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Jim Syvertsen and Yoseph Levy

Multiple stresses almost always have synergistic effects on plants. In citrus, there are direct and indirect interactions between salinity and other physical abiotic stresses like poor soil drainage, drought, irradiance, leaf temperature, and atmospheric evaporative demand. In addition, salinity interacts with biotic pests and diseases including root rot (Phytophthora spp.), nematodes, and mycorrhizae. Improving tree water relations through optimum irrigation/drainage management, maintaining nutrient balances, and decreasing evaporative demand can alleviate salt injury and decrease toxic ion accumulation. Irrigation with high salinity water not only can have direct effects on root pathogens, but salinity can also predispose citrus rootstocks to attack by root rot and nematodes. Rootstocks known to be tolerant to root rot and nematode pests can become more susceptible when irrigated with high salinity water. In addition, nematodes and mycorrhizae can affect the salt tolerance of citrus roots and may increase chloride (Cl-) uptake. Not all effects of salinity are negative, however, as moderate salinity stress can reduce physiological activity and growth, allowing citrus seedlings to survive cold stress, and can even enhance flowering after the salinity stress is relieved.

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John Carter and Kim E. Hummer

Black currant (Ribes nigrum L.) cultivars with heavy, light, and no gooseberry mite (Cecidophyopsis grossulariae Collinge) infestation levels (MIL) were tested for cold hardiness by visually determining the bud injury rating (BIR) after laboratory freezing in Jan. 1998. Lightly mite-infested cvs. Blackdown and Risager, usually thought of as less cold hardy than Nordic cultivars, survived -35 °C, while mite-infested buds of the Finnish cv. Brödtorp were injured at -35 °C. Heavily mite-infested buds of the Swedish R. nigrum L. cv. StorKlas from Corvallis, Ore., were injured at -20 °C while lightly infested buds were injured to -25 °C. Noninfested `StorKlas' buds from Pennsylvania and British Columbia survived laboratory freezing to -35 °C. Heavy mite infestation lowered the bud cold hardiness of `Brödtorp' and `StorKlas' by 10 °C, as estimated by a modified Spearman-Karber T50, relative to the hardiness of lightly mite-infested buds of these cultivars. Heavily mite-infested buds contained unusual tissues forming what appeared to be spherical blisters or eruptions, ≈100 μ in diameter. Other tissues in the region of heavy mite infestation appeared to be more turgid than their noninfested counterparts. Abiotic and biotic stresses can have a combined impact on field-grown black currants.

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Xiaojuan Zong, Jiawei Wang, Li Xu, Hairong Wei, Xin Chen, Dongzi Zhu, Yue Tan, and Qingzhong Liu

al., 2004 ). Their homologs in tobacco, WIPK and SIPK, have also been shown in many studies to regulate response to abiotic and biotic stresses ( Kallenbach et al., 2010 ; Katou et al., 2005 ; Kim et al., 2011 ; Kobayashi et al., 2010 ; Sharma et

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Jiao Chen, De-bao Yuan, Chao-zheng Wang, Yi-xing Li, Fen-fang Li, Ke-qian Hong, and Wang-jin Lu

family. A number of zinc finger proteins have been found to be involved in abiotic and biotic stress responses ( Cui et al., 2002 ; Huang et al., 2005 ; Kim et al., 2001 ; Mukhopadhyay et al., 2004 ; Sakamoto et al., 2004 ). One interesting zinc

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Zhigang Ouyang, Huihui Duan, Lanfang Mi, Wei Hu, Jianmei Chen, Xingtao Li, and Balian Zhong

reported to play roles in multiple aspects of plant life, such as developmental progress, and responses to abiotic and biotic stress. However, direct evidence is limited. Wang et al. (2017a) reported that overexpressing MhYTP1 or MhYTP2 in Malus

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Priscila L. Gratão, Carolina C. Monteiro, Lázaro E.P. Peres, and Ricardo Antunes Azevedo

The activity of catalase (CAT), guaiacol peroxidase (GPOX), ascorbate peroxidase (APX), glutathione reductase (GR), and the isoenzymes of superoxide dismutase (SOD) were determined in the organs of tomato (Lycopersicon esculentum) cultivar Micro-Tom after 104 days of development. The total activities of CAT, GPOX, and GR were higher in the stem than in others tissues, whereas the stem exhibited the lowest APX activity. Activity staining analysis following gel electrophoresis revealed the existence of four SOD isoenzymes in leaves, three in fruits, but only two in the roots and stems. This characterization is essential for an investigation into the effect of abiotic and biotic stresses on the oxidative stress responses by this plant model system.

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Ana B. Monteagudo, A. Paula Rodiño, Margarita Lema, María De la Fuente, Marta Santalla, Antonio M. De Ron, and Shree P. Singh

Availability of germplasm with high level of resistance is essential for broadening the genetic base and breeding crop cultivars resistant to abiotic and biotic stresses. The objective of this study was to determine reaction of a common bean core collection from the Iberian Peninsula to anthracnose, rust, common and halo blights, bean common mosaic virus (BCMV, a potyvirus) and bean common mosaic necrosis virus (BCMNV, a potyvirus) pathogens. Of 43 accessions evaluated, 14 large-seeded Andean type, seven small-seeded Middle American type and seven with intermediate characteristics or recombinant type between the two gene pools had resistant reaction to one or more diseases. Resistance to race 17 or 23 of anthracnose pathogen was present in 17 accessions and four accessions were resistant to both races. Resistance to race 38 or 53 of rust pathogen was shown by 22 accessions and five accessions were resistant to both races. All accessions were susceptible to common bacterial blight and 12 accessions had resistance to halo blight. Ten accessions showed resistance to BCMV, none to BCMNV, and two were variable to both viruses. Accessions such as PHA-0573 (pinto), PHA-0589 (marrow), PHA-0654 (favada pinto), and PHA-0706 (favada) showed resistance to two or more diseases. These accessions may be valuable in breeding Andean bean for enhancing simultaneous utilization of both large seed size and disease resistance.

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Stephen B. Ryu and Jiwan P. Palta

Lipids have been thought to be important largely in membrane structure and energy reserve. It is now evident that lipids and lipid-derived metabolites play a role in many critical cellular processes. Recent studies have shown that membrane lipid-based signaling mediated by phospholipases such as phospholipase A2 (PLA2), phospholipase C (PLC), and phospholipase D (PLD) constitutes a crucial step in plant responses to abiotic and biotic stresses. Phospholipases and their products also play a role during plant growth and development. For example, PLA2-derived lysophospholipids acted as growth regulators that retard senescence of plant tissues. Interestingly, the PLA2 products inhibited the activity of PLD, which has been suggested to be a key enzyme responsible for membrane lipid breakdown leading to plant senescence. Endogenous levels of lysophospholipids, such as lysophosphatidylethanolamine (LPE), could be increased in castor bean leaf discs by the treatment of auxin (50 μM), which is known to be a activator of PLA2. Pretreatment of leaf discs with a PLA2 inhibitor before auxin treatment nullified the auxin effect and rather resulted in accelerated senescence even compared to the nontreated control. Our recent results suggest a potential role of PLA2 products as biologically active molecules mediating hormonal regulation of growth and senescence. One such product LPE is being commercially exploited for retarding senescence and improving shelf life of fruits, vegetables, and cut flowers.

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Qiansheng Li, Jianjun Chen, Dennis B. McConnell, and Richard J. Henny

A simple and effective method for quantification of leaf variegation was developed. Using a digital camera or a scanner, the image of a variegated leaf was imported into a computer and saved to a file. Total pixels of the entire leaf area and total pixels of each color within the leaf were determined using an Adobe Photoshop graphics editor. Thus, the percentage of each color's total pixel count in relation to the total pixel count of the entire leaf was obtained. Total leaf area was measured through a leaf area meter; the exact area of this color was calculated in reference to the pixel percentage obtained from Photoshop. Using this method, variegated leaves of ‘Mary Ann’ aglaonema (Aglaonema x), ‘Ornate’ calathea (Calathea ornate), ‘Yellow Petra’ codiaeum (Codiaeum variegatum), ‘Florida Beauty’ dracaena (Dracaena surculosa), ‘Camille’ dieffenbachia (Dieffenbachia maculata), and ‘Triostar’ stromanthe (Stromanthe sanguinea) were quantified. After a brief training period, this method was used by five randomly selected individuals to quantify the variegation of the same set of leaves. The results were highly reproducible no matter who performed the quantification. This method, which the authors have chosen to call the quantification of leaf variegation (QLV) method, can be used for monitoring changes in colors and variegation patterns incited by abiotic and biotic stresses as well as quantifying differences in variegation patterns of plants developed in breeding programs.

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Jennifer K. Boldt, James C. Locke, and James E. Altland

Silicon (Si) is a plant beneficial element associated with the mitigation of abiotic and biotic stresses. Most greenhouse-grown ornamentals are considered low Si accumulators based on foliar Si concentration. However, Si accumulates in all tissues, and there is little published data on the distribution of Si in plants. This knowledge may be critical to using Si to mitigate tissue-specific plant stresses, e.g., pathogens. Therefore, we quantified Si accumulation and distribution in petunia (Petunia ×hybrida Hort. Vilm.-Andr. ‘Dreams Pink’), a low Si accumulator, and sunflower (Helianthus annuus L. ‘Pacino Gold’), a high Si accumulator. Plants were grown in a sphagnum peat: perlite substrate amended with 0% (−Si) or 20% (+Si) parboiled rice hulls for 53 (petunia) or 72 days (sunflower). Aboveground dry weight was greater in nonamended petunia (13%) and sunflower (18%), compared with rice hull–amended plants, but days to flower was unaffected. Sunflowers grown in the rice hull–amended substrate had the greatest Si concentration in leaves (10,909 mg·kg−1), whereas roots (895 mg·kg−1), stems (303 mg·kg−1), and flowers (252 mg·kg−1) had lower, but similar Si concentrations. In petunia, Si concentration was greatest in leaves (2036 mg·kg−1), then roots (1237 mg·kg−1), followed by stems (301 mg·kg−1), and flowers (247 mg·kg−1). The addition of rice hulls to the substrate increased Si concentration in sunflower 414% in roots, 512% in flowers, 611% in stems, and 766% in leaves. By contrast, Si concentration in petunia increased only 7% in flowers, 105% in stems, and 115% in leaves, but increased 687% in roots. In rice hull–amended sunflowers, the distribution of Si was 91% in leaves, 3% in stems, 3% in roots, and 3% in flowers, and in petunia, it was 72% in leaves, 17% in stems, 6% in roots, and 5% in flowers.