Effects of Low-Temperature Stress on Physiological Characteristics and Microstructure of Stems and Leaves of Pinus massoniana L.

<i>Pinus massoniana</i> L. is one of the most important conifer species in southern China and is the mainstay of the forest ecosystem and timber production, yet low temperatures limit its growth and geographical distribution. This study used 30-day-old seedlings from families of varying cold-tolerance to examine the morphological traits of needles and stems, chlorophyll fluorescence characteristics, protective enzymes, and changes in starch and lignin under different low-temperature stresses in an artificial climate chamber. The results showed that the seedlings of <i>Pinus massoniana</i> exhibited changes in phenotypic morphology and tissue structure under low-temperature stress. Physiological and biochemical indexes such as protective enzymes, osmoregulatory substances, starch, and lignin responded to low-temperature stress. The cold-tolerant family increased soluble sugars, starch grain, and lignin content as well as peroxidase activity, and decreased malondialdehyde content by increasing the levels of actual photochemical efficiency (<i>ΦPSII</i>), electron transport rate (ETR), and photochemical quenching (<i>qP</i>) to improve the cold tolerance ability. This study provides a reference for the selection and breeding of cold-tolerant genetic resources of <i>Pinus massoniana</i> and the mechanism of cold-tolerance, as well as the analysis of the mechanism of adaptation of <i>Pinus massoniana</i> in different climatic regions of China.