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Huai-Fu Fan, Chang-Xia Du, and Shi-Rong Guo

Nitric oxide (NO), an endogenous signaling molecule in plants and animals, mediates responses to abiotic and biotic stresses. This study was conducted in a nutrient solution to investigate 1) the effects of exogenous sodium nitroprusside (SNP), an NO donor, on free proline (Pro) and protein content; and 2) the enzymes involved in Pro metabolism [pyrroline-5-carboxylate synthetase (P5CS) and proline dehydrogenase (PDH)] in cucumber (Cucumis sativus) seedling leaves and roots under NaCl stress. The results showed that the increases in free Pro and protein were significantly higher in the 50 mm NaCl solution but highly significant with the addition of 100 μM SNP to the 50 mm NaCl solution for the entire treatment period. Moreover, leaves maintained higher levels of free Pro and protein content than roots throughout the experiments. The P5CS activity increased in the saline treatment compared with the control, and this increase was greater in the 50 mm NaCl + 100 μM SNP solution than in the other treatments. On the other hand, the PDH activity was inhibited under NaCl stress but the reduction in activity was greater in the 50 mm NaCl + 100 μM SNP solution than in the others. These findings suggest that Pro metabolism was significantly altered during the exogenously applied NO under salt stress and that this alteration prompted the accumulation of higher levels of free Pro, which, in turn, maintained the turgor in the cucumber seedlings and helped protect them from salt stress. Moreover, the toxic effects generated by 50 mm NaCl were partially overcome by the application of NO, which could be used as a potential growth regulator to improve plant salinity tolerance. Therefore, it was concluded that NO could alleviate salinity damage in cucumber seedlings by regulating Pro metabolism. Overall, the adverse effects of salt stress could be lessened by the exogenous application of NO to cucumber seedlings.

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Chang-Xia Du, Huai-Fu Fan, Shi-Rong Guo, and Takafumi Tezuka

To examine whether spermidine (SPD) modifies plant antioxidant enzyme expression in response to short-term salt stress, cucumber (Cucumis sativus) seedlings were treated with NaCl in the presence or absence of SPD for 3 days. Compared with untreated control plants, free radical production and malondialdehyde content in leaves and roots increased significantly and plant growth was suppressed under 50 mm NaCl stress. Exogenous SPD sprayed on leaves at a concentration of 1 mm alleviated salinity-mediated growth reduction. Salt stress caused a consistent increase in soluble protein content, as well as peroxidase (POD) and superoxide dismutase (SOD) activities in cucumber seedlings. By native polyacrylamide gel electrophoresis, five POD isozymes were detected in cucumber seedling leaves, and seven in roots. We detected five SOD isozymes in leaves and four in roots, and two catalase (CAT) isozymes in leaves and two in roots. Our results indicate that salt stress induced the expression of POD and SOD isozymes in cucumber seedlings, but inhibited the expression of CAT isozymes in roots. Application of exogenous SPD further increased POD and SOD expression and activity, and led to the differential regulation of CAT in leaves and roots. These data show that antioxidant enzymes, especially POD and SOD, appear to protect cucumber seedlings against stress-related damage, and they appear to function as the molecular mechanisms underlying the response of cucumber seedlings to salinity. Moreover, SPD has potential to scavenge directly free radical and to alleviate growth inhibition and promote the activity and expression of antioxidant system enzymes in cucumber seedlings under short-term salt stress.

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Qing Xu, Shi-Rong Guo, He Li, Nan-Shan Du, Sheng Shu, and Jin Sun

The use of grafted seedlings in vegetable crops has increased in recent years to enhance the resistance to biological and abiotic stresses, and improve yields. However, incompatibility restricts the wide application of grafting. In this study, two pumpkin (Cucurbita) cultivars, with great differences in grafting affinity and symbiotic affinity, were used as rootstocks and cucumber (Cucumis sativus) seedlings were used as the scion. The effects of compatibility or incompatibility on histological aspects, antioxidant enzyme activities, phenylpropanoid contents, and chlorophyll fluorescence were studied. The results showed that compatible graft combinations present a stronger resistance to the oxidative damage resulting from grafting and had relatively weak phenylpropanoid metabolisms. The results also indicated that the chlorophyll fluorescence levels of incompatible combinations were lower, except compared with the original fluorescence. Finally, a necrotic layer existed earlier in compatible graft combinations. These differences at the morphological, physiological, and cellular levels may govern compatibility and incompatibility, and may provide valuable information for determining the symbiotic affinity of grafted seedlings at an early stage.

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Jing Tian, Li-Ping Wang, Yan-Juan Yang, Jin Sun, and Shi-Rong Guo

Heat tolerance is considered to be an essential feature for cucumber (Cucumis sativus) production, and it has been suggested that higher antioxidant ability could prevent the oxidative damage in plants caused by high-temperature stress. We aimed to investigate whether the application of exogenous spermidine (Spd) increases antioxidant activities and, therefore, elevates the heat tolerance of cucumber. Cucumber seedlings (cv. Jinchun No. 4) showing moderate heat tolerance were grown in climate chambers to investigate the effects of exogenous Spd (1 mm) foliar spray treatment on the activities and isozyme levels of antioxidative enzymes under both high-temperature stress 42/32 °C (day/night) and normal temperature 28/18 °C (day/night). On high-temperature stress, the activities of superoxide dismutase and ascorbate peroxidase were significantly reduced; the catalase activity was initially lower and then increased, whereas the peroxidase activity was initially higher and then decreased. The levels of these isozymes also changed differently. On treatment with exogenous Spd, the activities of these antioxidant enzymes were noticeably enhanced, and the isozyme zymogram expression had some changes. It was concluded that foliar spray with Spd effectively improved the total antioxidant ability of cucumber seedlings and, therefore, enhanced the tolerance of the plants to high-temperature stress.

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Guo-Gui Ning, Xue-Ping Shi, Hui-Rong Hu, Yan Yan, and Man-Zhu Bao

A set of Petunia hybrida plants encompassing a range of ploidy levels was developed through colchicine-mediated induction of chromosome doubling. The resulting double-flower tetraploid plants were cross-hybridized with inbred single-flower diploid lines to generate F1 populations with segregation for ploidy level and flower type. The initial in vivo application of colchicine to seedling apical tips produced mixoploid plants of petunia at a high rate of efficiency. Thus, 95% of the shoot tips treated with colchicine for 48 h resulted in polyploid mutant plants, and no difference in this efficiency was observed using concentrations of colchicine between 0.2 and 2.0 mg·mL−1. Of the polyploid plants, 10% were found to be tetraploid and 85% were mixoploid (chimeric). Compared with their diploid counterparts, polyploid plants underwent reduced elongation growth during the first 2 weeks and had thicker stems and shorter internodes resulting in dwarfing of the whole plant. In extreme cases, very slow growth rates produced stunted plantlets. Polyploid plants also had larger, thicker leaves and, in some cases, the leaves that developed after 1 month of growth appeared seriously malformed. Octoploid plants were also obtained and these tended to have more extreme phenotypes. Pure tetraploid plants of double-flower petunia were isolated by the in vitro culture of explants from the initial chimeric tetraploid mutants. These were crossed with three inbred single-flower diploid lines (S1, S2, and S3) thereby generating F1 populations that showed segregation for flower type and ploidy level and included the generation of triploid plants. In the tetraploid plants, flower diameter and the number of flower petals were not changed significantly (P > 0.05) compared with the original diploid double-flower plants, but observation of the pollen grains revealed segregation for size consistent with the increased ploidy level. Analysis of the F1 progeny plants also indicated that chromosome number is not necessary but sufficient to cause the production of semidouble-flowered plants. Flower color and flower diameter were also analyzed in the F1 progeny and complex patterns of inheritance were inferred. In addition to single and double flowers, semidouble-flowered plants were also suggested to be generated by the hybridization of 2n or 3n pollen from the double-flower tetraploid plants with the single-flower diploid lines.