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Safwan Shiyab

This is the first study on the performance of sour orange (Citrus aurantium L.) in soil contaminated with chromium (Cr). A greenhouse experiment was conducted to determine the phytotoxic effect of Cr on seed germination and seedling growth of sour orange. Cr treatments were applied as Cr(NO3)3 in five concentrations (0, 50, 100, 150, and 200 ppm). A gradual increase in Cr concentration leads to inhibition of seed germination and other growth parameters. Phytotoxicity, relative water content (RWC), seed vigor index (SVI), and the tolerance index (TI) show a significant decrease up to 100 ppm as a result of the presence of metal. Biochemical constituents, nutrient uptake, antioxidative enzymes, and lipid peroxidation under Cr stress were also investigated. The results indicate that concentrations greater than 100 ppm Cr cause an increase in plant antioxidative enzymes—superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX)—and increased lipid peroxidation. On the other hand, sour orange effectively generated an enzymatic antioxidant defense system (especially CAT) to scavenge hydrogen peroxide (H2O2), resulting in less H2O2 in shoots with greater Cr concentrations. A majority of Cr accumulated in the shoots and low translocations to shoots. However, sour orange is the best universal rootstock for citrus because of its resistance to salinity and disease. It also grows well in heavy soils. Based on the results of this study, sour orange might be a potential candidate plant for phytofiltration of contaminated water and phytostabilization of Cr-contaminated soils.

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Milica Ćalović, Qibin Yu, Vladimir Orbović, Frederick G. Gmitter Jr, Jude W. Grosser and Chunxian Chen

Six mandarin cultivars, Ponkan (Citrus reticulata), Willowleaf (Citrus deliciosa), Kinnow (Citrus nobilis × C. deliciosa), Murcott (purported C. reticulata × Citrus sinensis), W. Murcott [purported (C. reticulata × C. sinensis) × C. reticulata)], and Snack (purported C. reticulata hybrid), were used in protoplast fusion with different parental combinations to generate somatic hybrids. Sixty-five somatic regenerants were obtained using optimized formulation of enzymes and molecular weight of polyethylene glycol for improved protoplast yield and heterokaryon fusion rate, respectively. Flow cytometry was used to determine the ploidy level of somatic regenerants, and nuclear expressed sequence tag–simple sequence repeat (EST-SSR) markers to determine their parental source. Of the 65 somatic regenerants, 46 were identified as autotetraploids, 18 allotetraploids, and one undefined. The EST-SSR markers also revealed that some ‘W. Murcott’ embryogenic callus lines that were presumed to be of nucellar origin were actually derived unexpectedly from individual ovules of zygotic origin. These mandarin-derived tetraploids are valuable as potential breeding parents for interploid crosses with an aim at seedlessness and easy-peeling traits.

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James T. Brosnan and Gregory K. Breeden

Pyrimisulfan is a sulfonanilide herbicidal inhibitor of acetolactate synthase (ALS) used to control grass and sedge weeds of rice (Oryza stricta L.) production. Penoxsulam is an ALS-inhibiting herbicide that provides early postemergence control of broadleaf weeds in managed turfgrass. Separate field trials were conducted in Knoxville, TN, during Summer 2017 and 2018 to evaluate the efficacy of pyrimisulfan + penoxsulam for control of white clover (Trifolium repens L.), yellow nutsedge (Cyperus esculentus L.), wild violet (Viola spp.), ground ivy (Glechoma hederacea L.), and virginia buttonweed (Diodia virginiana L.) in common bermudagrass (Cynodon dactylon L.) and tall fescue (Festuca arundinacea Schreb.) turf. All treatments were applied on a granular fertilizer carrier (mean particle size, 1.5 mm) that contained 21% N : 0% P2O5 : 3% K2O. Treatments were applied at an early postemergence growth stage during April of each year and were irrigated into the soil within 24 hours of application. Weed control was assessed from 4 to 10 weeks after initial treatment (WAIT) relative to untreated control plots in each replication. White clover and wild violet were controlled effectively with pyrimisulfan + penoxsulam at 70 + 70 g·ha−1 whereas sequential applications at either 70 + 70 g·ha−1 followed by 35 + 35 g·ha−1 or 52.5 + 52.5 g·ha−1 followed by 52.5 + 52.5 g·ha−1 were needed to control yellow nutsedge, ground ivy, and virginia buttonweed effectively. Future research should explore long-term control of these species, particularly wild violet, ground ivy, and virginia buttonweed with pyrimisulfan + penoxsulam applied over multiple seasons. Chemical names: 2′-[(4,6-dimethoxypyrimidin-2-yl)(hydroxy) methyl]-1,1-difluoro-6′-(methoxymethyl)methanesulfonanilide (pyrimisulfan); 2-(2,2-difluoroethoxy)-N-(5,8-dimethoxy1,2,4triazolo 1.5-c-pyrimidin-2-yl)-6-(trifluoromethyl)benzenesulfonamide (penoxsulam).

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Michele R. Warmund, Jeanne D. Mihail and Kaley Hensel

Elderberry rust (Puccinia sambuci Schwein.) Arthur (=P. bolleyana) (Arthur, 1921) disease is frequently found in commercial American elderberry (Sambucus nigra L. subsp. canadensis L.) plantings when an alternate host, Carex sp., is present. To evaluate potential infection periods of P. sambuci on elderberry plants, micrometeorological conditions were monitored. Rust symptoms were observed on elderberry on 5 Apr. 2016, and conditions favorable for possible infection were 9 to 18 °C, ≥3 hours of continuous leaf wetness, and ≥85% relative humidity. Studies were also conducted to ascertain whether P. sambuci with varying pustule numbers affects fruiting, berry puree quality, or vegetative growth. Fruit yield was reduced by 31% when potted ‘Bob Gordon’ elderberry averaged six rust pustules per plant compared with noninfected plants. In another experiment, field-grown ‘Wyldewood’ plants averaging 137 rust pustules/cane at harvest had 47% less fruit weight on canes than uninfected canes. Titratable acidity of fruit puree from plants was lower when plants had either 690 rust pustules/plant or 137/pustules/cane, but soluble solids and pH of puree were unaffected by P. sambuci infection. The effect of rust infection on vegetative growth of elderberry plants also varied with pustule numbers. With a low infection level (six pustules per plant), P. sambuci did not induce premature leaf loss on ‘Bob Gordon’ plants or adversely affect shoot dry weight at the end of the growing season. When P. sambuci infection on ‘Wyldewood’ plants was more severe (137 pustules/cane), greater leaf loss occurred on infected canes than on uninfected canes. At very high infection levels (690 pustules/plant), ‘Bob Gordon’ plant dry weight was reduced. Because of the potential for fruit yield loss on elderberry plants, control of P. sambuci at relatively low infection levels on this plant may be warranted. Strategies that eliminate or suppress the alternate host would likely reduce the P. sambuci inoculum and limit the potential for elderberry plant infection.

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Krishna Nemali and Marc W. van Iersel

Bedding plants are at increased risk for exposure to drought stress during production because they are grown in small containers. Physiological mechanisms of bedding plants at leaf and cellular scales that regulate whole-plant photosynthesis under drought conditions are not well understood. This information can be useful for screening bedding plant cultivars with improved drought-tolerance and generate guidelines to mitigate drought stress during production. We subjected drought-sensitive salvia (Salvia splendens ‘Bonfire Red’) and drought-tolerant vinca (Catharanthus roseus ‘Cooler Peppermint’) to gradual drought stress inside whole-plant gas exchange chambers. Substrate water content (Θ), whole-plant net photosynthesis (Pn_avg), whole-plant respiration (Rd_avg), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance (g S) to water, leaf water (ΨL), osmotic (ΨS), and turgor (ΨP) potentials were measured at the start and end of the drought phase. In addition, ΨS was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted ΦPSII) was measured after rehydrating plants. The results indicated that, at whole-plant scale, vinca continued to uptake water at lower Θ levels than the Θ level that resulted in wilting of salvia. There were no differences in Rd_avg; however, Pn_avg and DCG of salvia decreased at a higher Θ level than that of vinca. This indicated that salvia experienced drought stress at a higher Θ level than did vinca. At the leaf scale, there were no differences in ΨL; however, a more negative ΨS (P = 0.06) and significantly higher ΨP were observed in vinca (compared to salvia) under drought conditions. In addition, ΨS was not different between species before exposure to drought, whereas ΨS of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, ΨS of rehydrated leaves after exposure to drought was significantly lower than that observed before exposure to drought in vinca. This indicated osmotic adjustment (OA) in vinca under drought conditions. Dark-adapted ΦPSII was lower in salvia than in vinca after exposure to drought, indicating damage to photosynthetic mechanisms. Our results suggested that increased OA likely helped to maintain higher ΨP under drought conditions and continuation of water uptake at lower Θ in vinca compared to salvia. In addition, healthier photosynthetic mechanisms of vinca (compared to salvia) under drought conditions likely resulted in its higher Pn_avg and DCG at lower Θ. Screening for OA and dark-adapted ΦPSII may be useful for developing drought-tolerant bedding plant cultivars.

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Ming Li and Wei-tang Song

Long-term exposure to an elevated ambient carbon dioxide (eCO2) concentration could weaken or diminish the enhancement of plant photosynthesis and growth. To monitor this response and offer references for growth management, the whole-plant photosynthetic rate (P n,w) and dark respiration rate (R d,w) of Angelica acutiloba Kitagawa transplants were monitored with a growth chamber. The results showed that eCO2 increased both the P n,w and R d,w by (79 ± 42) % and (126 ± 51) %. The dry weight of transplants under eCO2 was 33.6% greater than that under aCO2. However, the photosynthetic acclimation to eCO2 occurred. The increase in the P n,w was maintained until the end of the experiment due to increased leaf area. Moreover, the increase in plant dry weight mainly occurred in the first 15 days of treatment. Furthermore, the dry weight estimated based on the P n,w and R d,w agreed well with the measured dry weight. The relative growth rate (RGR) calculated with the estimated dry weight demonstrated the response of transplant growth to eCO2. These results indicated that the proposed method can be used to monitor the response of plant growth to eCO2.

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Enrique I. Sánchez-González, J. Guadalupe Gutiérrez-Soto, Emilio Olivares-Sáenz, Adriana Gutiérrez-Díez, Alejandro F. Barrientos-Priego and Salvador Ochoa-Ascencio

Because of the low availability of avocado rootstocks with resistance to Phytophthora cinnamomi, it is necessary to search for genotypes that offer resistance and that could be used as commercial rootstocks. The objective of this study was to select progeny from the genotypes of Mexican race avocado plants that are resistant to P. cinnamomi. Seedlings from 12 avocado genotypes were placed in containers inoculated with a mycelial suspension of P. cinnamomi. Signs of disease in the upper part of the seedlings were registered every 3 days for 8 weeks using a visual scale of damage severity. The χ 2 test (P < 0.009) showed significant differences among the genotypes evaluated, with ‘Todo el Año’ being the most resistant, as demonstrated by its rating of 70% asymptomatic seedlings, followed by ‘Plátano’ with 40%. The most susceptible genotypes were ‘María Elena’, ‘Silvestre’, and ‘Hass’, with 100% mortality. Seedling inoculation facilitated the detection of resistance to P. cinnamomi. ‘Todo el Año’ showed resistance toward P. cinnamomi. Therefore, individuals of its offspring could be recommended for use as rootstocks after confirming their resistance with a second evaluation, as well as performing tests in multiple localities to demonstrate their productive behavior after grafting.

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Tongyin Li, Guihong Bi and Richard L. Harkess

Plant growth and nitrogen (N) uptake of Encore® azalea ‘Chiffon’ (Rhododendron sp.) grown in a traditional plastic container or a biodegradable container made from recycled paper were investigated over the 2013 growing season. Three hundred twenty 1-year-old azalea liners, grown in two types of containers, were fertilized twice weekly with 250 mL N-free liquid fertilizer with no N or 15 mm N from ammonium nitrate (NH4NO3). Biweekly from 10 May to 3 Dec., five plants from each N rate and container type were selected randomly to measure plant height, widths, and leaf chlorophyll content in terms of soil–plant analysis development (SPAD) readings, and were then harvested destructively for nutrient analyses. Leaf SPAD readings and tissue N concentration were influenced mostly by N rate rather than container type, with 15 mm N producing greater values than the no-N treatment. Leaf SPAD readings increased from May to August and decreased from September to December. Using 15 mm N, plastic containers generally resulted in similar or increased plant growth [plant growth index (PGI) and dry weight] and N uptake from May to August as in biocontainers, with greater SPAD readings, leaf and root dry weights, stem and root N concentrations, and leaf and root N content than biocontainers at some harvests. However, biocontainers resulted in greater PGI, dry weights, and N content (in leaves, stems, roots, and total plant) than plastic containers later in the season, from September to December. These differences appeared in September after plants grown in plastic containers ceased active growth in dry weight and N uptake by the end of August. Plants grown in biocontainers had extended active growth from 13 Sept. to 9 Nov., resulting in greater tissue N content and greater N uptake efficiency. The biocontainers used in this study produced azalea plants of greater size, dry weight, and improved N uptake by increasing growth rate and extending the plants’ active growth period into late fall. The beneficial effects likely resulted from greater evaporative cooling through container sidewalls and the lighter color of the biocontainers, and therefore led to lower substrate temperatures and improved drainage.

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Huan Xiong, Feng Zou, Sujuan Guo, Deyi Yuan and Genhua Niu

Chinese chestnut (Castanea mollissima), which is native to China, has been cultivated as a nontimber forest tree species for 4000 years. This species has been found to display self-sterility, which results in a significantly lower seed set following self-pollination (SP) compared with that following cross-pollination (CP). Self-sterility can be induced by prezygotic or postzygotic late-acting self-incompatibility (LSI) or early-acting inbreeding depression (EID). To elucidate the causes of self-sterility in chestnut, we investigated pollen–pistil interactions, fertilization, and early ovule development following SP and CP by using a paraffin section technique and fluorescence microscopy. The fruit set percentage and seed characteristics also were evaluated among different pollination treatments. The results show that there were no significant differences in pollen tube behavior following SP vs. CP, regardless of the stigmatic or stylar level. Double fertilization was significantly greater following CP (18.09%) than SP (2.58%). The significantly lower percentages of ovule penetration and double fertilization in the selfed vs. crossed ovules support a prezygotic LSI mechanism in C. mollissima. The fruit set resulting from chase-pollination (CHP; 53.85% to 63.64%) was greater than that resulting from SP (12.12% to 14.00%). In addition, the distribution of aborted seed sizes after SP showed a widely clumped pattern. Abortion occurred at different stages during seed development rather than at a uniform stage, which supported the idea that EID was operating in C. mollissima. Levels of self-sterility in the chinese chestnut trees ranged from 88.2% to 90.5%. Thus, partial prezygotic LSI and EID contributed to self-sterility in the C. mollissima ‘Yanshanzaofeng’, with prezygotic LSI rejecting part of the self-pollen in the ovary and EID aborting part of the self-fertilized seeds.

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Yuyao Kong, Ajay Nemali, Cary Mitchell and Krishna Nemali

High energy-use cost for electric lighting is one of the major issues challenging sustainability of the indoor lettuce-farming industry. Thus, maximizing electrical energy-use efficiency (EUE, g·KWh−1), defined as the ratio of dry matter production (g) to electrical energy consumption (EEC, KWh−1), is crucial during indoor production. Light-emitting diodes (LEDs) are energy efficient and highly suitable for indoor farms. Research on optimal spectral quality of LEDs for lettuce growth is extensive; however, there is limited research examining LED spectral quality effects on EEC and EUE. Photon efficiency, defined as the ratio of light output to electrical energy input (PE, µmol·J−1), generally is used for selection of LED fixtures. Because PE does not account for differences in emitted light spectrum, it is not clear whether light-fixture selection based on PE can maximize EUE in lettuce production. This study comprised two experiments. In Expt. 1, we used four “phosphor-converted” commercial LEDs with different light intensities and spectra to model the effect of light spectral quality on lettuce shoot dry weight (SDW), EEC, and EUE. We also evaluated relations between EUE vs. PE and EUE vs. PER (PE based on red light) for indoor lettuce production. Results indicated that light spectral quality affected SDW, EEC, and EUE in lettuce production. Fitted models indicated that EEC increased linearly with increasing percentage of red-light output and was unaffected by other spectral colors or ratios. However, EUE increased in a curvilinear fashion with an increasing ratio of red to blue (R:B) light and reached a maximum at a ratio of 4.47. Similar to EUE, SDW also responded in a curvilinear fashion to R:B. Results also indicated that EUE correlated poorly with PE but linearly to PER. In Expt. 2, we grew three lettuce varieties under two commercial LED fixtures. They had similar levels of PE but different percentages of red, R:B, and PER values. Regardless of the variety, fixtures with greater percentages of red, R:B, and PER significantly increased EUE. We conclude that red-light quality is an important determinant of EUE and growers should select fixtures based on R:B and high PER in indoor lettuce farming.