Ripening-related perturbations in apple cell wall nuclear spin dynamics

Spin-lattice (¹HT₁, ²³Na⁺T₁) and rotating frame spin-lattice (¹HT_1p, ¹³CT_1p) relaxation times were measured on intact, critical point dried apple tissue at various degrees of ripeness using cross polarization and magic angle spinning (CPMAS) NMR techniques. Solid state carbonyl (δ172)¹³CT_1p and ²³Na⁺-carboxylate anion T₁ values, which are inversely proportional to carboxylate reorientation rates, decreased 15–19% during the time course study. Carbonyl resonance ¹HT₁s diminished by 63% as the tissue softened; a maximal decline of 42% was also observed in the ¹HT₁s of nonspecific carbohydrate ring carbon signals (δ74) indicating an increase in both acidic and neutral polymer motion. Treatment of the cell wall with polygalacturonase resulted in a significant decrease in both carbonyl and ring carbon ¹HT₁s (57 and 42%, respectively) demonstrating the important structural function of polyuronides not only in the middle lamellae but also in the primary cell wall.