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  • Author or Editor: Luis-Alonso Valdez-Aguilar x
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Response to alkalinity was evaluated in two hibiscus cultivars, Bimini Breeze and Carolina Breeze, grown in a soilless growing medium and in hydroponic culture. For soilless growing medium, plants were potted in a sphagnum peat–perlite-based substrate and irrigated with solutions containing 0 to 10 mm NaHCO3 for 12 weeks. In hydroponic culture, bare-rooted plants were transferred to a 9-L tray containing a Hoagland's nutrient solution prepared with NaHCO3 at the concentrations previously indicated. In soilless growing medium, shoot dry weight was minimally affected by NaHCO3 concentration for `Bimini Breeze', but `Carolina Breeze' exhibited a significant decrease in shoot mass with increasing NaHCO3 concentration. In hydroponic culture, increasing concentration of NaHCO3 induced a decrease in shoot and root mass in both cultivars, but root mass decrease was more pronounced in `Bimini Breeze'. In soilless growing medium, increasing the concentration of NaHCO3 caused an increase in growing medium pH. The pH increase was less pronounced for `Bimini Breeze' than for `Carolina Breeze', indicating a higher capacity for root zone acidification by `Bimini Breeze'. Newly developed leaves of both cultivars showed increasing chlorosis with increasing NaHCO3 concentration. However, `Bimini Breeze' was more tolerant because, according to regression models, 5.7 mm NaHCO3 would be required to reduce chlorophyll levels by 10%, compared with 2.2 mm for `Carolina Breeze', when grown in soilless medium. Fe reductase activity decreased when `Carolina Breeze' plants were grown in 5 mm NaHCO3. However, in `Bimini Breeze', Fe reductase activity was enhanced. These observations indicate that the increased tolerance of `Bimini Breeze' to increasing alkalinity is the result of enhanced Fe reductase activity and increased acidification of the root zone.

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During Fall 2004, poinsettia plants were grown in a greenhouse (Texcoco, Edo. Mexico, 19°29'N). The objectives were to: 1) evaluate which soluble carbohydrates (mono- and disaccharides) are present in the cultivars Supjibi and Peter Star and their concentrations; 2) study the relationship between sugar content and flower induction; and 3) analyze the relationship between starch content and phenology of the plant. Apical meristems were prepared for microscopy, soluble sugars, and sugars from starch hydrolysis were studied by HPLC. Flower induction in `Supjibi' took place about 99 days after transplant (DAT), with no artificial short photoperiod. In `Peter Star', flower induction took place about 137 DAT, 19 days after initiation of short-day photoperiod. Soluble sugars found were: sucrose, maltose, glucose, and fructose (in order of the highest to lowest concentration). Concentration varied from 0.5% to 2.1% for `Supjibi' and from 1.1% to 2.9% for `Peter Star', based on fresh weight. Sucrose content is reduced in root and mature leaf during flower induction, probably sent to young leaves. During flower induction, there is also an increase in glucose in young leaves. Sugars from starch hydrolysis were: fucose, (6-desoxi-L-galactose), fructose, and galactose. Soluble sugars content generated from starch varies in each organ from 2.0% to 32% for `Supjibi', and from 2.0% to 39% in `Peter Star'. During induction, starch content is reduced (between 6% and 9%). After flower induction, there is an increase in leaf area and in starch content (from 32% to 39%), during bract development starch seems to be utilized in this plant part.

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Irrigation water high in alkalinity can severely compromise the growth and marketability of ornamental plants. In the present study we investigated the response of lisianthus to increased calcium (Ca) when irrigated with solutions containing high levels of bicarbonate (HCO3 )-induced alkalinity. Alkalinity in irrigation water reduced the growth of lisianthus; however, plants supplemented with an increased concentration of Ca at alkalinity levels from 4 to 7 meq·L−1 of HCO3 exhibited improved growth and dry mass (DM) accumulation or were not detrimentally affected, demonstrating that Ca contributes to the increase of the tolerance of lisianthus to alkalinity. Supplementary Ca did maintain a high stomatal conductance (g S) and transpiration rate when alkalinity was at 4 meq·L−1, which explained the lower water potential in young leaves. Plants irrigated with solutions containing supplementary Ca had higher total DM, which was associated with a higher g S; however, when conductance was higher than 0.280 cm·s−1, like in plants with no supplementary Ca, DM tended to decrease. At a typical Ca concentration, there was a disruption on stomata functioning as g S and transpiration rate increased, which was associated with a reduction in shoot potassium (K). Calcium ameliorated the uptake of K when alkalinity was 4 meq·L−1 by allowing a less marked reduction in shoot K concentration. Chlorophyll was reduced by increasing alkalinity as a result of a decrease in shoot iron (Fe); however, supplementary Ca also contributed in increasing plant tolerance to alkalinity at 4 meq·L−1 by sustaining a high shoot Fe concentration. Supplementary Ca increased catalase and peroxidase activities, indicating that lisianthus responded to the stress by enhancing the activity of these enzymes to reduce oxidative damage.

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