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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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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, Royal D. Heins, Arthur Cameron, and Will Carlson

The effects of temperature on flower size and number of flower buds of Campanula carpatica Jacq. 'Blue Clips', 'Deep Blue Clips', and Campanula 'Birch Hybrid' were investigated in four temperature and light-transfer experiments. In year 1, 'Blue Clips' and 'Birch Hybrid' plants were grown initially at 20 °C and then transferred at visible flower bud (VB) to 14, 17, 20, 23, or 26 °C until flower (Expt. 1). In Expt. 2, 'Blue Clips' and 'Birch Hybrid' plants were transferred from 14 to 26 °C or from 26 to 14 °C at various intervals after flower induction. Flower size of both species was negatively correlated with average daily temperature (ADT) after VB; flowers on plants grown at 14 °C were 35% larger than those on plants grown at 26 °C. In contrast, temperature before VB had only a small effect on final flower size in both species, although flower diameter of 'Birch Hybrid' plants grown at constant 26 °C was 20% smaller than that of the plants grown initially at 20°C and then transferred to VB to 26 °C. For both species, the longer the exposure to high temperature after VB, the smaller the flowers. Number of flower buds at flower in 'Birch Hybrid' decreased as ADT after VB increased. In year 2, 'Deep Blue Clips' plants were grown at constant 20 °C under high or low daily light integral (DLI, 17 or 5.7 mol·m-2·d-1) until VB, and then transferred to 14, 17, 20, 23, or 26 °C under high or low DLI (Expt. 3). In Expt. 4, 'Deep Blue Clips' plants were grown at 14, 17, 20, 23, or 26 °C until VB, and then transferred to constant 20 °C under high or low DLI until flower. Flower size (petal length) was negatively correlated with ADT both before and after VB, while flower bud number was negatively correlated with the ADT only after VB, regardless of DLI. In both experiments, petal length decreased by 0.3 to 0.5 mm per 1 °C increase in ADT before or after VB. Flowers were larger and more numerous under high than under low DLIs after VB, regardless of the DLI before VB.

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Genhua Niu, Royal Heins, Arthur Cameron, and William Carlson

The influence of vernalization temperature and duration and devernalization treatments on subsequent growth and flower development of Campanula `Birch Hybrid' and Leucanthemum ×superbum Bergman ex J. Ingram `Snow Cap' was determined. In the vernalization experiment, plants of `Birch Hybrid' were vernalized at 0, 2.5, 5, 7.5, or 10 °C for 2, 4, 6, or 8 weeks. `Snow Cap' was vernalized at 0, 2.5, 5, or 10 °C for 2, 4, 6, or 8 weeks. In another devernalization experiment, plants of both species were moved to a high temperature (30/10 °C, day/night) growth chamber for 2 or 4 days at various times during or after the 6-week vernalization period. A 6-week vernalization was necessary to obtain 100% flowering in `Birch Hybrid', and 8 weeks of vernalization decreased time to flower by 7 to 10 days compared with 6-week vernalization. Exposure to high temperature for 2 days during or immediately after vernalization did not devernalize `Birch Hybrid' plants, while a 4-day exposure decreased flowering percentage in some treatments and delayed flowering by 7 to 10 days. There were no significant differences in flowering characteristics of `Snow Cap' plants vernalized at 0 to 5 °C for 4 to 8 weeks. A 2-week vernalization at 0, 2.5, 5, or 10 °C or 4 to 8 week vernalization at 10 °C delayed flowering by 5 to 10 days compared with those vernalized at 0 to 5 °C for 4 to 8 weeks. Exposure to high temperature for 2 d did not devernalize `Snow Cap' plants regardless of exposure times, but a 4-day exposure delayed flowering by 4 to 5 days in some treatments. Combined, the data indicate that `Birch Hybrid' has an obligate 6-week vernalization requirement and `Snow Cap' has a facultative 4-week vernalization requirement that can be fulfilled in the 0 to 10 °C range. Exposure to temperatures of 30 °C (9 h·d-1) for 12 out of 42 days did not devernalize either species but in some cases caused a small delay in time to flower.

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Genhua Niu, Denise S. Rodriguez, Lizzie Aguiniga, and Wayne Mackay

Use of recycled water to irrigate urban landscapes and nursery plants may be inevitable as fresh water supplies diminish and populations continue to grow in the arid and semiarid southwestern United States. Lupinus havardii Wats. (Big Bend bluebonnet) has potential as a cut flower and Lupinus texensis Hook. (Texas bluebonnet) as a bedding plant, but little information is available on salt tolerance of these species. A greenhouse study was conducted to characterize the growth in response to various salinity levels. Plants were grown in 10-L containers and drip-irrigated with synthesized saline solutions at electrical conductivity levels of 1.6, 3.7, 5.7, 7.6, or 9.4 dS·m−1. Although shoot growth of L. texensis was reduced as salinity levels increased, it was visually acceptable (without any visual injury) when irrigated with salinity levels of less than 7.6 dS·m−1. All plants survived at 7.6 dS·m−1, whereas only 15% did at 9.4 dS·m−1. In contrast, L. havardii had leaf injury at 5.7 dS·m−1. No plants survived at 9.4 dS·m−1, and only 7% plants survived at 7.6 dS·m−1. In addition, growth of L. havardii was significantly reduced and plants were shorter at elevated salinity levels. Cut raceme yield of L. havardii decreased at salinity levels greater than 3.7 dS·m−1. However, no difference in cut raceme yield was observed between the control and 3.7 dS·m−1, although shoot growth was reduced. Overall, L. texensis was more salt-tolerant than L. havardii.

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Zejin Zhang, Dongxian He, Genhua Niu, and Rongfu Gao

Dendrobium officinale, endemic to China, is a rare and endangered medicinal herb. As a result of its high economic value, slow growth, and diminishing wild population, protected cultivation is preferred. However, little information is available on its growing environment and photosynthetic characteristics. In this study, the photosynthetic patterns of D. officinale were investigated under various environmental conditions by measuring the net CO2 exchange rates continuously for several days or weeks. Under non-stressed growth chamber conditions with 12-hour light and 12-hour dark periods, D. officinale had concomitance of C3 and crassulacean acid metabolism (CAM) photosynthesis patterns. Different degrees of CAM in D. officinale, expressed as the percentage of CO2 exchanges in the dark period to the daily amount of CO2 exchanges, were observed depending on environmental conditions. With decreasing substrate water content, a typical CAM pattern was found, and concomitance of C3 and CAM patterns was found again when plants were rewatered. The accumulation of leaf titratable acidity during a dark period increased as substrate dried out but decreased again as plants were rewatered. A shorter light–dark cycle (4-hour light and 4-hour dark periods) led to a C3 pattern alone. The substrate moisture and light–dark cycle were inducible factors for switching between C3 and CAM patterns in D. officinale. These results indicate that D. officinale is a facultative CAM plant and the C3 pathway can be induced by controlling the growing environment. Further studies are needed to identify the optimal environmental conditions to enhance the growth of D. officinale.

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Genhua Niu, Pedro Osuna, Youping Sun, and Denise S. Rodriguez

Ornamental chile peppers are popular bedding plants. As high-quality water supply becomes limited in many parts of the world, alternative waters such as municipal reclaimed water is encouraged to be used for landscape irrigation. The purpose of this study was to assess the relative salt tolerance of 10 cultivars of ornamental chile peppers by irrigating the mature plants with saline solutions and germinating seeds in saline substrate in a greenhouse. In the mature plant salt tolerance experiment, plants were irrigated with nutrient solution (no addition of salts, control) or saline solution at electrical conductance (EC) of 4.1 dS·m−1 or 8.1 dS·m−1 for 8 weeks. Plants in the EC of 4.1 dS·m−1 treatment did not have any foliar salt damage regardless of cultivar. At EC of 8.1 dS·m−1, ‘NuMex Memorial Day’ had the most severe foliar salt damage, whereas ‘NuMex April Fool’s Day’, ‘NuMex Cinco de Mayo’, ‘NuMex Thanksgiving’, and ‘NuMex Twilight’ had little or no foliar damage. Shoot dry weight (DW) reduction at EC of 8.1 dS·m−1 compared with control was smallest in ‘NuMex Thanksgiving’ (15%), whereas ‘NuMex Memorial Day’ had the greatest reduction of 74% followed by ‘NuMex Christmas’ of 61%. The highest shoot DW reduction in ‘NuMex Memorial Day’ coincided with lowest visual score, indicating that this cultivar was the least tolerant to salinity. The leaf Na+ and Cl concentrations increased dramatically with increasing EC of the irrigation water in all cultivars. The highest Na+ concentration of 10.9 mg·g−1 DW at EC of 8.1 dS·m−1 was observed in ‘NuMex Christmas’. The highest Cl concentration at EC of 8.1 dS·m−1 was found in ‘NuMex Memorial Day’ with 64.8 mg·g−1 DW, which was four times higher than the control. In the seedling emergence experiment, seeds of the 10 cultivars were germinated in substrate either moistened with reverse osmosis water (EC ≈0) or saline solution at EC of 17.1 dS·m−1. ‘NuMex Christmas’ and ‘NuMex Memorial Day’ had the lowest relative seedling emergence index, indicating that these two cultivars were the least tolerant to salinity during the seedling emergence stage. ‘NuMex Thanksgiving’ and ‘NuMex Cinco de Mayo’ had the highest relative seedling emergence index. Combining the results from both experiments, we concluded that ‘NuMex Cinco de Mayo’ and ‘NuMex Thanksgiving’ were the most tolerant cultivars, whereas ‘NuMex Christmas’ and ‘NuMex Memorial Day’ were the least tolerant ones.

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Ockert Greyvenstein, Terri Starman, Brent Pemberton, Genhua Niu, and David Byrne

The decline of garden rose sales over the past 20 years can be partially attributed to the lack of material adapted to a wide range of landscapes, which includes adaptation to high temperature stress. Current methods for evaluating high temperature susceptibility in garden roses are based on field observations, which are time consuming and subjected to ever-changing environmental conditions. A series of experiments were conducted to optimize protocols and compare the use of chlorophyll fluorescence (CFL) and cell membrane thermostability (MTS) by way of electrolyte leakage as methods to screen for high temperature susceptibility. Immature leaves proved better than mature leaves for both CFL and MTS measurements, using either detached leaf or whole plant stress assays. MTS measured on immature leaves stressed in a water bath at 50 °C for 45 minutes proved most consistent in separating rose clones based on high temperature susceptibility. Stressing actively growing plants with flower buds of 2 mm in diameter in a heat chamber at 44 °C for 3 hours resulted in increased flower abscission and leaf necrotic lesions on more susceptible clones when compared with those that were heat tolerant. Combining MTS measurements from immature leaves stressed in a water bath with the flower abscission and leaf necrosis responses 10 days after stress in a heat chamber could be the first step to screen and select against the more susceptible clones in a garden rose breeding program. Power analyses suggest that the proposed MTS protocol would be efficient in detecting differences between clones when the difference in electrolyte leakage is greater than 10%.

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Genhua Niu, Royal Heins, Arthur Cameron, and William Carlson

The influence of daily light integral (DLI) before vernalization and vernalization temperature and duration on growth and flower development was determined for seed-propagated perennials Aquilegia ×hybrida Sims `Remembrance', Coreopsis grandiflora Hogg ex Sweet `Sunray', and Lavandula angustifolia Mill. `Hidcote Blue'. Seedlings were grown under two DLIs (4 or 14 mol·m-2·d-l) for 5 weeks before being vernalized at -2.5, 0, 2.5, or 5 °C for 2,4,5, or 8 weeks. `Remembrance' and `Sunray' plants were vernalized in the dark, while `Hidcote Blue' plants were vernalized in light at 5 to 10 μmol·m-2·s-l for 9 hourslday. After vernalization, plants were forced under a 16-h photoperiod in the greenhouse at 20±2 °C. `Remembrance' plants flowered uniformly when vernalized at 0 to 2.5 °C for 2 weeks or longer, and flower number, plant height, time to visible bud or to flower were generally not influenced by vernalization temperature or duration. No `Sunray' plants flowered without vernalization, and only a low percentage flowered with 4-week vernalization. Compared with low DLI, a 14 mol·m-2·d-1 before vernalization delayed flowering by 7 to 20 days in `Remembrance', but there were no substantial differences in flowering characteristics of `Sunray'. `Hidcote Blue' plants were best vernalized in the light at 5 °C for 8 weeks to obtain rapid and uniform flowering and the highest number of inflorescences. Flowering and survival percentages of `Hidcote Blue' were much lower for plants at 14 mol·m-2·d-l DLI compared to 4 mol·m-2·d-1.

Open access

Qiang Liu, Youping Sun, James Altland, and Genhua Niu

Tatarian dogwood (Cornus alba) is an ornamental shrub with white fruits, creamy-white flowers, and red stems in fall through late winter and is widely used in residential landscape, public parks, and botanical gardens. Two greenhouse experiments were conducted to characterize the survival, morphological, aesthetic, and physiological responses of tatarian dogwood seedlings to salinity and drought stresses. In Expt. 1, tatarian dogwood seedlings grown in three soilless growing substrates (Metro-Mix 360, 560, and 902) were irrigated with a nutrient solution at an electrical conductivity (EC) of 1.2 dS·m−1 (control) or saline solution (by adding calculated amount of sodium chloride and calcium chloride) at an EC of 5.0 or 10.0 dS·m−1 once per week for 8 weeks. Results showed that substrate did not influence the growth of tatarian dogwood seedling. All plants irrigated with saline solutions at an EC of 10.0 dS·m−1 died, whereas those irrigated with saline solutions at an EC of 5.0 dS·m−1 exhibited severe foliar salt damage with an average visual score of 1.0 (on a scale of 0 to 5, with 0 = dead and 5 = excellent without foliar salt damage). Compared with the control, saline solutions at an EC of 5.0 dS·m−1 reduced plant height and shoot dry weight (DW) by 50.8% and 55.2%, respectively. Relative chlorophyll content [soil plant analysis development (SPAD) reading], chlorophyll fluorescence (Fv/Fm), and net photosynthesis rate (Pn) also decreased when plants were irrigated with saline solutions at an EC of 5.0 and 10.0 dS·m−1. Leaf sodium (Na+) concentration of tatarian dogwood seedlings irrigated with saline solutions at an EC of 5.0 and 10.0 dS·m−1 increased 11 and 40 times, respectively, compared with the control, whereas chloride (Cl-) concentration increased 25 and 33 times, respectively. In Expt. 2, tatarian dogwood seedlings were irrigated at a substrate volumetric water contents (volume of water/volume of substrate, VWC) of 15%, 20%, 25%, 30%, 35%, 40%, or 45% using a sensor-based automated irrigation system for 60 days. Results showed that drought stress decreased plant growth of tatarian dogwood seedlings with a reduction of 71%, 85%, and 87% in plant height, leaf area, and shoot DW, respectively, when VWC decreased from 45% to 15%, but all plants survived at all VWC treatments. Significant reductions of photosynthesis (Pn), stomatal conductance (g S), transpiration rate (E), and water potential were also found in plants at a VWC of 15%, compared with other VWCs. However, SPAD readings and Fv/Fm of tatarian dogwood seedlings were similar among the VWCs. In conclusion, tatarian dogwood seedlings were sensitive to the salinity levels tested in this study but could survive at all tested substrate volumetric water contents and exhibited resistance to drought conditions.