Gaillardia aristata Foug. is a hardy, drought-tolerant perennial found throughout much of the United States. Little information exists on the salt tolerance of this plant when grown in various growing media. A study was conducted to characterize the response of G. aristata to three salinity levels (0.8, 2.0, or 4.0 dS/m) and four growing media: 1) 100% perlite; 2) 1 perlite: 1 Sunshine mix No. 4 (v/v); 3) 100% Sunshine mix No. 4; or 4) 1 Sunshine mix No. 4: 1 composted mulch (v/v). The type of medium influenced the dry weight of roots but not shoots, while salinity significantly influenced the dry weight of both shoots and roots. The dry weight of shoots was higher in plants irrigated with tap water (0.8 dS/m) compared to those irrigated with saline solution at 2.0 or 4.0 dS/m except for those grown in 100% Sunshine mix. The ratio of root to shoot dry weight was not influenced by salinity, but was highest in the plants grown in 100% perlite. Both medium and salinity affected plant height. Elevated salinity reduced plant height. Plants were taller when grown in 100% perlite and in 1 Sunshine mix: 1 composted mulch. However, plants had fewer lateral shoots when grown in 100% perlite or 1 Sunshine mix: 1 composted mulch. Some of the flower buds aborted when grown in 100% Sunshine mix or 1 perlite: 1 Sunshine mix compared to none in plants grown in 100% perlite or 1 Sunshine mix: 1 composted mulch. These results indicate that growth and morphology of G. aristata were affected by not only salinity, but also the type of medium.
Genhua Niu and Denise S. Rodriguez
Genhua Niu and Denise S. Rodriguez
Salt-tolerant garden roses (Rosa L.) are needed for arid and semiarid regions where high-quality water supply is limited and soil salinization often occurs. This greenhouse study evaluated growth, ion uptake characteristics, and the daily evapotranspiration rate (ET) of four rose rootstocks [‘Dr. Huey’ (Rosa ×hybrida L.), R. ×fortuniana Lindl., R. multiflora Thunb., and R. odorata (Andr.) Sweet] irrigated with saline solutions with chloride or sulfate as the dominant salts. After 16 weeks of treatment, the elevated salinities reduced growth of all rootstocks, but the magnitude varied with the rootstock and dominant salt type. At moderate [3.9 dS·m−1 electrical conductivity (EC)] and high salinities (7.9 to 8.2 dS·m−1), chloride-dominated salinity led to a greater growth reduction in R. × fortuniana, followed by R. odorata and R. multiflora. At high salinity dominated by sulfate, R. odorata had a greater growth reduction, followed by R. multiflora, ‘Dr. Huey’, and R. ×fortuniana. For R. multiflora, growth was reduced more in chloride-dominated salinity at high salinity levels, but no differences were found in the growth between the two salt types at moderate salinity. Rosa multiflora accumulated more Na than R. odorata, and R. ×fortuniana accumulated the least. However, R. multiflora retained most the Na in the roots, whereas R. odorata transported 57% of the Na to shoots. All rootstocks had a similar high leaf Cl concentration at high salinity dominated by chloride, while R. ×fortuniana had the most severe foliar salt damage, indicating that R. ×fortuniana had a lower threshold concentration of tissue Cl. At moderate salinity, all rootstocks had acceptable visual quality. At high salinity, the appearance of all rootstocks declined with typical salt damage on lower, older leaves, and the plants had lower visual scores in chloride-dominated salinity, especially in R. ×fortuniana. Salinity treatment did not affect the daily ET per unit leaf area, regardless of rootstock and dominant salt type. Daily ET per pot was the smallest in R. ×fortuniana among the four rootstocks due to its smaller total leaf area. The four rootstocks responded differently to salinity and dominant salt type.
Genhua Niu and Denise S. Rodriguez
Drought-tolerant garden roses (Rosa spp.) are needed in arid and semiarid regions where irrigation water is scarce. The vast majority of garden rose cultivars are budded or grafted onto rootstocks and are seldom grown on their own roots. The objective of this study was to compare the growth and physiological responses of four rose rootstocks to drought stress. Rosa ×hybrida ‘Dr. Huey’, R. ×fortuniana, R. multiflora, and R. odorata grown in 12-L containers were well-irrigated throughout the experiment or were subjected to mild drought stress for five or six cycles, depending on rootstocks, over 10 weeks. Following the mild drought stress cycles, plants that received the mild drought treatment were subjected to a severe dry-down before termination of the experiment. In R. ×fortuniana, drought stress did not affect any growth parameter. Compared with the well-irrigated plants, shoot dry weight of ‘Dr. Huey’, R. multiflora, and R. odorata were reduced by 22%, 33%, and 38%, respectively, by the drought treatment. The final leaf area of R. multiflora and R. odorata was reduced by 42% and 59%, respectively, compared with the control plants. The final leaf area of ‘Dr. Huey’ was not influenced by the drought treatment. Root to shoot ratio in ‘Dr. Huey’ was unaffected, while that of R. multiflora and R. odorata increased as a result of the drought treatment. As substrate moisture content decreased, leaf relative water content (RWC) of all rootstocks decreased linearly, with differences in decreasing slope or intercept of the linear regression lines among rootstocks. Predawn leaf water potential during the dry-down began to decrease rapidly when substrate moisture content decreased to below 20% (25% in R. odorata) in ‘Dr. Huey’, R. ×fortuniana, and R. multiflora. Leaf net photosynthetic rate (Pn), transpiration rate (E) and stomatal conductance (gs) of all rootstocks decreased rapidly during the dry-down as substrate moisture content decreased from 25%. In ‘Dr. Huey’ and R. ×fortuniana, Pn, E, and gs were closely correlated with leaf RWC, while E and gs of R. odorata were not. Correlations between gas exchange rates (Pn, E, and gs) and leaf RWC in R. multiflora were weaker than those in ‘Dr. Huey’ and R. ×fortuniana. At low substrate moisture content (below 15%), Pn, E, and gs in R. odorata were lower than those in other rootstocks. Growth reduction was greatest in R. odorata, regardless of the least water deficit of the substrate, followed by R. multiflora and ‘Dr. Huey’. The results of this study suggest that R. ×fortuniana was the most tolerant and R. odorata was the least tolerant to drought stress.
Genhua Niu and Denise S. Rodriguez
Use of recycled water to irrigate urban landscapes may be inevitable, because the freshwater supply has been diminishing and the population continues to grow in the arid and semiarid southwestern United States. However, little information exists on the performance of landscape plants irrigated with nonpotable water. Two greenhouse studies were conducted during the summer and the fall to characterize the relative salt tolerance of five herbaceous perennials by irrigating the plants with a saline solution at an electrical conductivity (EC) of 0.8 dS·m–1 (tap water), 2.0 dS·m–1, or 4.0 dS·m–1. In the summer study, after 10 weeks of treatment, Achillea millefolium L., Gaillardia aristata Foug., and Salvia coccinea Juss ex J. had an aesthetically acceptable appearance for landscape performance (visual quality scores of 4 points or more), whereas Agastache cana (Hook.) Woot. & Standl. and Echinacea purpurea (L.) Moench had relatively low tolerance to salinity. Dry weight of shoots of A. millefolium, A. cana, and G. arstata was lower at elevated salinity levels. In the fall study, A. millefolium, E. purpurea, G. arstata, and S. coccinea had acceptable growth and visual quality at elevated salinity levels, whereas A. cana had lower quality and reduced growth. Dry weight of shoots was lower in G. arstata and A. millefolium at an EC of 2.0 dS·m–1 or 4.0 dS·m–1. Leaf osmotic potential of all species in the summer experiment was significantly lower at higher salinity compared with the control. In the fall experiment, leaf osmotic potential in A. millefolium, E. purpurea, and G. aristata at 4 dS·m–1 was lower compared with lower salinity treatment and the control. Leaf osmotic potential in the fall was higher than that of the same species at the same salinity level in the summer experiment, indicating that plants in the fall were less stressed than in the summer. Combined the results from both experiments, the authors concluded that A. millefolium, G. arstata, and S. coccinea had a relatively high salt tolerance (as much as 4 dS·m–1 of irrigation water under greenhouse conditions) among the tested species, whereas A. cana and E. purpurea were not tolerant to salt and should not be irrigated with low-quality water.
Genhua Niu and Denise S. Rodriguez
In order to use reclaimed water to irrigate landscape plants and minimize damage and loss, salinity tolerance of commonly used landscape plants needs to be identified and characterized. Eight herbaceous perennials and groundcovers were obtained from a nursery, transplanted to 2.6-L plastic containers, and grown in the greenhouse for 2 weeks before salinity treatments (1.0, 3.2, 6.4, and 12 dS·m-1) were initiated. Plants were irrigated with measured amounts of saline solutions to obtain a 30% leaching when ≈50% water was depleted. After 12 weeks, half of the plants in each treatment were destructively harvested and dry weights of shoots and roots were taken. Three Penstemon species (pseudospectabilis, eatoni, and strictus) and Lavandula angustifoliaat 6.4 and 12 dS·m-1 and most of them at 3.2 dS/m did not survive. Shoot dry weight of Delosperma cooperidecreased by 25% at 12 dS·m–1, but there were no significant differences among the rest of the treatments. All plants of Teucrium chamaedryssurvived, but growth was reduced significantly with lower visual scores as salinity of irrigation water increased. Although growth was reduced in Gazaniarigensas salinity increased, no other signs of stress were observed. Ceratostigma plumbaginoides had less growth at 3.2 dS·m–1, and older leaves showed reddish pigmentation at 6.4 dS·m-1, whereas those at 12 dS·m-1 did not survive. Among the tested species, D.cooperiand G.rigensindicated a high tolerance to salinity; T. chamaedrysand C. plumbaginoides were moderately tolerant; and the rest were salt sensitive.
Genhua Niu and Denise S. Rodriguez
Salvia greggii (salvia) and Dalea frutescens (dalea) are two popular shrubs. However, little information is available on their drought tolerance. The objectives of this study were to investigate the effect of various degrees of water stress on growth and to characterize the dynamics of water relations to root substrate water content for developing best irrigation management. Salvia and dalea plants in 12-L plastic containers were grown in a greenhouse and pruned to one node at the base of the soft shoots for salvia or at the same height for dalea prior to the start of the experiment. There were three irrigation regimens: plants were irrigated daily (control), or irrigation was withheld until moderate or severe water stress signs exhibited. After several weeks of intermittent cyclic dry-down irrigation regimens, total shoot number per container was reduced by 40% to 50% for salvia and 35% to 40% for dalea. Average shoot length was reduced by 35% to 45% for salvia and 50% to 65% for dalea in moderate and severe stressed treatments compared to the control. Drought stress resulted in less shoot elongation and fewer new shoots in both species. To examine the relationship between plant water status and substrate water content, a dry down test was performed on five well-watered plants by withholding irrigation until midday water potential dropped to below –4 MPa. As substrate water contents in both species reached 8%, the predawn water potentials did not recover from the midday water potential of the previous day, indicating there was no available water in the substrate for roots to take up. The drought tolerance of these two species needs further study using various growing media.
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.
Genhua Niu, Denise S. Rodriguez, and Wayne Mackay
Oleander (Nerium oleander L.), native to southern Asia and the Mediterranean region, is a fast-growing evergreen shrub planted widely in the southern United States. A greenhouse study was conducted to quantify the growth and physiological responses of two cultivars, Hardy Pink and Hardy Red, and two breeding lines, EP1 and EP2, of oleander to a 12-week cyclic drought stress. Drought stress was imposed by irrigating the plants to near container capacity and then withholding irrigation until predetermined container weights were reached. Compared with the control where plants were well-irrigated throughout the experiment, shoot dry weight (DW) was reduced by 52%, 41%, 34%, and 11% in EP1, EP2, ‘Hardy Red’, and ‘Hardy Pink’, respectively. Root-to-shoot DW ratio was higher for the drought-treated plants than the control, regardless of cultivar or breeding line (hereafter, clone). The increase in root-to-shoot DW ratio from the drought treatment was highest in EP1, followed by EP2, ‘Hardy Pink’, and ‘Hardy Red.’ New shoot growth was greatest in ‘Hardy Pink’, followed by ‘Hardy Red’, EP1, and EP2. The number of newly developed shoots during the drought treatment period was 6.8, 3.0, 0.7, and 0.0 in ‘Hardy Pink’, ‘Hardy Red’, EP1, and EP2, respectively. As substrate volumetric moisture content decreased from 30%, leaf net photosynthetic rate (Pn), evapotranspiration rate (E), and stomatal conductance (gs) decreased in all clones. A curvilinear relationship between Pn and gs was found in all clones. EP1 had a lower maximum Pn (Pm) than those of ‘Hardy Pink’ and EP2 but was not different from that of ‘Hardy Red’. Predawn leaf water potential began to decrease rapidly when substrate moisture content dropped below 15% in all clones. During the dry-down, compared with the control, increases in minimal fluorescence (F0) or decreases in maximal fluorescence (Fm) and Fv/Fm (Fv = Fm – F0) in drought-stressed plants were observed in all clones, indicating some damage in photosystem II from the drought treatment. However, compared with growth parameters, the differences in physiological responses to drought stress among the clones were much smaller. ‘Hardy Pink’ was more tolerant to drought stress than ‘Hardy Red’ and the other two clones in terms of productivity because it maintained greatest growth during the drought-stress period. However, EP2 and EP1 may be more tolerant if survival is concerned because they had a higher root-to-shoot DW ratio with minimal new growth.
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.
Genhua Niu, Denise S. Rodriguez, and Terri Starman
Bedding plants are extensively used in urban landscapes. As high-quality water supply becomes limited in many parts of the world, the use of recycled water with high salt levels for landscape irrigation is being encouraged. Therefore, information on salt tolerance of bedding plants is of increasing importance. Two experiments were conducted, one in a 25% light exclusion shadehouse in summer (Expt. 1) and the other in a greenhouse in winter (Expt. 2). Plants were irrigated with saline solution at electrical conductivities of 0.8, 2.8, 4.0, 5.1, or 7.4 dS·m−1 created by adding NaCl, MgSO4, and CaCl2 to tap water to simulate the composition of local reclaimed water. In Expt. 1, shoot dry weight (DW) at the end of the experiments was reduced in all species at 7.4 dS·m−1 compared with the control (0.8 dS·m−1). The magnitude of reduction varied with species and cultivars. The salinity thresholds of irrigation water in which growth reduction occurred were 4.0 dS·m−1 for angelonia (Angelonia angustifolia) cultivars and ornamental pepper (Capsicum annuum) ‘Calico’ and 4.0 to 5.1 dS·m−1 for helenium (Helenium amarum), licorice plant (Helichrysum petiolatum), and plumbago (Plumbago auriculata). Shoot DW and growth index of ornamental pepper ‘Black Pearl’ and vinca (Catharanthus roseus) ‘Rose’ decreased linearly as salinity increased. All plants survived in Expt. 1 regardless of treatment, except for ornamental pepper ‘Purple Flash’. No visual injuries were observed in Expt. 1 regardless of treatment. Leaf sodium (Na) and chlorine (Cl) concentrations varied with species and treatments. Ornamental pepper ‘Black Pearl’ had the highest leaf Cl concentrations at higher salinities compared with other species and cultivars. Leaf Na concentrations in licorice plant and plumbago were in the range of 10 to 30 g·kg−1 DW, higher than those in other species. In Expt. 2, shoot DW was reduced by salinity treatments in ornamental pepper ‘Black Pearl’, plumbago, and angelonia but not in other species. The three ornamental peppers, ‘Black Pearl’, ‘Calico’, and ‘Purple Flash’, exhibited slight foliar injuries on some plants in Expt. 2 as a result of high salinity in the root zone in the highest salinity treatment. Ornamental pepper ‘Black Pearl’ was most sensitive to salinity stress. In general, the bedding plants tested in this study are moderately tolerant to salt stress and may be irrigated with saline water up to 4.0 dS·m−1 with little reduction in aesthetical appearance.