Shading has been used to produce high-quality lettuce (Lactuca sativa) in locations where production conditions are not optimal for this cool-season crop. To learn what additional benefits shading provides if heat-tolerant cultivars are used and to understand the effects of shading on growth, sensory quality, chemical content, and transcriptome profile on heat-tolerant lettuce, we grew two romaine lettuce cultivars with and without shading using 50% black shadecloth in 2018 and 2019. Shading reduced plant leaf temperatures, lettuce head fresh weights, glucose and total sugars content, and sweetness, but not bitterness, whereas it increased lettuce chlorophyll b content compared with unshaded controls. Transcriptome analyses identified genes predominantly involved in chlorophyll biosynthesis, photosynthesis, and carbohydrate metabolism as upregulated in unshaded controls compared with shaded treatments. For the tested cultivars, which were bred to withstand high growing temperatures, it may be preferable to grow them under unshaded conditions to avoid increased infrastructure costs and obtain lettuce deemed sweeter than if shaded.
Camila M.L. Alves, Hsueh-Yuan Chang, Cindy B.S. Tong, Charlie L. Rohwer, Loren Avalos, and Zata M. Vickers
Zhenghai Zhang, Hai Sun, Cai Shao, Huixia Lei, Jiaqi Qian, Yinyin Ruan, and Yayu Zhang
Calcium (Ca) is necessary for plant growth and stress resistance, which are essential for the successful cultivation of Panax quinquefolium L. (American ginseng). However, information about the physiology of Ca nutrition in this species is limited. Therefore, the objective of this study was to determine the effect of Ca on the growth and physiological performance of American ginseng. Two-year-old American ginseng plants were supplemented with the following Ca concentrations [Ca2+] in a hydroponic system: 0, 160.17, 320.34, 640.68, and 961.02 mg⋅L−1. Measurements included growth biomass accumulation, chlorophyll (Chl) content and fluorescence, photosynthetic parameters, antioxidant enzyme activity, root activity, and malondialdehyde content. Biomass, stem height, leaf area, maximum photochemical efficiency, and superoxide dismutase activity peaked at [Ca2+] of 640.68 mg⋅L−1. Actual photochemical efficiency, minimum saturating irradiance, photosynthetic rate, catalase and peroxidase activities, and root activity reached their maximum at [Ca2+] of 320.34 mg⋅L−1. Stem diameter and regulated thermal energy dissipation increased with [Ca2+]. The sum of nonregulated heat dissipation and fluorescence emission and malondialdehyde content decreased to a minimum at [Ca2+] of 320.34 mg⋅L−1. The Chl content reached a maximum at [Ca2+] of 160.17 mg⋅L−1, but the Chl a/b ratio increased with [Ca2+]; the actual photochemical efficiency and photosynthetic rate reached their maximum level at Chl a/b ratios of 2.04 and [Ca2+] of 320.34 mg⋅L−1. Therefore, the optimal [Ca2+] for American ginseng growth was 320.34 mg⋅L−1. Furthermore, an appropriate increase [Ca2+] in the growth medium may improve biomass accumulation, light energy utilization efficiency, and stress resistance in American ginseng.
Jenny C. Moore, Brian Leib, Zachariah R. Hansen, and Annette L. Wszelaki
Growers seeking alternatives to traditional polyethylene plastic mulch may use biodegradable plastic mulches (BDMs). However, plasticulture systems typically also use plastic drip tape underneath the mulch, which must be removed from the field and disposed of at the end of the season, making tilling the BDM into the soil more difficult and expensive. A potential solution to this dilemma may be to use other irrigation methods, such as overhead sprinklers, that could be more easily removed from the field and reused from year to year. At Knoxville, TN, in 2019 and 2020, we grew three cultivars of romaine lettuce (Lactuca sativa) on BDM with two irrigation systems (overhead sprinklers above the mulch and drip irrigation tape under the mulch) to compare water use, disease, and yield in these two irrigation systems. Water use was higher in overhead vs. drip irrigation in both years; however, the difference in water use was much smaller in 2019 due to higher rainfall amounts during the time period the lettuce was growing in the field (March to May). Disease incidence and severity were very low both years for both irrigation systems. There were no differences in marketable yield (number of heads) between irrigation treatment in 2019. In 2020, marketable yield by number was greater in the drip vs. overhead irrigation treatment. Unmarketable yield in 2019 was due to heads that were too small; in 2020, unmarketability was predominantly due to tipburn in overhead irrigated ‘Jericho’. Overall, marketable lettuce yield did not differ between irrigation treatments in 2019 and was similar for ‘Parris Island Cos’ in 2020. Although quantitative weed counts were not made, observations of weed pressure between rows showed that weed pressure was higher in overhead irrigated compared with drip irrigated subplots. This highlights the need to have a between-row weed management program in place. The results of this study suggest that with attention to cultivar and weed management, overhead irrigation could be a viable alternative to drip irrigation for lettuce production on BDM, especially for early spring lettuce when rainfall is historically more plentiful.
Prashant Bhandari, Reza Shekasteband, and Tong Geon Lee
The first consensus genetic map in fresh-market tomato (Solanum lycopersicum) was constructed, combining genetic recombination data from two biparental F2 segregating populations derived from four different fresh-market tomatoes. Each F2 population was nominated by different academic tomato breeding programs located in major fresh-market tomato-producing areas of the United States, and chromosome-wide variation in recombination rates was observed between tomato populations based on the origin of their breeding programs. A consensus map constructed using 335 common single nucleotide polymorphism (SNP) sites found in both populations spanned 737.3 cM across 12 tomato chromosomes, with chromosome 2 containing more than 40% of the total SNPs and chromosomes 4, 5, 7, and 10 together representing less than 10% of the SNPs. There was a high degree of collinearity between the genetic and physical positions of those 335 SNP markers. The integration of 6553 SNP sites that were detected in either of the two populations with 335 common sites resulted in an extended consensus genetic map. The total length of the extended map was estimated to be 1997.9 cM, which was compatible with a previous estimate for large-fruited fresh-market tomato. A linkage panel for fresh-market tomato was also established using the combined dataset of the consensus map of 335 SNP loci and 73 SNP-genotyped core fresh-market tomatoes. An empirical genetic mapping study of the tomato brachytic trait using the linkage panel demonstrated the value of the consensus map and linkage panel for tomato research. The allelic information in the linkage panel will serve as a basis for SNP marker implementation, such as genotyping platforms and genomic association map, in tomato.
Shahrzad Bodaghi, Gabriel Pugina, Bo Meyering, Kim D. Bowman, and Ute Albrecht
Grafting a scion onto a rootstock results in physical and physiological changes in plant growth and development, which can affect tree vigor, productivity, and tolerance to stress and disease. Huanglongbing (HLB) is one of the most destructive citrus diseases and has become endemic in Florida since its introduction in 2005. It is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), which cause severe metabolic disruptions in affected plants. Although most scion cultivars are highly susceptible, some rootstock cultivars are tolerant and allow the grafted tree to cope better with the disease. The objectives of this study were to identify rootstock traits that can be used to assess cultivars under controlled greenhouse conditions in advance of longer-term field trials. We used 10 commercially important rootstocks with different genetic backgrounds and known field performance in graft combination with ‘Valencia’ sweet orange scion. Trees were graft-inoculated with CLas and compared against mock-inoculated trees. Tree health and CLas populations were assessed regularly, and root growth was monitored using a minirhizotron imaging system. Plants were excavated and destructively sampled 21 months after inoculation to assess biomass distributions and other CLas-induced effects. We found significant differences between healthy and infected trees for most variables measured, regardless of the rootstock. In contrast to leaf CLas titers, root titers were significantly influenced by the rootstock, and highest levels were measured for ‘Ridge’ sweet orange and sour orange. Root growth and root biomasses were reduced upon infection but differences among rootstocks did not always agree with reported field performances. Despite severe biomass reductions plants maintained their relative distribution of biomass among different components of the root system, and no dead roots were observed. Root respiration was reduced by CLas infection and was overall higher in tolerant cultivars suggesting its potential as a physiological marker. This study improves our knowledge about the strengths and weaknesses of assessing rootstock traits of grafted trees in a controlled greenhouse setting. Results from the study suggest that in addition to HLB tolerance, other rootstock traits will ultimately have major contributions to field survival and productivity of the grafted trees in an HLB endemic production environment.
Shahrzad Bodaghi, Bo Meyering, Kim D. Bowman, and Ute Albrecht
The devastating citrus disease huanglongbing (HLB) associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas) has caused a more than 70% reduction in citrus production since its discovery in Florida in 2005. Most citrus scion cultivars are sensitive to HLB, whereas some cultivars used as rootstocks are tolerant. Using such tolerant rootstocks can help trees to cope better with the disease’s impact. Evaluating rootstock effects on a grafted scion in the field takes many years, but shorter-term evaluation is imperative to aid in rootstock selection for an HLB-endemic production environment. In this study, we investigated grafted healthy and CLas-infected citrus trees under controlled greenhouse conditions. The objectives were to identify traits suitable for assessing grafted tree tolerance in advance of longer-term field studies and aiding in the selection of superior rootstock cultivars. We assessed 10 commercially important rootstocks grafted with ‘Valencia’ sweet orange scion and with known field performance. At 6, 9, 15, and 21 months after graft inoculation (mai), leaf CLas titers were determined and canopy health was evaluated. Plants were destructively sampled at 21 mai to assess plant biomasses and other physiological and horticultural variables. There was little influence of the rootstock cultivar on CLas titers. Surprisingly, few HLB foliar disease symptoms and no differences in soluble and nonsoluble carbohydrate concentrations were measured in infected compared with healthy plants, despite high CLas titers and significant reductions in plant biomasses. Most trees on rootstocks with trifoliate orange parentage were less damaged by HLB than other rootstocks, although results did not always agree with reported field performance. Among the different variables measured, leaf size appeared to be most predictive for grafted tree assessment of HLB sensitivity. The results of this study provide a better understanding of the strengths and weaknesses of assessing rootstock influence on grafted tree performance in a controlled greenhouse environment. Although such studies provide valuable information for cultivar tolerance to HLB, other rootstock traits will ultimately contribute to field survival and productivity in an HLB endemic production environment.
Yuvraj Khamare, S. Christopher Marble, James E. Altland, Brian J. Pearson, Jianjun Chen, and Pratap Devkota
Substrate stratification is a method of filling nursery containers with “layers” of different substrates, or different textures of the same substrate. Recently, it has been proposed as a means to improve drainage, substrate moisture dynamics, and optimize nutrient use efficiency. Substrates layered with larger particle bark as the top portion and smaller particle bark as the bottom portion of the container profile would theoretically result in a substrate that dries quickly on the surface, thereby reducing weed germination, but that would also retain adequate moisture for crop growth. The objective of this study was to evaluate the effect of stratified substrates on the growth of common nursery weeds and ornamental crops. This study evaluated the use of coarser bark (<0.5 or 0.75 inches) as the top substrate and finer bark (<0.38 inches) as the bottom substrate with the goal of reducing the water-holding capacity in the top 2 to 3 inches of the substrate to reduce weed germination and growth. Results showed that substrate stratification with more coarse bark on the top decreased the growth of bittercress (Cardamine flexuosa) by 80% to 97%, whereas liverwort (Marchantia polymorpha) coverage was reduced by 95% to 99%. Substrate stratification initially reduced the growth of ligustrum (Ligustrum japonicum) and blue plumbago (Plumbago auriculata), but there was no difference in the shoot or root dry weights of either species in comparison with those of nonstratified industry standard substrates at the end of 24 weeks. The data suggest substrate stratification could be used as an effective weed management strategy for container nursery production.
Yiyun Lin and Michelle L. Jones
Microbial biostimulants can promote ornamental plant growth during production and improve crop performance under abiotic stresses. Even though biostimulants have shown potential in many agricultural applications, the effectiveness and specificity of many products are not well understood. The objective of this study was to analyze the growth-promoting effects of microbial biostimulants during the greenhouse production of floriculture crops. We evaluated 13 biostimulant products in greenhouse-grown zinnia (Zinnia elegans ‘Magellan Ivory’) and petunia (Petunia ×hybrida ‘Carpet White’) at low fertility (one-third of the optimal fertilizer concentration). Biostimulant products 1 and 2 containing multiple species of beneficial bacteria and fungi, and product 10 containing Bacillus subtilis QST 713, were found to increase various aspects of plant growth, including the growth index, leaf chlorophyll content (SPAD index), and shoot biomass. Both flower biomass and numbers were greater in petunia treated with product 1, and leaf size increased in zinnia treated with products 1, 2, and 10. Plants treated with these effective biostimulants at low fertility had similar or better growth and quality than untreated plants grown under optimal fertility. The concentration of various nutrient elements in leaves was higher in zinnia plants treated with biostimulant products 1, 2, or 10 compared with the negative control. Some putative mechanisms for biostimulant effectiveness, the possible reasons for biostimulant ineffectiveness, and the potential for using biostimulants as a sustainable cultural strategy are discussed. This study provides useful information about microbial biostimulant effectiveness, which is important for the development and utilization of biostimulants in the greenhouse production of floriculture plants.
Heather Kalaman, Sandra B. Wilson, Rachel E. Mallinger, Gary W. Knox, and Edzard van Santen
Diverse floral resources impart immense value for pollinating insects of all types. With increasing popularity and demand for modern ornamental hybrids, cultivation by breeders has led to selection for a suite of traits such as extended bloom periods and novel colors and forms deemed attractive to the human eye. Largely understudied is pollinator preference for these new cultivars, as compared with their native congeners. To address this gap in understanding, 10 species of popular herbaceous flowering plants, commonly labeled as pollinator-friendly, were evaluated at two sites in Florida [U.S. Department of Agriculture (USDA) cold hardiness zones 8b and 9a] and across three seasons for their floral abundance and overall attractiveness to different groups of pollinating insects. Each genus, apart from pentas, encompassed a native and nonnative species. Native species included blanket flower (Gaillardia pulchella), lanceleaf coreopsis (Coreopsis lanceolata), pineland lantana (Lantana depressa), and scarlet sage (Salvia coccinea). Nonnative species included Barbican™ yellow-red ring blanket flower (G. aristata ‘Gaiz005’), Bloomify™ rose lantana (L. camara ‘UF-1011-2’), mysty salvia (S. longispicata ×farinacea ‘Balsalmysty’), Lucky Star® dark red pentas (Pentas lanceolata ‘PAS1231189’), ruby glow pentas (P. lanceolata ‘Ruby glow’) and Uptick™ Gold & Bronze coreopsis (Coreopsis × ‘Baluptgonz’). Flower-visiting insects were recorded during five-minute intervals in the morning and categorized into the following morpho-groups: honey bees, large-bodied bees (bumble and carpenter bees), other bees (small to medium-bodied native bees), butterflies/moths, and wasps. Floral abundance and pollinator visitation varied widely by season, location, and species. Of the plant species evaluated, nonnative plants produced nearly twice as many flowers as native plants. About 22,000 floral visitations were observed. The majority of visits were by native, small to medium-bodied bees (55.28%), followed by butterflies and moths (15.4%), large-bodied native bees (11.8%), wasps (10.0%), and honey bees (7.6%). Among plant genera, both native and nonnative coreopsis and blanket flower were most attractive to native, small to medium-bodied bees (e.g., sweat bees, leafcutter bees) with the greatest number of visitations occurring during the early and midmonths of the study (May–August). Across the study, butterflies and moths visited lantana more frequently than all other ornamentals evaluated, whereas pentas were most attractive to wasps. Large-bodied bees visited plants most frequently in May and June, primarily foraging from both native and nonnative salvia. While results from this study showed nominal differences between native and nonnative species in their ability to attract the studied pollinator groups, care should be taken to making similar assessments of other modern plant types.
Heather Kalaman, Sandra B. Wilson, Rachel E. Mallinger, Gary W. Knox, Taehoon Kim, Kevin Begcy, and Edzard van Santen
Consumer demand for novel, visually attractive ornamentals has often overshadowed the functional value plants may provide for flower-visiting insects. As native and nonnative species are hybridized for form, color, flowering, and disease resistance, it is important to assess whether some of these alterations influence plant nutrient quality for foraging insect pollinators. A study was conducted to ascertain the resource value of ornamental cultivars compared with their native congeners. The nectar volume and pollen quantity, viability, and protein content of 10 species of popular herbaceous flowering plants, commonly advertised as pollinator-friendly, were evaluated in northcentral Florida. Each genus encompassed a native and nonnative species, apart from pentas. Native species included blanket flower (Gaillardia pulchella), lanceleaf coreopsis (Coreopsis lanceolata), pineland lantana (Lantana depressa), and scarlet sage (Salvia coccinea). Nonnative species included Barbican™ yellow-red ring blanket flower (Gaillardia aristata ‘Gaiz005’), Bloomify™ rose lantana (Lantana camara ‘UF-1011-2’), mysty salvia (Salvia longispicata × farinacea ‘Balsalmysty’), Lucky Star® dark red pentas (Pentas lanceolata ‘PAS1231189’), ruby glow pentas (Pentas lanceolata ‘Ruby glow’) and UpTick™ Gold & Bronze coreopsis (Coreopsis × ‘Baluptgonz’). Floral rewards differed significantly across species. The native scarlet sage exhibited the largest nectar volume per flower in the summer (2.13 ± 0.17 µL), followed by the nonnative mysty salvia (1.26 ± 0.17 µL). In the fall, ruby glow pentas exhibited the largest nectar volume per flower (1.09 ± 0.17 µL) compared with all other ornamentals. The composite flowers of the native and nonnative blanket flower and coreopsis species had the lowest nectar volume per flower regardless of sampling date. Likewise, ruby glow pentas displayed the highest quantity of pollen grains (96.29 ± 0.12) per sample, followed by Lucky star pentas (52.33 ± 0.12), and Barbican blanket flower (50.98 ± 0.12). Pollen viability was similarly high (92% to 98%) among all species, apart from Bloomify rose lantana (20%) and pineland lantana (48%). Pollen protein content was highest in Uptick coreopsis (11.378 ± 1.860 μg/mg dry weight) and Lucky star pentas (10.656 ± 3.726 μg/mg dry weight), followed by lanceleaf coreopsis (7.918 ± 1.793 μg/mg dry weight). These results largely showed that the nonnative ornamentals selected provided resource-rich floral rewards, comparable to native congeners. Still, care should be taken in making similar assessments of other modern floral types.