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Kristine M. Lang and Ajay Nair

High tunnels allow vegetable growers to extend the growing season, increase crop production, and improve produce quality. Tomatoes (Solanum lycopersicum L.) are the most widely grown crops in high tunnels; however, tomato production in high tunnels can be challenging. Continuous cropping in high tunnels can increase soil-borne disease pressure and can lead to soil salinity or nutrient depletion issues. Based on preliminary research, we hypothesized that use of the rootstock ‘RST-04-106-T’ would increase yield and quality of heirloom and hybrid tomato scions compared with nongrafted plants. To test this hypothesis, our research objectives were to assess marketable yields, fruit quality and nutritional value, and plant growth of grafted and nongrafted hybrid and heirloom tomatoes in a high tunnel production system. Grafted and nongrafted ‘Cherokee Purple’ (heirloom) and ‘Mountain Fresh Plus’ (hybrid) tomatoes were grown in the same high tunnel for two seasons (7 May–20 Oct. 2015 and 29 April–7 Oct. 2016) at the Horticulture Research Station in Ames, IA. Grafted plants produced significantly more marketable fruit, although marketable and total fruit weight did not increase significantly. Individual fruit size was unaffected by grafting. Across cultivars, mean soluble solids content (SSC) in fruit was 0.3 °Brix lower in grafted plants as compared with the nongrafted control. Grafting did not affect lycopene content of fruit. Grafting increased stem diameter by an average of 0.8 mm, but overall plant biomass was unaffected. The effect of grafting on leaf chlorophyll concentration (SPAD readings) was mixed. In addition, grafting increased leaf chlorophyll concentration in ‘Cherokee Purple’ but decreased it in ‘Mountain Fresh Plus’ plants. Grafting is a valuable tool in tomato production, but the impact of ‘RST-04-106-T’ rootstock use appears to be specific to certain soil types with high incidence of bacterial wilt.

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Keun H. Cho, Veronica Y. Laux, Nathan Wallace-Springer, David G. Clark, Kevin M. Folta and Thomas A. Colquhoun

Coleus (Plectranthus scutellarioides) is an attractive and popular ornamental plant with propagation mainly achieved through vegetative cuttings. For commercial purposes, it is of interest to enhance the speed of establishment while maintaining high quality. Light quality has been shown to influence adventitious root development, so these experiments examined the effect of narrow-bandwidth light treatments on root growth and overall plant quality for seven coleus cultivars with vegetative cuttings in potting soil and one cultivar with shoot tip in vitro cultures onto Murashige and Skoog (MS) agar medium. During the 28 days of the propagation period, the cuttings grown under narrow-bandwidth red light (R; 663.4 nm at peak) more than doubled in the adventitious root number compared with those under blue light (B; 445.7 nm at peak) and green light (G; 530.0 nm at peak) in five cultivars. R light also increased fresh weight of the cuttings by 55.6% more than G light. In comparison, the cuttings grown under G light yielded significantly lower root and shoot dry mass than other light treatments. R light cuttings showed more dry mass content (9.63%) than those under white light (W; 437.4 nm and 559.5 nm at peak) and G light (7.85% and 5.86%, respectively). A positive correlation (R 2 = 0.598, P < 0.001) was found between the formation of adventitious roots and gained fresh weight of cuttings. R light made the reddish color of leaves significantly stronger in most cultivars, whereas the cuttings exposed to G light became less vivid compared with other light conditions. When the shoot tips were propagated in vitro onto MS medium, R light treatment initiated the root development more rapidly than other lights, with significantly greater rooting rate (20.0% and 63.6%, respectively) at day 5 and 10. The shoot tips under R light also formed significantly more roots (12.3 per cutting) than those grown under narrow-bandwidth B light (5.8 per cutting). The shoot tips showed browning at an early stage and newly emerged leaves grew very compactly under B light. The combination of red and green light (R+G) increased more than twice as much roots and dry mass compared with W light. In addition, the R+G light led to morphological changes, including larger leaves and longer petioles and internodes than those in other light treatments. The exposure to R+G+B and B light made the shoots very compact for the 28 days of in vitro culture period and significantly increased the chlorophyll contents resulting in dark green leaves.

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Sawyer N. Adams, Walter O. Ac-Pangan and Lorenzo Rossi

The accumulation of salt deposits in the soil is becoming a major problem in agriculture. The problem can affect citrus production, which is already seriously hindered in Florida by the disease known as Huanglongbing (HLB, or citrus greening). With the citrus industry declining, it is imperative to study all potential environmental stresses affecting the citrus cultivation and to conduct screening tests to evaluate which rootstocks perform best under these challenging new conditions. This study aims to determine how the ‘US-942’ rootstock reacts to salinity. Thirty-six 3-month-old ‘US-942’ citrus rootstock seedlings were grown in a greenhouse. After 15 d of acclimation, plants were assigned to one of six groups and treated for 30 d with solutions of 25% Hoagland solution amended with 0 (control), 30, 60, 90, 120, or 150 mm NaCl. Higher NaCl concentrations significantly hindered plant growth and negatively affected some physiological processes [i.e., stomatal conductance (g S)] and chlorophyll contents. Conversely, plants treated with mild concentrations (30 mm NaCl) had higher plant biomass and exhibited higher photosynthetic efficiency. Free hand sections of fresh roots were taken at the end of the experiment, and the suberin lamellae development was examined under a fluorescence microscope. In conclusion, results reported that ‘US-942’ rootstock is tolerant to mild salt stress and confirmed the hypothesis that the formation of root apoplastic barriers and the increase in the root biomass could be two possible mechanisms that give the ‘US-942’ a mild NaCl stress tolerance.

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Yuejin Weng, Jun Qin, Stephen Eaton, Yufeng Yang, Waltram Second Ravelombola and Ainong Shi

Cowpea [Vigna unguiculata (L.) Walp] is an annual legume crop grown worldwide to provide protein for human consumption and animal feed. The objective of this research was to evaluate the seed protein content in U.S. Department of Agriculture (USDA) cowpea germplasm for use in cowpea breeding programs. A field experiment was conducted with a randomized complete block design (RCBD) with three duplications in two locations, Fayetteville and Alma, in Arkansas, United States. A total of 173 USDA cowpea accessions were evaluated with the Elementar Rapid N analyzer III for their seed protein contents. The results showed that there was a wide range of seed protein content among the 173 cowpea genotypes, ranging from 22.8% to 28.9% with an average of 25.6%. The broad-sense heritability for seed protein among the 173 cowpea genotypes was 50.8%, indicating that seed protein content was inheritable and can be selected in breeding processing. The top five cowpea accessions with the highest seed protein contents were USDA accession PI 662992 originally collected from Florida (28.9%), PI 601085 from Minnesota (28.5%), and PI 255765 and PI 255774 from Nigeria and PI 666253 from Arkansas (28.4% each). PI 339587 from South Africa had the lowest protein content with 21.8%. The were also significant differences in seed protein contents observed among different seedcoat colors; the accessions with cream color exhibited higher protein content (27.2%) than others. This research could provide information for breeders to develop cowpea cultivars with higher seed protein content in a cowpea breeding program.

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Huihui Liu, Ke Cao, Gengrui Zhu, Weichao Fang, Changwen Chen, Xinwei Wang and Lirong Wang

Anthocyanins are important molecules that are responsible for fruit color formation and are also beneficial to human health. To date, numerous structural and regulatory genes associated with anthocyanin biosynthesis in peach (Prunus persica) have been reported based on linkage analysis. In this study, we sought to identify further genes associated with anthocyanin content in peach by conducting a genome-wide association analysis of 129 peach accessions to detect markers associated with the trait. Significant association signals were detected when anthocyanin content was considered a qualitative character but not when it was considered a quantitative trait. We detected an association region located between 11.7 and 13.1 Mb in chromosome 1, a region in which only 133 of 146 genes have previously been functionally annotated. Gene ontology annotation of the genes in this region showed that membrane-associated genes (including one gene encoding a chloride channel protein and 17 sugar transport/carrier-associated genes) were significantly enriched, and we focused on these in subsequent analyses. Based on in vitro induction of anthocyanins in fruit flesh using different exogenously applied sugars and subsequent culture, we found that the expression level of 3 of the 18 membrane-associated genes, Prupe.1G156300, Prupe.1G156900, and Prupe.1G157000, increased during induction treatment. Furthermore, during the fruit development period of a white-fleshed and a red-fleshed peach cultivar, the expression of one gene encoding a transmembrane sugar transport protein was observed to be positively correlated with anthocyanin biosynthesis. These results will facilitate understanding of the molecular mechanism of anthocyanin biosynthesis in peach.

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Yanjun Guo, Terri Starman and Charles Hall

This study analyzed the effects of two ranges of drying down of substrate moisture content (SMC) before re-watering on plant growth and development, postproduction quality, and economic value of bedding plants grown in 1.67-L containers during greenhouse production. The two SMC treatments were wide-range (WR) SMC (WR-SMC) for dry-down from container capacity (CC) of 54% SMC dried down to 20% SMC or narrow-range (NR) SMC (NR-SMC) for dry-down from CC of 54% SMC dried down to 40% SMC. Six bedding plant cultivars were used [Solenostemon scutellarioides ‘French Quarter’ (coleus); Petunia ×hybrida ‘Colorworks Pink Radiance’ (petunia); Lantana camara ‘Lucky Flame’ (lantana); Impatiens ×hybrida ‘Sunpatiens Compact Hot Coral’ (SCC); ‘Sunpatiens Spreading Lavender’ (SSL) (impatiens); and Salvia splendens ‘Red Hot Sally II’ (salvia)]. Shoot dry weight was reduced with WR-SMC on petunia, lantana, impatiens SCC, and salvia at the end of production. With WR-SMC, the petunia and impatiens SCC root ball coverage percentages were greater on the bottom of the container, whereas those of impatiens SSL and salvia were reduced. The WR-SMC increased petunia postproduction quality by increasing the flower number. Lantana and impatiens SCC inflorescence/flower and/or bud number were reduced with WR-SMC. The impatiens SSL flower number was unaffected by SMC treatment. Salvia grown with WR-SMC had increased postproduction quality. WR-SMC reduced postproduction water potential in petunia, lantana, and coleus, suggesting that plants with WR-SMC during production were acclimated to reduced irrigation administered during postproduction. WR-SMC saved labor due to less frequent watering and overhead-associated costs due to reduced bench space, with the exception of coleus and impatiens SSL, which used the same bench space as NR-SMC. Considering production and/or postproduction quality, using WR-SMC during greenhouse production is beneficial as an irrigation method for coleus, petunia, impatiens SSL, and salvia, but not for impatiens SCC or lantana grown in 1.67-L containers.

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Lauren Lindsey, Raymon Shange, Ramble O. Ankumah, Desmond G. Mortley and Sangita Karki

Organic fertilization techniques have become an attractive alternative to conventional techniques, but there remains interest in their impact on rhizosphere ecology. This study was aimed at assessing the impacts of various organic fertilizer amendments on storage root yield, chemical, biochemical, and microbial factors in the rhizosphere ecosystem and the bacterial community composition in the rhizosphere ecosystem. Four sweetpotato cultivars (J6/66, NCC-58, TU Purple, and Whatley/Loretan) and four organic fertilizer treatments [poultry litter, Megabloom (fish protein), NPK, and an untreated control] were used in the study. The experiments were conducted as a randomized complete block design with a 4 × 4 factorial treatment arrangement and three replications. Fertilizer treatments were split-applied at the rate of 134–67–67 kg·ha−1 NPK equivalent based on soil test recommendations 1 and 4 weeks after planting as single bands 15 cm from the plants and organic amendments were calculated based on total N content. Rhizosphere soil samples were collected at harvest and analyzed for soil pH, soil organic carbon (SOC), bacterial 16S rDNA, and selected soil enzymes. Organic amendments did not affect storage root yield or percent dry matter but enhanced both the mass and number of US#1 storage roots. Rhizosphere pH varied depending on cultivar and cultivar response varied with pH and ranged from 6.1 to 6.8, whereas SOC was similar regardless of the amendment. The impact of fertilizers was evident as Megabloom (fish protein) treatment suppressed the relative abundance (RA) of nitrifiers (Nitrosococcus and Nitrosomonadaceae). Also, the rhizosphere of ‘Whatley/Loretan’ seemed to have been a beneficial habitat for populations of common nitrogen-fixing bacteria Bradyrhizobium elkanii, and Rhodospirillaceae sp. as their RA increased significantly in the rhizosphere. That bacteria associated with carbon and nitrogen cycling under aerobic conditions were found to be ubiquitous in the rhizosphere of sweetpotato, suggesting that certain amendments positively impacted the populations of nitrogen-cycling bacteria, thus making them a viable alternative to NPK when considering increasing or sustaining yield while promoting long-term soil health.

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Jiangbo Dang, Tingrong Wu, Guolu Liang, Di Wu, Qiao He and Qigao Guo

A loquat (Eriobotrya japonica) seedling obtained from an open-pollinated triploid variety ‘Wuheguoyu’ (2n = 3x = 51) was verified as aneuploid and designated H39. It was shown to have five extra chromosome copies (2n = 39) compared with the diploid plant (2n = 2x = 34), one additional copy each for the 2nd, 4th, 7th, 9th, and 11th chromosomes. A number of novel features of leaf morphology was observed for H39 in comparison with ‘Ruantiaobaisha’ (2x, female progenitor) and ‘Wuheguoyu’ (3x, female parent), including increased leaf width, reduced leaf thickness, and narrowed palisade mesophyll and wax coat. Total chlorophyll content in unit area of H39 leaves was close to or slightly less than the diploid and triploid parent lines. Chlorophyll content in unit mass showed the opposite trend, with H39 having higher amount than the 2x and 3x. As we expected, H39 had the lowest net photosynthetic rates among the three lines. Furthermore, 8-month-old scions of H39 grew more slowly than those of the diploid and triploid lines, especially in plant height, which was much reduced (P < 0.01). These results indicated that the aneuploid H39 was a potential germplasm for breeding dwarfing loquat rootstock or interstock.

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Rui Li, Lu Fan, Jingdong Lin, Mingyang Li, Daofeng Liu and Shunzhao Sui

Kalanchoe (Kalanchoe blossfeldiana) is a common potted flower that is popular throughout the world. Brown spot (caused by Stemphylium lycopersici) is one of the common foliage diseases in kalanchoe. This disease tends to infect leaves of kalanchoe plants in hot and humid environments, reducing their aesthetic value. The current investigation aimed to generate mutations resistant to brown spot in ‘Mary’ kalanchoe through chemical mutagenesis followed by molecular marker identification. Putative mutants were developed by treating embryogenic calluses with ethyl methanesulfonate (EMS) at median lethal doses (LD50)–either a 0.8% concentration for 2 hours or a 1.0% concentration for 0.5 hours. Brown spot crude toxin solution was used as the selection agent to identify disease-resistant calluses during tissue culture. The optimal crude concentration (60%) was determined by soaking calluses with different concentrations of crude pathogen: 0%, 20%, 40%, 60%, and 80% (v/v). A total of 32 anti-brown spot lines were regenerated and tested for disease resistance with detached leaves. Three regenerated EMS mutant lines showed no obvious brown spot lesions on their leaves after the disease resistance assay and were subjected to polymorphism identification by start codon targeted (SCoT) molecular markers. Three (SCoT40, SCoT71, and SCoT72) of 45 selected primers were chosen to identify the mutants. This work may lay the foundation for further development of new disease-resistant cultivars of kalanchoe.

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Deron Caplan, Mike Dixon and Youbin Zheng

Controlled application of drought can increase secondary metabolite concentrations in some essential oil-producing crops. To evaluate the effects of drought on cannabis (Cannabis sativa L.) inflorescence dry weight and cannabinoid content, drought stress was applied to container-grown cannabis plants through gradual growing substrate drying under controlled environment. Fertigation was withheld during week 7 in the flowering stage until midday plant water potential (WP) was approximately −1.5 MPa (drought stress threshold). This occurred after 11 days without fertigation. A well-irrigated control was used for comparison. Leaf net photosynthetic rate (Pn), plant WP, wilting (leaf angle), and volumetric moisture content (VMC) were monitored throughout the drying period until the day after the drought group was fertigated. At the drought stress threshold, Pn was 42% lower and plant WP was 50% lower in the drought group than the control. Upon harvest, drought-stressed plants had increased concentrations of major cannabinoids tetrahydrocannabinol acid (THCA) and cannabidiolic acid (CBDA) by 12% and 13%, respectively, compared with the control. Further, yield per unit growing area of THCA was 43% higher than the control, CBDA yield was 47% higher, ∆9-tetrahydrocannabinol (THC) yield was 50% higher, and cannabidiol (CBD) yield was 67% higher. Controlled drought stress may therefore be an effective horticultural management technique to maximize both inflorescence dry weight and cannabinoid yield in cannabis, although results may differ by cannabis cultivar or chemotype.