Low temperature combined with low light (LL) affects crop production, especially the yield and quality of peppers, in northwest China during the winter and spring seasons. Zeaxanthin (Z) is a known lipid protectant and active oxygen scavenger. However, whether exogenous Z can mitigate LL-induced inhibition of photosynthesis and oxidative stress in peppers remains unclear. In this study, we investigated the effects of exogenous Z on photosynthesis and the antioxidant machinery of pepper seedlings subject to LL stress. The results showed that the growth and photosynthesis of pepper seedlings were significantly inhibited by LL stress. In addition, the antioxidant machinery was disturbed by the uneven production and elimination of reactive oxygen species (ROS), which resulted in damage to the pepper. For example, membrane lipid peroxidation increased ROS content, and so on. However, exogenous application of Z before LL stress significantly increased the plant height, stem diameter, net photosynthetic rate (Pn), and stomata, which were obviously closed at LL. The activities of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), mono de-hydroascorbate reductase (MDHAR), de-hydroascorbate reductase (DHAR), ascorbate peroxidase (APX), and ascorbate oxidase (AAO) improved significantly due to the increased expression of CaSOD, CaCAT, CaAPX, CaMDHAR, and CaDHAR. The ascorbic (AsA) and glutathione (GSH) contents and ascorbic/dehydroascorbate (AsA/DHA) and glutathione/oxidized glutathione (GSH/GSSG) ratios also increased significantly, resulting in the effective removal of hydrogen peroxide (H₂O₂) and superoxide anions (O₂^•−) caused by LL stress. Thus, pre-treatment with Z significantly reduced ROS accumulation in pepper seedlings under LL stress by enhancing the activity of antioxidant enzymes and accumulation of components of the ascorbate–glutathione (AsA–GSH) cycle and upregulated key genes in the AsA–GSH cycle.

To determine the optimum parameters for extracting three carotenoids including zeaxanthin, lutein epoxide, and violaxanthin from pepper leaves by response surface methodology (RSM), a solvent of acetone and ethyl acetate (1:2) was used to extract carotenoids with four independent factors: ultrasound time (20–60 min); ratio of sample to solvent (1:12–1:4); saponification time (10–50 min); and concentration of saponification solution (KOH–methanol) (10–30%). A second-order polynomial model produced a satisfactory fitting of the experimental data with regard to zeaxanthin (R² = 75.95%, p < 0.0197), lutein epoxide (R² = 90.24%, p < 0.0001), and violaxanthin (R² = 73.84%, p < 0.0809) content. The optimum joint extraction conditions of zeaxanthin, lutein epoxide, and violaxanthin were 40 min, 1:8, 32 min, and 20%, respectively. The optimal predicted contents for zeaxanthin (0.823022 µg/g DW), lutein epoxide (4.03684 µg/g dry; DW—dry weight), and violaxanthin (16.1972 µg/g DW) in extraction had little difference with the actual experimental values obtained under the optimum extraction conditions for each response: zeaxanthin (0.8118 µg/g DW), lutein epoxide (3.9497 µg/g DW), and violaxanthin (16.1590 µg/g DW), which provides a theoretical basis and method for cultivating new varieties at low temperatures and weak light resistance.

The Mass Ratio Hypothesis states that the growth dynamics of dominant species influence forest species diversity by regulating the niches of subordinate and transient species. However, this prediction has not yet been empirical confirmed in subtropical forests over long term. Using data from 1995 to 2017, we examined how dominant tree species regulate species evenness and richness by analyzing their height and diameter growth in three clear-cut forests (Castanopsis carlesii (Hemsl.) Hayata, Castanopsis fissa (Champ. ex Benth.) Rehder & E. H. Wilson, and Cunninghamia lanceolata (Lamb.) Hook. stands), combined with the mean value of species niche breadth (measures the diversity of resources a species utilizes) across the community, including separate analyzes for subordinate (persistently coexisting with dominants species) and transient species (temporarily occurring species). Our results showed that an increase in height and diameter growth of dominant species had a negative effect on niche breadth of subordinate species, which in turn reduced species evenness (p < 0.01) but showed no significant relationship with species richness (p ≥ 0.05). Growth dynamics of dominants had no significant influence on the niche breadth of transient species. The early-fast growing dominant C. lanceolata significantly restricted the niche breadth of subordinate species (1.16 ± 0.23), resulting in relatively low evenness (0.49 ± 0.11). Conversely, the late-fast growing dominant C. carlesii promoted niche expansion (6.62 ± 1.20), resulting in higher evenness (0.81 ± 0.02). C. fissa -dominated strands with intermediate growth increments, exhibited moderate species evenness. These findings provide long-term empirical support for the Mass Ratio Hypothesis by demonstrating that growths of dominant species modulate niche partitioning in subordinates and thereby shape species diversity in subtropical forest communities.