Differentiating Leaf Structures and Physiological Responses to Freezing Stress of Mangrove Kandelia obovata from Different Provenances

Kandelia obovata (Rhizophoraceae) is the most cold-tolerant mangrove species and has been widely used in coastal wetland ecological restoration due to its specific viviparous phenomenon, beautiful shape, and unique floral pattern. Due to long-term adaptation to their local environment, the phenotypic characteristics and stress resistance of widely distributed plants of the same species often differentiate across different locations. The capacity for cold resistance is closely linked to the physiological and structural characteristics of plants. Herein, we explored the temporal variations in the leaf structure and physiological status of K. obovata under -5.5 C-degrees from different areas such as Jiulongjiang Estuary (JLJ, 24(degrees)25' N), Fujian Province, and Longgang City (LG, 27(degrees)34' N) and Jiaojiang District (JJ, 28(degrees)67' N), Zhejiang Province. The morphological variations implied that the cold resistance of K. obovata obviously strengthened after the northward migration and acclimatization, in the following order: LG > JJ > JLJ. More specifically, after exposure to a sustained low temperature, the relative conductivity (REC), an index widely used to evaluate the degree of plant damage, remarkably increased from 33.62 +/- 2.39 to 63.73 +/- 3.81, 31.20 +/- 1.63 to 49.48 +/- 1.12, and 23.75 +/- 0.13 to 54.24 +/- 1.45 for JLJ, LG, and JJ, respectively (p < 0.05). Additionally, the palisade-to-spongy tissue ratio (P/I) of JLJ and JJ decreased from 0.78 +/- 0.05 and 0.75 +/- 0.03 to 0.5 +/- 0.04 and 0.64 +/- 0.02 (p < 0.05), whereas no significant changes were found in LG (p > 0.05). The SOD activity of LG significantly kept increasing, with values increased from 352.49 +/- 10.38 to 477.65 +/- 1.78 U center dot g(-1), whereas no apparent changes in JLJ and JJ were observed with the sustained low temperature. The results of this study improved our understanding of the response of K. obovata to freezing stress, which could provide a sound theoretical foundation for cultivating cold-resistant varieties, as well as expanding mangrove plantations in higher latitudes.