Rare earth elements (REEs) can affect the growth and development of plants. However, few studies have been carried out on the effects of REEs on citrus seedlings. In this study, the growth parameters, toxicity symptoms, chlorophyll content, and La content of three citrus rootstocks are analyzed under different concentrations of La, a representative REE. The results show that the growth of citrus rootstock seedlings was stimulated at La ≤ 0.5 mmol·L⁻¹ and inhibited at concentrations above 1 mmol·L⁻¹. The chlorophyll and carotenoid contents of trifoliate orange (Poncirus trifoliata L. Raf.) and Ziyang Xiangcheng (C. junos Sieb. ex Tanaka) leaves of plants grown at low concentrations of La (≤1.5 mmol·L⁻¹) were similar to those of the control but were significantly reduced at 4 mmol·L⁻¹ La. Toxic symptoms gradually appeared with increasing La concentrations, with yellowed leaves and burst veins appearing at 4 mmol·L⁻¹ La. The symptoms of toxicity were most severe in trifoliate orange, followed by Shatian Pomelo (Citrus grandis var. shatinyu Hort) and then Ziyang Xiangcheng. Moreover, in leaves, the Ca content was significantly negatively correlated with La content (p < 0.01). These results indicate that La has a hormesis effect on the growth of citrus rootstocks. Of the studied citrus seedlings, Ziyang Xiangcheng is the most resistant to La.

The Florida citrus industry is going through major changes and considerable replanting. Many growers are planting varieties, especially rootstocks, that are potentially productive under Huanglongbing (HLB; Candidatus Liberibacter asiaticus) prevalent conditions. However, the high demand for new plant material has put tremendous pressure on citrus nurseries and has created a bottleneck in production. Often it can take more than one year to produce field-transplant ready plants in nurseries; therefore, there is a critical need to accelerate plant production. This three-part study was conducted with the following objectives: (1) determine citrus nursery trends; (2) evaluate use of compost for rootstock germination; and, (3) evaluate use of compost for rootstock seedling growth. According to the nursery survey, rootstock seed germination and seedling growth were the most time-consuming, taking 6–8 months. Moreover, it was apparent from the survey results that 44% of the citrus nurseries were experimenting with potting mixes to achieve adequate plant growth and quality. Our greenhouse study demonstrated successful use of yard waste compost in place of peat moss in a potting mix. With use of 37% to 50% of compost in the potting mix, the overall germination rate and mean emergence time were improved to 70% in less than four weeks for US-897 rootstock as compared to no compost in the potting mix. In addition, 37% to 50% compost resulted in higher biomass accumulation in seedlings. When seedlings of rootstocks C-32 and Flying Dragon were grown with 37% to 50% compost, the growth rate and, therefore, percentage of successful budding were significantly increased as compared to no compost. In addition, substrate analysis indicated that a high compost potting media was rich in mineral nutrients, hence the use of fertilizer in nurseries could be minimized. Altogether, use of compost in place of peat moss seems promising and could accelerate germination and growth of rootstocks, reducing the production time as well as cost.

Boron (B) is an essential trace nutrient element for citrus, but excessive B levels are frequently encountered in citrus production, potentially resulting in citrus toxicity. To better understand the tolerance and physiological responses of citrus rootstocks to excess B levels, Trifoliate orange, Ziyangxiangcheng, Carrizo citrange, and Red tangerine were treated with four B concentrations (0.05, 0.2, 0.8, and 3.2 mmol/L). High B concentrations resulted in leaf yellowing and shedding and eventual plant death. Chlorophyll content and photosynthetic capacity declined in response to high B concentrations, and relative leaf cell conductivity rose significantly. Trifoliate orange was the first to exhibit symptoms of B toxicity, with the highest levels of B-associated injury. As B concentrations rose, the height increment ratio declined, as did belowground and aboveground dry fresh weight. Soluble protein content initially rose and then fell, while proline content, SOD activity, and POD activity rose with B concentrations. B levels in these rootstocks also increased significantly, with the greatest increases in the leaves. Principal component analysis and subordinate function results revealed that the relative rank order for the B tolerance of citrus rootstocks was: Red tangerine > Carrizo citrange > Ziyangxiangcheng > Trifoliate orange.

‘Albedo bluing’ of fruits occurs in many varieties of citrus, resulting in a significant reduction in their commercial value. We first presented a breakthrough method for successfully extracting and purifying the ‘albedo bluing’ substance (ABS) from citrus fruits, resulting in the attainment of highly purified ABS. Then, HPLC and UPLC-QTOF-MS were used to prove that ABS in the fruits of three citrus varieties (Citrus reticulate Blanco cv. ‘Gonggan’, ‘Orah’, and ‘Mashuiju’) are identical. However, the chemical structure of ABS remains elusive for many reasons. Fortunately, a more stable derivative of ABS (ABS-D) was successfully obtained. Through various analytical techniques such as HRESIMS, 1D and 2D NMR, and chemical shift calculation, ABS-D was identified as 2,4-dihydroxy-6-(β-D-glucopyranosyloxy)phenyl(5,6-dihydroxy-7-(β-D-glucopyranosyloxy)benzo[d]thiazol-2-yl)methanone, indicating that both ABS and its derivative belong to a rare category of benzothiazole glucosides. Furthermore, both ABS and ABS-D demonstrated potent antioxidant abilities. These findings lay the groundwork for further elucidating the chemical structure of ABS and the causative mechanism of the ‘albedo bluing’ phenomenon in citrus fruits.

An issue of common concern in lemon production is finding a safe and efficient alternative to 2,4-dichlorophenoxyacetic acid (2,4-D). In this study, ‘Eureka’ lemon fruits were treated with three concentrations (1, 2 and 3) of fluroxypyr-meptyl (FME), a combination of fluroxypyr-meptyl and fluroxypyr (FLFM), 2,4-dichlorophenoxypropionic acid (2,4-DP), and 2-methyl-4-chlorophenoxyacetic acid (MCPA). Water and 2,4-D served as controls. We measured the storage performance indicators of fruit, such as weight loss rate and decay rate, and shelf-life quality parameters, such as juice yield, flavor compounds and pesticide residues. During storage, weight loss was significantly higher in water than under all other treatments. Weight loss rates under FME2 and 2,4-DP3 were significantly lower than under 2,4-D. Decay rates in FLFM1, 2,4-DP2, and the FME series were significantly lower than in 2,4-D and water, whereas those in 2,4-DP1 and the MCPA series were significantly higher than in 2,4-D at 200 days after treatment. Regarding shelf-life performance, juice yield in water (65.14%) and 2,4-D (68.26%) was significantly lower than under most other treatments. The highest juice yield was observed in FME2 (77.84%). Treatments 2,4-DP1, 2,4-DP2, and FME2 were superior to 2,4-D and water in maintaining total soluble solids, titratable acid, and vitamin C contents, while other treatments showed no negative effects on internal quality. Fruits under MCPA2, 2,4-DP3, 2,4-DP2, and FME2 maintained better flavor compound profiles than those in water. Notably, MCPA2 resulted in significantly higher levels of terpenes (e.g., D-limonene) and aldehydes (e.g., citral); FME2 effectively maintained linalool, geraniol, and α-terpineol; and 2,4-DP3 performed well in maintaining D-limonene, sesquiterpenes, and alcohols compared with other treatments. All treated fruits complied with Chinese National Food Safety Standard Maximum Residue Limits for Pesticides in Food GB 2763-2026 and meet the EU standard limits for citrus. Overall, FME2 treatment resulted in the best storage performance and quality, followed by 2,4-DP3, indicating that these treatments may serve as effective postharvest alternatives for lemon preservation.