Cold stress seriously inhibits plant growth and development, geographical distribution, and yield stability of plants. Cold acclimation (CA) is an important strategy for modulating cold stress, but the mechanism by which CA induces plant resistance to cold stress is still not clear. The purpose of this study was to investigate the effect of CA treatment on the cold resistance of citrus seedlings under cold stress treatment, and to use seedlings without CA treatment as the control (NA). The results revealed that CA treatment increased the content of photosynthetic pigments under cold stress, whereas cold stress greatly reduced the value of gas exchange parameters. CA treatment also promoted the activity of Rubisco and FBPase, as well as led to an upregulation of the transcription levels of photosynthetic related genes (rbcL and rbcS)，compared to the NA group without cold stress. In addition, cold stress profoundly reduced photochemical chemistry of photosystem II (PSII), especially the maximum quantum efficiency (F_v/F_m) in PSII. Conversely, CA treatment improved the chlorophyll a fluorescence parameters, thereby improving electron transfer efficiency. Moreover, under cold stress, CA treatment alleviated oxidative stress damage to cell membranes by inhibiting the concentration of H₂O₂ and MDA, enhancing the activities of superoxide dismutase (SOD), catalase (CAT), ascorbic acid peroxidase (APX) and glutathione reductase (GR), accompanied by an increase in the expression level of antioxidant enzyme genes (CuZnSOD1, CAT1, APX and GR). Additionally, CA also increased the contents of abscisic acid (ABA) and salicylic acid (SA) in plants under cold stress. Overall, we concluded that CA treatment suppressed the negative effects of cold stress by enhancing photosynthetic performance, antioxidant enzymes functions and plant hormones contents.

High temperatures significantly injure the flowering, pollination, fruit growth, and quality of plants. Photosynthesis, the fundamental process supporting plant life, is crucial. Nevertheless, the quantitative evaluation of the physiological activity of the photosynthetic system of Nanfeng tangerine (NT) plants under high-temperature conditions remains a challenge. This research utilized NT plants, a distinctive citrus variety in Jiangxi Province, as the experimental subject. The study investigated the effects of varying degrees of high-temperature stress and duration on 16 photosynthetic physiological parameters of NT plants. The study examined the impact of four varying high-temperature treatment levels (32/22 °C, 35/25 °C, 38/28 °C, and 41/31 °C) for durations of 2, 4, 6, and 8 days, respectively. Principal component analysis was utilized to identify the key indicators of photosynthetic physiological activity in NT plants, with F_v/F_m, P_max, LCP, H₂O₂, MDA, and POD being selected as key parameters. The high-temperature stress index model previously constructed was used to calculate the high-temperature stress index value of the NT plants exposed to varying degrees and durations of high temperature, in order to provide a comprehensive assessment of the photosynthetic system of NT plants under high-temperature stress. Subsequently, the high-temperature stress levels were categorized into five levels based on the calculated values: Level 0 for 0 < HSI ≤ 2, Level 1 for 2 < HSI ≤ 4, Level 2 for 4 < HSI ≤ 6, Level 3 for 6 < HSI ≤ 8, and Level 4 for HSI > 8. The research results provide valuable data for agricultural meteorological departments to carry out disaster risk zoning and risk assessment in the future.

Low-temperature freezing stress constitutes the most significant meteorological disaster during the overwintering period in the Nanfeng Tangerine (NT) production area, severely impacting the normal growth and development of the plants. Currently, the accuracy of meteorological disaster warnings and forecasts for NT orchards remains suboptimal, primarily due to the absence of quantitative meteorological indicators for low-temperature freezing stress. Therefore, this study employed NT plants as experimental subjects and conducted controlled treatment experiments under varying intensities of low-temperature freezing stress (0 °C, −2 °C, −5 °C, −7 °C, and −9 °C) and durations (1 h, 4 h, and 7 h). Subsequently, physiological and biochemical parameters were measured, including photosynthetic parameters, chlorophyll fluorescence parameters, reactive oxygen species, osmoregulatory substances, and antioxidant enzyme activities in NT plants. The results demonstrated that low-temperature freezing stress adversely affected the photosynthetic system of NT plants, disrupted the dynamic equilibrium of the antioxidant system, and compromised cellular stability. The severity of freezing damage increased with decreasing temperature and prolonged exposure. Chlorophyll (a/b) ratio (Chl (a/b)), maximum quantum yield of photosystem II (F_v/F_m), soluble sugar, and malondialdehyde (MDA) were identified as key indicators for assessing physiological and biochemical changes in NT plants. Utilizing these four parameters, a comprehensive score (CS) model of freezing damage was developed to quantitatively evaluate the growth status of NT plants across varying low-temperature freezing damage gradients and durations. Subsequently, the freezing damage grade index for NT plants during the overwintering period was established. Specifically, Level 1 for CS ≤ −0.50, Level 2 for −0.5 < CS ≤ 0, Level 3 for 0 < CS ≤ 0.5, and Level 4 for 0.5 < CS. The research results provide valuable data for agricultural meteorological departments to carry out disaster monitoring, early warning, and prevention and control.

The mandarin is an important fruit crop worldwide, and ‘Xinyu mandarin’—a local Citrus reticulata variety—is widely cultivated in Jiangxi Province, in China. Autopolyploidy has frequently been used for the improvement of crop varieties. In a previous study, we identified a tetraploid (4X) material of ‘Xinyu mandarin’ generated from its diploid (2X) mother seedling via a spontaneous mutation. However, the physiological and metabolic variations after polyploidy are not clear enough, which is not conducive to the early screening. In the present study, we analyzed the morphological, physiological, and metabolic differences between the tetraploid and diploid plants. The tetraploids had larger and thicker leaves, and the activities of key enzymes in photosynthesis, the total chlorophyll, phenolic, and proline contents, were all higher in tetraploids. In the non-targeted metabolomic profile, a total of 522 metabolites were identified, of which 61 were significantly different between diploids and tetraploids. The differential metabolites included similar proportions of primary and secondary metabolites, and most of these were up-regulated in tetraploids, especially stress-related metabolites such as phenolic acids, flavonoids, alkaloids, and so on, indicating that tetraploids may possess better stress tolerance ability and vigor. Therefore, the tetraploids of ‘Xinyu mandarin’ may serve as an excellent female parent for the improvement of citrus varieties.