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Neil S. Mattson and W. Roland Leatherwood

Silicon (Si) is a beneficial nutrient that improves biotic and abiotic stress tolerance of several crop species. Previous Si research with container-grown floriculture crops has either focused on a limited number of species or has been conducted in hydroponics using purified water, but little research has been conducted with plants grown in soilless substrates. The objective of this experiment was to examine whether weekly potassium silicate drenches would alter leaf Si concentration or affect morphological traits of several floriculture species grown in soilless substrate. Rooted liners of 21 cultivars were transplanted into a peat-based substrate. Control plants received no Si supplementation, whereas treated plants were given weekly drenches of 100 mg·L−1 Si from potassium silicate for 10 weeks. Leaf Si concentration of control plants ranged from 211 mg·L−1 for petunia (Petunia ×hybrida Vilm. ‘Cascadias Cherry Spark’) to 2606 mg·L−1 for argyranthemum [Argyranthemum frutescens (L.) Sch. Bip. ‘Sunlight’]. Si supplementation increased leaf Si concentration of 11 cultivars; leaf Si concentrations for these supplemented plants were 13% to 145% greater than control plants. Among the taxa studied, Si supplementation response was variable; Si either increased or decreased height, diameter, fresh weight, dry weight, flower diameter, and leaf thickness. For three cultivars, these morphological traits were apparently unaffected by Si supplementation despite accumulating Si. Similarly, significant morphological differences were observed in four cultivars that did not accumulate Si. Eight cultivars both accumulated Si and showed significant morphological differences. Our results demonstrate that many common floriculture species grown in a peat-based substrate do take up Si and that SI may have an effect on plant development. Consequently, more work is needed to determine the appropriate rate of Si supplementation and to examine additional species.

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Jinrong Liu, W. Roland Leatherwood and Neil S. Mattson

In the United States, overhead irrigation is common to apply water and dissolved nutrients to vegetable transplants during greenhouse production. Overhead irrigation allows for the control of salt accumulation in the growing medium because excess water can leach salts out of the container. Alternatively, subirrigation saves labor and improves water use efficiency, but soluble salts can accumulate in the upper profile of the containers. Consequently different sets of fertilizer and electrical conductivity (EC) guidelines are required for overhead and subirrigation systems. The objective of this project was to determine the influence of fertilizer concentration and irrigation method (subirrigation vs. overhead irrigation) on the growth of several vegetable transplant crops intended for retail sale. Seedlings of collards (Brassica oleracea var. acephala ‘Vates’), kale (B. oleracea var. acephala ‘Nagoya Mix’), lettuce (Lactuca sativa ‘Buttercrunch’), pepper (Capsicum annuum ‘Sweet Banana’), and tomato (Solanum lycopersicum ‘Sweet 100’) were transplanted into 4-inch-diameter containers and grown in a greenhouse for 4 weeks. Irrigation was provided via ebb and flow benches (subirrigation) or hand-watering (overhead irrigation). Plants received a complete fertilizer solution provided at a concentration of 50, 100, 200, 350, and 500 mg·L−1 nitrogen (N). The treatments resulting in maximum shoot dry weight (DW) for overhead irrigated plants were 100 mg·L−1 N for pepper, 200 mg· L−1 N for tomato, and 350 mg·L−1 N for collards, kale, and lettuce. Irrigation method and fertilizer treatment significantly affected fresh weight (FW) and DW for kale, lettuce, and pepper. For kale and lettuce, regression analysis indicated that maximum DW was reached at a lower fertilizer concentration with overhead irrigation than subirrigation. The treatments resulting in maximum DW for subirrigated plants were 200 mg·L−1 N for kale, lettuce, pepper, and tomato and 350 mg·L−1 N for collards. Reducing fertilizer concentration was an effective method for controlling plant height for all crops we examined except for ‘Sweet Banana’ pepper. However, in many cases height control via nutritional limitation comes at substantial expense to other growth parameters. Our results suggest that, in some cases, fertilizer concentration guidelines for overhead irrigation can be reduced when growing vegetable transplants with subirrigation due to reduced leaching of nutrients and greater potential for accumulation of fertilizer salts.

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W. Roland Leatherwood, John M. Dole and James E. Faust

Ethephon [(2-chloroethyl) phosphonic acid] is used to increase stock plant cutting productivity through increased flower and flower bud abscission and branching. However, ethylene evolution resulting from ethephon application is suspected to cause leaf abscission of unrooted cuttings during shipping. It was the objective of this study to assess ethylene evolution from ethephon-treated cuttings during storage and shipping of unrooted cuttings. Impatiens hawkeri W. Bull ‘Sonic Red’ and ‘Sonic White’ stock plants were treated with 0, 250, 500, or 1000 mg·L−1 ethephon. Cuttings were harvested from 1 to 21 days later and each harvest was stored at 20 °C in sealed jars for 24 h before ethylene measurement. Higher ethephon doses resulted in greater ethylene generation. Cuttings harvested 1 day after treatment with 0, 250, 500, or 1000 mg·L−1 ethephon evolved 0.07, 1.3, 1.7, or 5.8 μL·L−1·g−1 (fresh weight) ethylene in the first 24 h of storage at 20 °C, respectively. Twenty-one days after treatment, cuttings from the same plants evolved 0.05, 0.05, 0.15, or 0.14 μL·L−1·g−1 (fresh weight) ethylene in the first 24 h of storage at 20 °C, respectively. As cuttings were harvested from Day 1 to Day 21, ethylene concentrations evolved within the first 24 h of storage decreased exponentially. Rinsing cuttings, treated 24 h earlier with 500 mg·L−1 ethephon, by gently agitating for 10 s in deionized water reduced ethylene evolution to 0.7 μL·L−1·g−1 (fresh weight) as compared with 1.7 for unrinsed cuttings. Cuttings harvested 24 h after treatment with 500 mg·L−1 ethephon stored at 10, 15, 20, and 25 °C for 24 h evolved 0.37, 0.81, 2.03, and 3.55 μL·L−1·g−1 (fresh weight) ethylene. The resulting mean temperature coefficient (Q10) for the 10 to 25 °C range from all replications was 5.15 ± 0.85. Thus, ethylene continues to evolve from ethephon-treated Impatiens hawkeri stock plants for up to 21 days and can accumulate to high concentrations during cutting storage.

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W. Roland Leatherwood, D. Mason Pharr, Lisa O. Dean and John D. Williamson

Sugars and sugar alcohols have well-documented roles in salt tolerance of whole plants and maturing seeds. Less is known, however, about possible effects of these compounds during germination. Seeds from mannitol-accumulating salt-tolerant celery [Apium graveolens L. var. dulce (P. Mill.) DC], non-mannitol-accumulating salt-tolerant cabbage (Brassica oleracea L. var. capitata L. ‘Golden Acre’), and salt-sensitive non-mannitol-accumulating tobacco (Nicotiana tabacum L.) and arabidopsis [Arabidopsis thaliana (L.) Heynh.] were placed on vertical Phytagel plates containing 0 to 300 mm NaCl. Germination percentage, root elongation, and carbohydrate content of seeds and seedlings were assessed. With the exception of cabbage, there was no positive relationship between ability to germinate in NaCl and the reported species salt tolerance of the mature plant. For instance, while cabbage seeds germinated in 300 mm NaCl, germination of two celery cultivars was inhibited completely by 150 mm NaCl. In contrast, seeds from salt-sensitive tobacco and arabidopsis germinated in 200 mm NaCl. There was also no obvious relationship between the observed salt tolerance and total soluble carbohydrates in either non-imbibed seeds or in seedlings germinated in salt. For example, the most-salt tolerant species in these studies, cabbage, had the third highest seed and seedling carbohydrate concentration, while the next most tolerant, arabidopsis, had the lowest. However, both species contained significant amounts of the osmoprotective oligosaccharides raffinose or stachyose. In addition, although celery seedling mannitol concentration initially increased at low NaCl concentrations (50 mm), germination and mannitol concentration decreased at higher NaCl concentrations (100 mm). Finally, the broadest response observed was a large increase in seedling sucrose at the lowest salt concentration that significantly inhibited germination. Although most seeds, with the notable exception of cabbage, did not germinate at 150 mm NaCl, they were still metabolically active because the sucrose content was two to eight times higher than in non-imbibed seeds, suggesting a possible role for sucrose in salt-stressed germinating seeds. These results not only suggest that mechanisms providing salt tolerance in mature plants are different from those in germinating seeds, but also that, even when the same mechanisms are employed, they may be less effective in seeds.

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W. Roland Leatherwood, John M. Dole, Ben A. Bergmann and James E. Faust

Knowing which herbaceous taxa are ethylene sensitive and managing exposure of unrooted terminal stem cuttings to ethylene in those taxa are critical for maintaining high-quality propagules that root readily. Of 59 taxa surveyed, freshly harvested terminal cuttings of Begonia hybrid ‘Snowcap’, Lantana camara L. ‘Patriot Sunbeam’, and Portulaca oleracea L. ‘Fairytales Sleeping Beauty’ were sensitive to exogenous application of 1 μL·L−1 ethylene, as demonstrated by leaf abscission within 24 hours of treatment. Exposure to 1-methylcyclopropene (1-MCP) at 700 μL·L−1 for 4 hours before ethylene treatment prevented ethylene injury in these species/cultivars. Exposing unrooted cuttings to 700 μL·L−1 1-MCP induced significant endogenous ethylene biosynthesis in terminal cuttings of the five taxa tested: Euphorbia pulcherrima Willd. ex Klotzsch ‘Visions of Grandeur’, Impatiens hawkeri W. Bull ‘Sonic Red’, Pelargonium peltatum (L.) L’Hérit. ‘Mandarin’, Pelargonium ×hortorum Bailey (pro sp.) [inquinans × zonale] ‘Rocky Mountain White’, and Petunia ×hybrida Vilm. ‘Suncatcher Coral Prism’. Exogenous 1 μL·L−1 ethylene improved adventitious rooting in two cultivars: Begonia hybrid Anita Louise and Fuchsia triphylla L. Honeysuckle. Other trials showed that 1-MCP exposure reduced root number and length of P. ×hortorum ‘Kardino’ and delayed adventitious rooting in all six cultivars tested: Angelonia angustifolia Benth. ‘Carita Lavender’, Calibrachoa ×hybrida Llave & Lex. ‘Terra Cotta’, I. hawkeri ‘Sonic Red’, P. oleracea ‘Fairytales Sleeping Beauty’, Sutera cordata Kuntze ‘Abunda Blue Improved’, and Verbena ×hybrida Groenl. & Ruempl. ‘Aztec Wild Rose’. Subsequent exposure to 1 μL·L−1 ethylene partially mitigated the negative effects on rooting from exposing cuttings to 1-MCP.