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Genhua Niu, Denise S. Rodriguez, and Mengmeng Gu

Use of reclaimed water to irrigate urban landscapes will likely increase because fresh water supply is diminishing and the population continues to grow in the semiarid southwestern United States. Salt tolerance of two native landscape woody ornamentals, Texas mountain laurel (Sophora secundiflora) and Mexican redbud (Cercis canadensis var. mexicana), was investigated in a greenhouse experiment. Seedlings of the two species were grown in two substrates mixed with composted mulch and a commercial potting mix at two ratios and irrigated with saline solutions at three salinity levels: 1.6 (control, nutrient solution), 3.0, or 6.0 dS·m−1 electrical conductivity (EC). There was no interaction between substrate and EC of irrigation water. Foliar salt damages such as leaf drop, leaf curl, and edge burn were observed in Mexican redbud when the plants were irrigated with solutions at EC of 3.0 and 6.0 dS·m−1. No symptoms were observed on Texas mountain laurel plants, although plants irrigated at EC of 3.0 and 6.0 dS·m−1 were smaller compared with controls. Shoot growth and elongation of both species were reduced by the elevated salinity of irrigation water, and the reduction in Mexican redbud was greater than Texas mountain laurel. Leaf photosynthesis rate and leaf stomatal conductance were also reduced in Texas mountain laurel by the elevated salinity of irrigation water. Tissue Na+ and Cl concentrations were higher in Texas mountain laurel irrigated with water of elevated salinity.

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Youping Sun, Genhua Niu, and Christina Perez

Salt tolerance of seven Texas Superstar® perennials [Malvaviscus arboreus var. drummondii (Turk’s cap), Phlox paniculata ‘John Fanick’ (‘John Fanick’ phlox), Phlox paniculata ‘Texas Pink’ (‘Texas Pink’ phlox), Ruellia brittoniana ‘Katie Blue’ (‘Katie Blue’ ruellia), Salvia farinacea ‘Henry Duelberg’ (‘Henry Duelberg’ salvia), Salvia leucantha (mexican bush sage), and Verbena ×hybrida ‘Blue Princess’ (‘Blue Princess’ verbena)] was evaluated in a greenhouse experiment. Plants were irrigated with a nutrient solution at electrical conductivity (EC) of 1.1 dS·m−1 (control) or a salt solution at EC of 5.0 or 10.0 dS·m−1 (EC 5 or EC 10) for 8 weeks. ‘John Fanick’ and ‘Texas Pink’ phlox plants in EC 5 had severe salt foliage damage, while those in EC 10 were died. Mexican bush sage in EC 10 had severe salt foliage damage. Turk’s cap, ‘Katie Blue’ ruellia, ‘Henry Duelberg’ salvia, and ‘Blue Princess’ verbena had minor foliar damage regardless of treatment. EC 5 reduced the shoot dry weight (DW) by 45% in ‘Texas Pink’ phlox and 11% to 18% in ‘Katie Blue’ ruellia, ‘Henry Duelberg’ salvia, and mexican bush sage, but did not impact the shoot DW of Turk’s cap and ‘John Fanick’ phlox. EC 10 further decreased the shoot DW of ‘Katie Blue’ ruellia, ‘Henry Duelberg’ salvia, and mexican bush sage plants by 32%, 29%, and 56%, respectively. EC 5 decreased leaf net photosynthesis (Pn) of ‘Texas Pink’ phlox and mexican bush sage, while EC 10 reduced Pn of all species except ‘Henry Duelberg’ salvia and ‘Blue Princess’ verbena. ‘Katie Blue’ ruellia and ‘Blue Princess’ verbena had relatively lower leaf Na concentration and ‘John Fanick’ phlox, ‘Texas Pink’phlox, and mexican bush sage had higher leaf Cl concentrations. In summary, Turk’s cap, ‘Katie Blue’ ruellia, ‘Henry Duelberg’ salvia, and ‘Blue Princess’ verbena were the most tolerant perennials, and ‘John Fanick’ phlox, ‘Texas Pink’ phlox, and mexican bush sage were the least tolerant to salinity.

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Youping Sun, Joseph Masabni, and Genhua Niu

Excessive salinity in soil and irrigation water in combination with waterlogging in coastal regions can significantly reduce the productivity of many agricultural crops. To evaluate the plant growth responses to simulated seawater (SSW) flooding, seedlings of 10 vegetables (broccoli, chinese cabbage, chinese greens, cucumber, eggplant, kale, radish, ‘Red Crunchy’ radish, spinach, and tomato) were flooded with SSW at electrical conductivity (EC) of 44.0 ± 1.3 dS·m−1 or tap water at EC of 0.8 ± 0.1 dS·m−1 for 24 hours and grown subsequently for 2 weeks in a greenhouse. Chinese greens and cucumber plants died shortly after flooding with SSW, whereas other vegetables exhibited various degrees of visible salt damage. Chinese cabbage suffered the strongest reduction, whereas spinach, tomato, and eggplant exhibited the least decrease in dry weight (DW) due to SSW flooding in comparison with their perspective control. Two weeks after flooding treatment with SSW, net photosynthetic rate of broccoli, kale, spinach, and tomato was reduced by 43% to 67%, transpiration rate by 35% to 66%, and stomatal conductance (g S) by 51% to 82%. In summary, spinach, eggplant, and tomato were the most tolerant, whereas chinese cabbage, chinese greens, and cucumber were the least tolerant to SSW flooding.

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Genhua Niu, Terri Starman, and David Byrne

The responses of garden roses to irrigation water with elevated salts are unknown. Two experiments were conducted to evaluate the relative salt tolerance of 13 self-rooted rose cultivars by irrigating the plants with nutrient solutions at an electrical conductivity (EC) of 1.4 dS·m−1 (control) or nutrient saline solutions at EC of 3.1, 4.4, or 6.4 dS·m−1. In Expt. 1, ‘Belinda’s Dream’, ‘Caldwell Pink’, ‘Carefree Beauty’, ‘Folksinger’, ‘Quietness’, and ‘Winter Sunset’ plants were grown in a greenhouse from 13 Aug. to 21 Oct. (10 weeks). Shoot dry weight of all cultivars decreased as EC of irrigation water increased. ‘Winter Sunset’ was most sensitive among these cultivars to salt stress followed by ‘Carefree Beauty’ and ‘Folksinger’ with severe leaf injury at EC of 3.1 dS·m−1 or higher or death at EC of 6.4 dS·m−1. No visual damage was observed in ‘Belinda’s Dream’ or ‘Caldwell Pink’, regardless of the salinity level. In Expt. 2, ‘Basye’s Blueberry’, ‘Iceberg’, ‘Little Buckaroo’, ‘The Fairy’, ‘Marie Pavie’, ‘Rise N Shine’, and ‘Sea Foam’ plants were grown in the greenhouse from 29 Sept. to 16 Nov. (7 weeks) and irrigated with the same nutrient or nutrient saline solutions. Salinity treatment did not affect shoot dry weight of ‘Basye’s Blueberry’, ‘Little Buckaroo’, ‘Sea Foam’, and ‘Rise N Shine’. Shoot dry weight of ‘Iceberg’, ‘The Fairy’, and ‘Marie Pavie’ decreased as EC of irrigation water increased. No or little visual damage was observed in ‘Little Buckaroo’, ‘Sea Foam’, and ‘Rise N Shine’. Leaf tip burns were seen in ‘Iceberg’, ‘Marie Pavie’, ‘Basye’s Blueberry’, and ‘The Fairy’ at EC 6.4 of dS·m−1. Generally, these symptoms were less severe than those observed in Expt. 1, probably attributable partially to the shorter treatment period. Whereas shoot Na+ and Cl varied greatly among the rose cultivars, the shoot concentrations of Ca2+, K+, and Mg2+ did not. Generally, salinity-tolerant cultivars had higher shoot Na+ and Cl concentrations. In summary, in Expt. 1, ‘Belinda’s Dream’ was the most tolerant cultivar, whereas ‘Winter Sunset’ was the least tolerant followed by ‘Carefree Beauty’. In Expt. 2, ‘Iceberg’, ‘Marie Pavie’, and ‘The Fairy’ were less tolerant to salinity as compared with other cultivars, although the differences were small.

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Genhua Niu, Denise S. Rodriguez, and Cynthia McKenney

Wildflowers are good candidates for water-wise landscapes because many of them are drought-tolerant after establishment. Little information is available regarding whether these herbaceous wildflowers are tolerant to salt stress. Container experiments were carried out in a greenhouse and a shadehouse under semiarid climate conditions to investigate the salt tolerance of six native wildflowers: Salvia farinacea (mealy cup sage), Berlandiera lyrata (chocolate daisy), Ratibida columnaris (Mexican hat), Oenothera elata (Hooker’s evening primrose), Zinnia grandiflora (plains zinnia), and Monarda citriodora (lemon horsemint). In the greenhouse experiment, mealy cup sage, Hooker’s evening primrose, and plains zinnia were irrigated with a saline solution with an electrical conductivity (EC) of 1.5 (control, nutrient solution), 2.8, 4.1, 5.1, or 7.3 dS·m−1 for 45 days. All plants survived except for plains zinnia at EC of 7.3 dS·m−1. Shoot dry weights decreased as EC of irrigation water increased for all three species. In the shadehouse experiment (second year), plants of all species (plains zinnia was not included) were irrigated with saline solutions at EC of 0.8 (control, tap water), 2.8, 3.9, 5.5, or 7.3 dS·m−1 for 35 days. Plants were fertilized with slow-release fertilizer in the shadehouse experiment. After 5 weeks of treatment, all plants of lemon horsemint in the elevated salinity treatments, regardless of EC levels, were dead. The visual foliar salt damage rating was lowest for lemon horsemint. Chocolate daisy had low survival percentages and low foliar ratings at EC of 5.5 dS·m−1 and 7.3 dS·m−1. For the other three species, survival percentages were 80% and 90% at EC of 7.3 dS·m−1. Hooker’s evening primrose and mealy cup sage had similar low foliar visual ratings at EC of 7.3 dS·m−1, whereas Mexican hat plants had high foliar visual ratings regardless of salinity treatment. All species had similar high uptake of Na+ in shoots, whereas Hooker’s evening primrose had slightly higher Cl concentrations compared with other species. Based on these results, lemon horsemint was most sensitive to salinity stress followed by chocolate daisy. Hooker’s evening primrose and mealy cup sage were moderately tolerant and may be irrigated with low salinity water at EC of less than 3.9 dS·m−1. Mexican hat was the most tolerant among the six species.

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Genhua Niu, Denise S. Rodriguez, and Lissie Aguiniga

Salt-tolerant landscape plants are needed for arid and semiarid regions where the supply of quality water is limited and soil salinization often occurs. This study evaluated growth, chloride (Cl) and sodium (Na) uptake, relative chlorophyll content, and chlorophyll fluorescence of three rose rootstocks [Rosa ×fortuniana Lindl., R. multiflora Thunb., and R. odorata (Andr.) Sweet] irrigated with saline solutions at 1.6 (control), 3.0, 6.0, or 9.0 dS·m−1 electrical conductivity in a greenhouse. After 15 weeks, most plants in 9.0 dS·m−1 treatment died regardless of rootstock. Significant growth reduction was observed in all rootstocks at 6.0 dS·m−1 compared with the control and 3.0 dS·m−1, but the reduction in R. ×fortuniana was smaller than in the other two rootstocks. The visual scores of R. multiflora at 3.0 and 6.0 dS·m−1 were slightly lower than those of the other rootstocks. Rosa odorata had the highest shoot Na concentration followed by R. multiflora; however, R. multiflora had the highest root Na concentration followed by R. odorata. All rootstocks had higher Cl accumulation in all plant parts at elevated salinities, and no substantial differences in Cl concentrations in all plant parts existed among the rootstocks, except for leaf Cl concentration in R. multiflora, which was higher than those in the other two rootstocks. The elevated salinities of irrigation water reduced the relative chlorophyll concentration, measured as leaf SPAD readings, and maximal photochemical efficiency of photosystem II (PSII) and minimal fluorescence (Fo)/maximum fluorescence (Fv/Fm), but the largest reduction in Fv/Fm was only 2.4%. Based on growth and visual quality, R. ×fortuniana was relatively more salt-tolerant than the other two rootstocks and R. odorata was slightly more salt-tolerant than R. multiflora.

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Genhua Niu, Toyoki Kozai, and Chieri Kubota

A system was designed for measuring the CO2 exchange rates [net photosynthetic rate (Pn) and dark respiration rate] of in vitro plantlets in situ (in the vessel with natural ventilation). The system, excluding gas cylinders, was placed in a growth chamber so that the desired photosynthetic photon flux (PPF) and temperature could be maintained during the measurement. The CO2 concentration inside the culture vessel (Ci) was indirectly controlled by controlling the CO2 concentration outside the vessel (Co). The Pn of the plantlets was estimated based on the measured Ci and Co at steady state using a gas chromatograph according to the method described by Fujiwara et al. (1987). The performance of the system was demonstrated by measuring the in situ Pn of sweetpotato [Ipomoea batatas (L.) Lam., cv. Beniazuma] and tomato (Lycopersicon esculentum Mill., cv. Hana Queen) plantlets in vitro under a range of CO2 concentrations and PPF. The photosynthetic parameters of the Pn model (Niu and Kozai, 1997) for the plantlets were then estimated based on the measured Pn. The preliminary measurements demonstrated the potential application of the system.

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Shasha Wu, Youping Sun, and Genhua Niu

To provide more species for landscapes where poor-quality irrigation water is used, salt tolerance of commonly used landscape plants should be characterized. Nine ornamental species, including six herbaceous and three woody, were irrigated with nutrient solution at electrical conductivity (EC) of 1.2 dS·m−1 (control) or saline solution at EC of 5.0 or 10.0 dS·m−1 (EC 5 or EC 10) for 8 weeks and their growth and physiological responses were determined. Although growth was reduced in orange peel jessamine (Cestrum ‘Orange Peel’) and mexican hummingbird bush (Dicliptera suberecta) as salinity increased, no obvious signs of stress or injury were observed, indicating that orange peel jessamine and mexican hummingbird bush were the most salt tolerant. Flame acanthus (Anisacanthus quadrifidus var. wrightii), rock rose (Pavonia lasiopetala), and ‘Dark knight’ bluebeard (Caryopteris ×clandonensis ‘Dark Knight’) had more growth reduction than that of orange peel jessamine and mexican hummingbird bush with minimal or no foliar damage in EC 5 and slight foliar damage in EC 10. Cardinal flower (Lobelia cardinalis) and mexican false heather (Cuphea hyssopifolia) exhibited mortality rates of 30% and 20%, severe foliar damage, and greater than 70% reduction in leaf area and dry weight in EC 10 compared with their respective controls. Although the growth reductions in butterfly blue (Scabiosa columbaria) were not as great as cardinal flower and mexican false heather, 40% of butterfly blue plants were dead with moderate foliar damage in EC 10. Therefore, cardinal flower, mexican false heather, and butterfly blue plants were considered as moderately salt sensitive. Eastern red columbine (Aquilegia canadensis) was the most salt sensitive among the species investigated with moderate foliar damage in EC 5 and all plants died in EC 10. Four out of the nine species tested had significant differences in net photosynthetic rate (Pn), stomatal conductance (g s), and/or relative chlorophyll content between the control and EC 10, and the difference varied with species. Shoot ion concentrations of the nine ornamentals were also affected by salinity levels and varied among species.

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Genhua Niu, Royal Heins, and Will Carlson

Late-season height control of poinsettia (Euphorbia pulcherrima) is difficult since most chemical growth retardants adversely reduce bract size when applied after first bract color. Paclobutrazol (Bonzi) controls stem elongation late in poinsettia crop development but can excessively reduce bract size if improperly applied. Two experiments were conducted to quantify how paclobutrazol application influenced height and bract area of `Freedom' poinsettia. In the first experiment, paclobutrazol was applied at 1 mg·L-1 (ppm) in 118-mL (4.0-fl oz) volumes per pot [(a.i.) 0.12 mg/pot (28,350 mg = 1.0 oz)] as a drench to a new group of plants weekly from the initiation of short days until 1 week before anthesis. Maximum reduction in height and bract area was obtained when paclobutrazol was applied immediately after short days, and the response to paclobutrazol decreased as application time was increasingly delayed toward anthesis. In the second experiment, paclobutrazol was applied weekly after first bract color as either a drench or subapplication at various concentrations. Plant height and bract area were reduced by 23% when 2 mg·L-1 [(a.i.) 0.24 mg/pot) paclobutrazol was applied through subapplication at first color. The effects of paclobutrazol on height and bract area reduction decreased as application time was progressively delayed. Concentrations lower than 1 mg·L-1 had no significant effect on height or bract area reduction, regardless of application time or method. Generally, the reduction in height and bract area was larger when paclobutrazol was applied through subapplication. The combined results from both experiments indicate that paclobutrazol drench applications after flower initiation concomitantly reduce plant height (internode extension) and bract area. Therefore, drench applications should be delayed as long as possible to limit reduction in bract size.

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Genhua Niu, Denise Rodriguez, and Mengmeng Gu

Texas mountain laurel (Sophora secundiflora) is a native shrub tolerating drought, heat, windy conditions, and alkaline or wet soils. However, its availability is somewhat low and little information is available on nutrient requirement and other culture information. Two greenhouse experiments were conducted to quantify the responses of Texas mountain laurel to different forms and rates of nitrogen (N) fertilizer. In Expt. 1, 1-year old seedlings were treated for 194 days with three NO3:NH4 ratios at 25:75, 50:50, and 75:25 and two rates of N at 100 and 200 mg·L−1 in a factorial design. There was no interaction between the N rate and form on any growth parameters. Nitrogen form did not significantly affect shoot dry weight, root dry weight, root–to-shoot ratio, or the total dry weight. There was no significant difference between N rate of 100 and 200 mg·L−1 on root dry weight, root-to-shoot ratio, or the total dry weight. The shoot dry weight of Texas mountain laurel fertilized with 100 mg·L−1 was higher compared with that of the plants fertilized at 200 mg·L−1. The reduced shoot dry weight at N of 200 mg·L−1 was the result of the higher substrate salinity. In Expt. 2, seedlings were fertilized with five N rates (50, 100, 150, 200, and 250 mg·L−1) for 203 days. Plants watered with 150, 200, and 250 mg·L−1 were taller than those fertilized with 50 mg·L−1. The shoot height of plants watered with 100 mg·L−1 was only significantly different from 50 mg·L−1. For rapid growth of Texas mountain laurel, a N rate range of ≈150 mg·L−1 was recommended supplied with a combination of NO3-N and NH4-N in the ratios of 0.3 to 3.0.