Cold hardiness evaluation and physiological mechanism elucidation in apricot-plum hybrid (Prunus salicina × P. armeniaca) germplasm

【Objective】The Prunus salicina × P. armeniaca hybrid is a novel interspecific hybrid derived from the cross between plum (P. salicina) and apricot (P. armeniaca), renowned for its exceptional fruit quality and economic value. P. salicina × P. armeniaca hybrid trees are widely planted in Xinjiang, where they are known as the climatically marginal zone for economic cultivation due to its harsh cold winters characterized by prolonged subzero temperatures. In recent years, recurrent freezing injuries have severely constrained the sustainable development of P. salicina × P. armeniaca hybrid plantations, positioning low- temperature stress as a critical abiotic factor limiting its cultivation. However, systematic investigations into cold tolerance evaluation and physiological response mechanisms of thishybrid fruit crop remain scarce. This study aimed to evaluate the cold tolerance capacity of P. salicina × P. armeniaca hybrid germplasms and elucidate the physiological mechanisms underlying genotype-specific responses to low-temperature stress, thereby facilitating the breeding of cold- resistant varieties. 【Methods】Ten P. salicina × P. armeniaca hybrid genotypes comprising six commercial cultivars and four elite lines selected through our previous efforts were used for this study. In the winter of 2023, their annual dormant branches were collected and were subjected to controlled low-temperature treatments at - 10, - 15, - 20, - 25 and - 30 ℃ for 12 h, respectively. The relative electrical conductivity (REC) at different temperatures was measured immediately, and the half-lethal temperature (LT50) was calculated by combining the logistic equation to identify their cold tolerance. Furthermore, comparative physiological analysis was conducted on two strong cold-tolerant genotypes (Weiwang and XL9) and two cold-sensitive cultivars (Konglongdan and Weidi). The malondialdehyde (MDA) content was measured using thiobarbituric acid method. The determination of soluble sugar (SS) content was carried out using the anthrone method. The proline (Pro) content was determined using the indene ketone colorimetric method. In addition, the superoxide anion (O2 - ·) content, hydrogen peroxide (H2O2) content, superoxide dismutase (SOD) activity, and peroxidase (POD) activity were determined using commercial reagent kits purchased from Suzhou Mengxi Biomedical Technology Co., Ltd. Finally, their cold tolerance was comprehensively evaluated through membership function analysis.【Results】With the decreasing of cold treatment temperature, the relative conductivity of the annual dormant branches of 10 P. salicina × P. armeniaca hybrid genotypes gradually increased, with LT50 values ranging from - 23.02 ℃ to -31.67 ℃, and the fit of the logistic equation for each genotype was R2 between 0.822 and 0.996. Based on these data, their cold tolerance is divided into three levels: strong cold tolerance (LT50＜ -30 ℃, including Weiwang and XL9), moderate cold tolerance (-30 ℃＜LT50＜-25 ℃, including Fengweihuanghou, Fengweimeigui and XL8), and low temperature sensitivity (LT50＞-25 ℃, including Weihou, Weidi, Konglongdan, XL7 and XL10). Two strong cold-tolerant genotypes (Weiwang and XL9) and low-temperature sensitive genotypes (Konglongdan and Weidi) were selected to comparatively analyze the physiological response mechanism to low-temperature stress. Physiological responses diverged significantly between cold-tolerant and sensitive genotypes. In Weiwang and XL9, antioxidant enzymes (SOD and POD) and osmolytes (SS and Pro) displayed coordinated regulation, peaking at intermediate temperatures (- 20 ℃ to - 25 ℃ ) before declining, suggesting adaptive stress mitigation. Conversely, sensitive genotypes exhibited progressive oxidative damage, evidenced by linearly increasing O2 - · , H2O2, and MDA levels (up to 195.4% increase in H2O2 at -30 ℃), coupled with declining SS (44.2% reduction) and Pro contents, which indicated the oxidative damage they suffered increased gradually and the cell osmotic balance was disrupted. Additionally, SOD and POD activity of Konglongdan and Weidi fluctuated significantly. Membership function analysis confirmed the cold tolerance hierarchy: XL9＞ Weiwang＞Konglongdan＞Weidi, aligning with LT50 rankings.【Conclusion】LT50 serves as a reliable indicator for cold tolerance evaluation in P. salicina × P. armeniaca hybrids. The superior cold tolerance of Weiwang and XL9 primarily stems from enhanced antioxidant capacity and osmotic adjustment in dormant branches. These findings provide valuable germplasm resources for cold-tolerant breeding programs and offer theoretical guidance for the introduction and cultivation of P. salicina × P. armeniaca hybrids in cold-prone regions.