Which structural carbohydrate component increases the indigestibility of older plants?

The increase in indigestibility in older plants is primarily due to lignin. Lignin is a complex organic polymer found in the cell walls of many plants, and it serves several important structural roles. As plants mature, the amount of lignin in their tissues increases, which enhances their strength and rigidity. This increased lignification makes the plant material tougher and more resistant to decomposition and digestion by microorganisms in the digestive systems of herbivores.

In contrast, cellulose is a major component of plant cell walls and contributes to structural integrity but is more digestible than lignin when considering how much it impedes digestion. Hemicellulose, while also a structural carbohydrate, typically has lower molecular weight and is more readily digestible compared to lignin. Starch is a storage carbohydrate that is highly digestible for animals, especially in younger plants, and does not contribute to the indigestibility associated with older plant materials. Therefore, the presence of lignin is the primary factor that increases indigestibility in older plants, making it the correct choice in this context.