methyl Transferase, an enzyme involved in lignin biosynthesis; accordingly, whole lignin content was decreased in cra1 roots. This correlated with differential accumulation of specific flavonoids and decreased polar auxin transport in cra1 mutants. Exogenous application of the isoflavone formononetin to wild-type plants mimicked the cra1 root phenotype, whereas decreasing flavonoid content through silencing chalcone synthases restored the polar auxin transport capacity of the cra1 mutant. The CRA1 gene, therefore, may control legume root growth through the regulation of lignin and flavonoid profiles, leading to changes in polar auxin transport...

Plant lignin and nitrogen contents control carbon dioxide production and

nitrogen mineralization in soils incubated with Bt and non-Bt corn residues..PDF enclosed for  further reading...

How the lignin and nitrogen concentration in roots affect the plant physiology ?