Adequate chill is of great importance for successful production of deciduous fruit trees. However, temperate fruit trees grown under tropical and subtropical regions may face insufficient winter chill, which has a crucial role in dormancy and productivity. The objective of this review is to discuss the challenges for dormancy and chilling requirements of temperate fruit trees, especially in warm winter regions, under climate change conditions. After defining climate change and dormancy, the effects of climate change on various parameters of temperate fruit trees are described. Then, dormancy breaking chemicals and organic compounds, as well as some aspects of the mechanism of dormancy breaking, are demonstrated. After this, the relationships between dormancy and chilling requirements are delineated and challenging aspects of chilling requirements in climate change conditions and in warm winter environments are demonstrated. Experts have sought to develop models for estimating chilling requirements and dormancy breaking in order to improve the adaption of temperate fruit trees under tropical and subtropical environments. Some of these models and their uses are described in the final section of this review. In conclusion, global warming has led to chill deficit during winter, which may become a limiting factor in the near future for the growth of temperate fruit trees in the tropics and subtropics. With the increasing rate of climate change, improvements in some managing tools (e.g., discovering new, more effective dormancy breaking organic compounds; breeding new, climate-smart cultivars in order to solve problems associated with dormancy and chilling requirements; and improving dormancy and chilling forecasting models) have the potential to solve the challenges of dormancy and chilling requirements for temperate fruit tree production in warm winter fruit tree growing regions.

This is the first report to study the impacts of potassium sources on apricot fruit yield, quality and storability as a preharvest foliar application. Five sources of potassium (K-humate, K-sulphate, K-nitrate, K-silicate and K-citrate), plus water as a control treatment, were applied individually at 0.2% three times on ‘Canino’ apricot over the 2019 and 2020 seasons. The results showed that all potassium salts, applied foliarly, have potential to improve yield, fruit color, and some fruit physical attributes, such as: weight, size and firmness, as well as a reduced lipid peroxidation, accompanied by a low fruit malondialdehyde content reflected in a high tolerance during storage. The K-nitrate treatment was more effective in the improvement of fruit yield, preharvest quality parameters and keeping fruit postharvest quality characteristics from sharp decline during cold storage. Concerning fruit anatomy, K-nitrate and K-citrate showed thicker cuticle and epidermal parenchyma cell diameters, while the K-silicate induced the highest cell wall thickness. K-nitrate was the most economical, and could be recommended for apricot growers in the Nubaria region of Egypt.

Ethylene plays a pivotal role in the climacteric fruit ripening and senescence process. The effect of three ethylene inhibitors on the yield, quality, and storability of ‘Canino’ apricot fruit was studied. Foliar sprays of distilled water (control), aminoethoxyvinylglycine (AVG) (150 and 100 mg·L⁻¹), salicylic acid (SA) (4 and 2 mM), and chitosan (2.5% and 1.5%) were applied 30 and 15 days before harvest. Results indicated that the high concentrations of AVG and SA recorded the lowest percentage of preharvest fruit drop and, hence, the highest yield. Trees receiving either concentration of AVG showed the highest fruit firmness. High concentrations of all three ethylene inhibitors reduced fruit weight loss, total carotenoids, and soluble solid content (SSC), but increased total acidity (TA) during cold storage (2 °C). A high score of overall taste acceptability was observed with a higher concentration of SA, which was also recorded the lowest fruit malondialdehyde content (MDA) at harvest and during storage. The highest concentrations of SA and chitosan recorded no decay for 28 days of storage. Gene expression analysis reflected higher expression of PaACS1 gene with the highest concentrations of ethylene inhibitors, suggesting that SA (4 mM) is recommended for optimal yield, quality, and storability of ‘Canino’ apricot fruit grown under Egyptian conditions.

A two-year experiment using ‘Murcott’ mandarin (Citrus reticulata) was conducted under deficit irrigation (DI) strategies with an anti-stressor application and then stored. The three DI regimes were 70% crop evapotranspiration (70% ET_C), 85% ETc, and full irrigation (100% ETc). Anti-stressor treatments consisted of a foliar application of either sodium nitroprusside (SNP), selenium nanoparticles (NanoSe), microalgae (Spirulina platensis), or a non-treated control. Mean water productivity values were highest for trees at 70% ETc and 85% ETc when they were sprayed with microalgae, which was 42% and 51% greater, respectively, compared to control trees at 100% ETc, but only 6.5% and 13% greater, respectively, compared to trees sprayed with microalgae at 100% ETc. Trees sprayed with microalgae at 100% ETc or 85% ETc had the highest and second-highest fruit yields. The percentage of fruit water loss after 15 d storage at either 1.5 °C + 90% RH or 16 ± 2 °C + 60–65% RH was reduced at all DI regimes when trees were sprayed with an anti-stressor compared to control trees at the same DI regime. Trees sprayed with 150 μmol L⁻¹ SNP had the only fruit peel that maintained the carotenoid content after cold storage. In general, trees sprayed with SNP were most successful at each DI regime for reduced Malondialdehyde (MDA), but after 15 d of cold storage, only trees sprayed with 150 μmol L⁻¹ SNP at 85% ETc or 100% ETc and/or trees sprayed with NanoSe at 70% ETc had fruit that maintained low MDA content. Catalase and peroxidase enzyme activities and proline content were enhanced when trees were sprayed with 150 μmol L⁻¹ SNP during DI strategies. This study demonstrates the effectiveness of deficit irrigation combined with anti-stressor applications to improve water productivity, fruit yields, and post-storage quality of ‘Murcott’ mandarin trees. These findings offer valuable insights into sustainable citrus production under limited water resources.

Considering climate change predictions, it is logical to anticipate detrimental effects on the mandarin tree, an essential citrus crop. Therefore, scientists should promptly focus on developing methods to enhance its resistance to climatic stress effects such as sunscald. This study assesses the strategies employed in ‘Balady’ mandarin trees when covered by shading nets of varying colors and percentages (white 50%, green 50% or 63%, black 50% or 63%), as well as the application of reflective materials (kaolin at 4% and CaCO₃ at 3%) on the micro-climate of orchards, leaf, and fruit surface temperatures, fruit sunburn%, productivity, and fruit quality. The results indicated that shade nets effectively reduced temperature and enhanced humidity, especially during the period from June to September, when compared to open-field treatments. Black shade nets, particularly those with a shading level of 63%, demonstrated the most notable decrease in canopy temperature and an elevation in humidity, surpassing the performance of green and white shade nets. The present study found that shade nets and reflecting materials like kaolin and calcium carbonate significantly reduced fruit sunburn. Trees without shade had a sunburn rate of 8.74%, while those with shade treatments suffered no sunburn. Kaolin foliar spray at a concentration of 4% and calcium carbonate at a concentration of 3% reduced sunburn incidence to 3.64% and 7.32%, respectively. These treatments also reduced the intensity of sunburn. All treatments increased fruit yield % compared to the control and yield efficiency (kg/m²), especially the trees covered with white shade net of a 50% shading rate provided the highest values (43.70 and 40.17%) and (5.24 and 5.47 kg/m²) compared to other treatments in both seasons, respectively. Trees covered with a white shade net of a 50% shading rate, followed by a green shade net of 50% and a 63% shading rate, as well as a black shade net of 50% and a 63% shading rate, tended to improve the physical and chemical fruit properties. Therefore, it could be recommended that trees be covered with a white shade net of a 50% shading rate or a green shade net of a 50 and 63% shading rate in summer months due to its beneficial impact on mitigating fruit sunburn damage and enhancing the productivity and quality of ‘‘Balady’’ mandarin trees. Hence, shade nets can be a beneficial technology to protect citrus fruits from sunburn without affecting fruit quality in commercial citrus farms.

Obtaining 'Balady' mandarin fruits with fewer seeds is a valuable target for consumers as it is one of the most important cultivars in Egypt because of its high eating properties. So, a field experiment was conducted during 2016 and 2017 seasons at a private orchard situated in Dacult area, Kafr El-Sheikh governorate, Egypt, to study the effect of foliar spray with gibberellic acid (GA3) at 25 and 50 mg L−1 , copper sulfate (CuSO4) at 25 mg L−1 alone or in combination on yield, fruit quality , fruit seed number .and shelf life. The obtained results showed that the combination between gibberellic acid (GA3) and copper sulfate (CuSO4) treatment reduced the seed number per fruit ranged (4.66-7) and improved yield and fruit quality such as markedly increase in fruit weight, volume, SSC/acid ratio and ascorbic acid. Simultaneously, it reduced the weight loss % and decay loss % during shelf life in comparison to other treatments and control.