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  • Author or Editor: Xiaoyan Zhang x
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Estimating chilling requirements is crucial for identifying appropriate cultivars for a given site, for timing applications of dormancy-breaking chemical agents, and for predicting consequences of climate change. For temperate-zone fruit species such as japanese apricot, productivity is reduced when chilling requirements are not adequately satisfied. In our study, we obtained chilling and heat requirements for flowering of six japanese apricot cultivars, which spanned the range of flowering times in this species for three successive years. Different methods for determining chilling requirements were evaluated and compared, and correlations among chilling requirements, heat requirements, and flowering date were established. The dynamic model proved to be the best for determining the chilling requirements of japanese apricot. The results showed a range of chilling requirements ranging from 26.3 to 75.7 chill portions and a narrow range of heat requirements, from 1017.7 to 1697.3 growing degree-hours (GDH). A very high correlation (R = 0.9797) between flowering date and chilling requirements and a low correlation (R = 0.4298) between flowering date and heat requirements suggest that flowering date in japanese apricot is mainly a consequence of the chilling requirements of the different genotypes, whereas heat requirements contribute a limited effect to the variation in flowering dates. Chilling requirements and heat requirements were positively related with a low correlation coefficient (R = 0.4211).

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Cornus florida seeds show strong dormancy. In this study, we investigated the causes of the dormancy by assessing the permeability of the stony endocarp, the germination of seeds after mechanical dissection, and the effect of endogenous inhibitors. Water uptake by intact and cracked seeds during imbibition showed that the endocarp formed a strong barrier for water absorption. Meanwhile, extracts from endocarp decreased the germination frequency of chinese cabbage seeds from 99.3% (control) to 2.7%. Therefore, the endocarp was the mechanical barrier and contained endogenous inhibitors for seed germination. However, the germination percentage of decoated seeds and dissected seeds with the exposed radicle were only 13.3% and 28.7%, respectively. It was found that the endosperm also played a role in seed dormancy. Extracts from endosperm decreased the germination frequency of chinese cabbage seeds from 99.3% (control) to 53.0%. By contrast, extracts from embryo did not affect the germination of chinese cabbage seeds. When tested with the excised embryos, germination percentage was up to 85.3% at the 16th day of incubation. Taking these results together, we concluded that the endocarp and endosperm were responsible for seed dormancy in C. florida. To break the seed dormancy of C. florida, stratification and soaking in sulfuric acid are the effective means. The highest germination frequency was achieved by immersing seeds in 98% sulfuric acid for 10 minutes, then soaking the seeds in 500 mg·L−1 gibberellic acid (GA3) for 72 hours before cold stratification at 5 °C for 60 days.

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Kiwifruit (Actinidia deliciosa) is a typical climacteric fruit, and its ripening is closely associated with ethylene. In this study, we present evidence that H2S alleviated ethylene-induced ripening and senescence of kiwifruit. Kiwifruit were fumigated with ethylene released from 0.4 g·L−1 ethephon solution or H2S with 1 mm sodium hydrosulfide (NaHS) as the donor or in combination. Fumigation with ethylene was found to accelerate kiwifruit ripening and H2S treatment effectively alleviated ethylene-induced fruit softening in parallel with attenuated activity of polygalacturonase (PG) and amylase. Ethylene + H2S treatment also maintained higher levels of ascorbic acid, titratable acid, starch, soluble protein, and reducing sugar compared with ethylene group, whereas suppressed the increase in chlorophyll and carotenoid. Kiwifruit ripening and senescence under ethylene treatment was accompanied by elevation in reactive oxygen species (ROS) levels, including H2O2 and superoxide anion and malondialdehyde (MDA), but combined treatment of ethylene plus H2S alleviated oxidative stress in fruit. Furthermore, the activities of antioxidative enzymes catalase (CAT) and ascorbate peroxidase (APX) were increased by ethylene + H2S treatment in comparison with ethylene alone, whereas the activities of lipoxygenase (LOX) and polyphenol oxidase (PPO) were attenuated by H2S treatment. Further investigations showed that H2S repressed the expression of ethylene synthesis-related genes AdSAM, AdACS1, AdACS2, AdACO2, and AdACO3 and cysteine protease genes, such as AdCP1 and AdCP3. Taken together, our findings suggest that H2S alleviates kiwifruit ripening and senescence by antagonizing the effect of ethylene through reduction of oxidative stress and inhibition of ethylene synthesis pathway.

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Phosphorus (P) is an essential nutrient element that is necessary for plant growth and development. However, most of the P exists in insoluble form. Aspergillus aculeatus has been reported to be able to solubilize insoluble forms of P. Here, to investigate the P-solubilizing effect of A. aculeatus on the performance of perennial ryegrass (Lolium perenne) under P-deficiency stress, we created four treatment groups: control [i.e., no Ca3(PO4)2 or A. aculeatus], A. aculeatus only (F), Ca3(PO4)2 and Ca3(PO4)2 + A. aculeatus [Ca3(PO4)2 + F] treatment, and Ca3(PO4)2 at concentrations of 0 and 3 g per pot (0.5 kg substrate per pot). In our results, the liquid medium inoculated with A. aculeatus exhibited enhanced soluble P and organic acid content (tartaric acid, citric acid, and aminoacetic acid) accompanied with lower pH, compared with the noninoculated regimen. Furthermore, A. aculeatus also played a primary role in increasing the soluble P content of substrate (1 sawdust: 3 sand), the growth rate, turf quality, and photosynthetic capacity of the plant exposed to Ca3(PO4)2 + F treatment, compared with other groups. Finally, in perennial ryegrass leaves, there was a dramatic increase in the valine, serine, tyrosine, and proline contents, and a remarkable decline in the glutamic acid, succinic acid, citric acid, and fumaric acid contents in the Ca3(PO4)2 + F regimen, compared with other groups. Overall, our results suggested that A. aculeatus may play a crucial role in the process of solubilizing Ca3(PO4)2 and modulating perennial ryegrass growth under P-deficiency stress.

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Aspergillus niger is a common pathogenic fungus causing postharvest rot of fruit and vegetable, whereas the knowledge on virulence factors is very limited. Superoxide dismutase [SOD (EC 1.15.1.10)] is an important metal enzyme in fungal defense against oxidative damage. Thus, we try to study whether Cu/Zn-SOD is a virulence factor in A. niger. Cu/Zn-SOD encoding gene sodC was deleted in A. niger [MA70.15 (wild type)] by homologous recombination. The deletion of sodC led to decreased SOD activity in A. niger, suggesting that sodC did contribute to full enzyme activity. ΔsodC strain showed normal mycelia growth and sporulation compared with wild type. However, sodC deletion markedly increased the cell’s sensitivity to intracellular superoxide anion generator menadione. Besides, spore germination under menadione and H2O2 stresses were significantly retarded in ΔsodC mutant compared with wild type. Further results showed that sodC deletion induced higher superoxide anion production and higher content of H2O2 and malondialdehyde (MDA) compared with wild type, supporting the role of SOD in metabolism of reactive oxygen species (ROS). Furthermore, ΔsodC mutant had a reduced virulence on chinese white pear (Pyrus bretschneideri) as lesion development by ΔsodC was significantly less than wild type. The determination of superoxide anion, H2O2, and MDA in A. niger-infected pear showed that chinese white pear infected with ΔsodC accumulated less superoxide anion, H2O2, and MDA compared with that of wild type A. niger, implying that ΔsodC induced an attenuated response in chinese white pear during fruit–pathogen interaction. Our results indicate that sodC gene contributes to the full virulence of A. niger during infection on fruit. Aspergillus niger is one of the most common species found in fungal communities. It is an important fermentation industrial strain and is also known to cause the most severe symptoms in fruit during long-term storage (). Meanwhile, plants activate their signaling pathways to trigger defense responses to limit pathogen expansion. One of the earliest host responses after pathogen attack is oxidative burst, during which large quantities of ROS are generated by different host enzyme systems, such as glucose oxidase (). ROS such as singlet oxygen, superoxide anion, hydroxyl (OH), and H2O2 are released to hinder the advance of pathogens (). ROS can react with and damage cellular molecules, such as DNA, protein, and lipids, which will limit fungal propagation in the host plant ().

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