Comparative Physiological Analysis of Lignification, Anthocyanin Metabolism and Correlated Gene Expression in Red <i>Toona sinensis</i> Buds during Cold Storage

The characteristics of anthocyanin and lignin are important parameters in evaluating the quality of red <i>Toona sinensis</i> buds. Red <i>T. sinensis</i> buds are prone to senescence during postharvest storage, which subsequently affects their quality and sales. However, the mechanism of senescence in red <i>T. sinensis</i> buds under low-temperature conditions remains unclear. In this study, red <i>T. sinensis</i> buds were stored at 4 °C, and their anthocyanin and lignin contents as well as the enzyme activities of PAL (phenylalanine ammonia lyase), 4CL (4-coumarate-CoA ligase), CAD (cinnamyl alcohol dehydrogenase) and POD (peroxidase) were determined at 0, 1, 2 and 3 d after handing. Meanwhile, the cellular structure of postharvest red <i>T. sinensis</i> buds was observed by microscopy. The relative expression of lignin-related and anthocyanin-related genes was analyzed using qRT-PCR. The results show that the anthocyanin content of the leaves was higher than that of the petioles. After 3 d of storage, the anthocyanin content of the leaves was 4.66 times that of the petioles. Moreover, the lignin content of the red <i>T. sinensis</i> buds gradually increased. Compared with 0 d, the lignin content of the leaves and petioles increased by 331.8 and 94.14 mg·g⁻¹, respectively. The enzyme activities of PAL, 4CL, CAD and POD increased during cold storage. The intercellular space and the arrangement of the palisade tissue and sponge tissue in the mesophyll of red <i>T. sinensis</i> buds became smaller and closer, respectively. The secondary cell wall of xylem cells thickened, the number of xylem cells increased, and the arrangement number of the xylem cells became closed in the leaf vein and petioles during red <i>T. sinensis</i> bud storage. The expression levels of anthocyanin-related (Except for <i>TsCHS</i> and <i>TsANS</i>) and lignin-related genes increased during red <i>T. sinensis</i> bud storage and are highly consistent with the accumulation patterns of anthocyanins and lignin. This study may serve as a reference for exploring the molecular mechanisms of senescence, regulating the quality and cultivating new varieties of red <i>T. sinensis</i> buds that have low lignin content but high anthocyanin content after harvest.