We examined the effectiveness of an elevated capillary mat system to maintain constant and different moisture levels in the growing medium and verify the potential of drought stress conditioning in producing small and compact bedding plants. To differentiate between plant height and compactness, we determined compactness as the leaf area or dry mass per unit stem length. Marigold `Queen Sophia' (Tagetes erecta L.) seedlings were grown in square, 9-cm-wide, 10-cm-high containers filled with a soilless growing medium. A capillary mat was laid on top of a greenhouse bench which was raised by 15 cm on one side compared to the other side to create an elevation effect. Seedlings were subirrigated by immersing the low end of the capillary mat in a reservoir of water. The amount of water moving to the higher end of the mat progressively decreased with elevation. The moisture content in the growing medium averaged from 26 to 294 mL/pot at different elevations. Regression analysis indicated that growth parameters including, shoot dry mass, leaf area, leaf number, and plant height decreased linearly with decreasing soil moisture content in the growing medium. Of all the measured growth parameters, plant height was found to be least sensitive to decreasing moisture content in the growing medium. Plants in high moisture treatments had more dry mass and leaf area per unit length of the stem compared to those in low moisture treatments. Our results indicate that drought stress can produce small, but not truly compact bedding plants.
Marc W. van Iersel and Krishna S. Nemali
Jong-Goo Kang and Marc W. van Iersel
To evaluate the effects of nutrient concentration and pH of the fertilizer solution on growth and nutrient uptake of salvia (Salvia splendens F. Sellow ex Roem. & Schult. `Scarlet Sage'), we grew plants with five different concentrations of Hoagland nutrient solution [0.125, 0.25, 0.5, 1.0, and 2.0× full strength; electrical conductivity (EC) of 0.4, 0.7, 1.1, 2.0, and 3.7 dS·m-1, respectively]. In a concurrent experiment, plants were subirrigated with modified Hoagland solution at 0.5× concentration and one of five solution pH values: 4.4, 5.4, 6.4, 7.2, and 8.0. Shoot and total dry weight and leaf area increased greatly with increasing nutrient solution concentrations from 0.125 to 1.0×, while leaf photosynthesis (Pn), transpiration, and stomatal conductance decreased with increasing nutrient solution concentrations. Treatment effects on growth apparently were caused by changes in carbon allocation within the plants. Shoot: root ratio and leaf area ratio increased with increasing fertilizer concentration. Plants flowered 8 days later at low concentrations of nutrient solution than at high concentrations. Shoot tissue concentrations of N, P, K, and B increased, while C, Al, Mo, and Na decreased with increasing concentration of the nutrient solution. The pH of the nutrient solution had no effect on the growth or gas exchange of the plants, while its effects on nutrient concentration in the shoot tissue generally were smaller than those of fertilizer concentration. These results indicate that 1.0 to 2.0× concentrations of Hoagland solution result in maximum growth, apparently because the plants produce leaf area more efficiently at high fertilizer concentrations.
Krishna S. Nemali and Marc W. van Iersel
We have developed a completely automated irrigation system that measures and maintains substrate volumetric water content (θ) at a target level for any length of time. Advantages of this system include complete automation of irrigation and simulation of precise levels of drought stress for potted plants. This system uses ECH2O moisture sensors interfaced with a CR10X datalogger and solenoid valves connected to the datalogger by a SDM CD16 AC/DC controller. The datalogger measures the θ of the substrate hourly. When the θ of the substrate drops below the set point, the datalogger opens the solenoid valves, which results in irrigation. Substrate θ is maintained at a constant level as the datalogger is programmed to increase θ by 2% to 3% during each irrigation. When the system was validated for its accuracy, we determined that the θ measured in the substrate within the range of 0.15 to 0.35 m3·m-3 was close (2% to 3%) to the θ determined by the conventional volumetric analysis. The daily average θ maintained in the substrate was slightly higher (within 3%) than the target level. Using this system, we were able to maintain four distinct levels of substrate θ for a prolonged period (40 days), regardless of differences in plant size and environmental conditions. Significant increases in number of irrigations, total water-use, and transpiration rate of impatiens, salvia, vinca, and petunia were noticed with increasing target θ of the substrate. For all species, highest and lowest water-use efficiency (WUE) were seen at 0.09 and 0.32 m3·m-3, respectively, while WUE was not different between 0.15 and 0.22 m3·m-3.
Erin C. James and Marc W. van Iersel
Nitrate pollution and water conservation are two of the most important environmental concerns for greenhouse growers. Closed irrigation systems, such as ebb and flow, can minimize these problems. The objective of this study was to determine optimal fertilizer concentrations for petunia (Petunia×hybrida Hort. Vilm-Andr.) and begonia (Begoni××emperflorens-cultorum Hort.) grown with ebb-and-flow irrigation. `Ambassador Scarlet' begonia and `Dreams Mix' petunia were grown as bedding plants in three soilless media. Plants were fertilized with solutions of a 20N-4.4P-16.6K water-soluble fertilizer with electrical conductivities (EC) of 0.15, 0.6, 1.2, 1.8, 2.4, or 3.0 dS·m-1. Maximum growth occurred with a fertilizer EC of 2.2 dS·m-1 for petunia and 1.6 dS·m-1 for begonia. Petunia growth was best in the medium with the highest porosity (Metro-Mix 220), but choice of medium had little effect on begonia growth. Leachate EC and pH were determined throughout the experiment, using the pour-through method. Leachate EC rose with increasing fertilizer concentration, and increased over time. The pH of the leachate decreased with increasing fertilizer concentration and dropped 0.5 to 1 unit over the course of the experiment with the higher fertilizer concentrations (≥0.6 dS·m-1). Plant growth was not very sensitive to leachate EC. Begonia and petunia grew well when the EC at the end of the production cycle was between 1.7 to 6.1 and 2.1 and 5.4 dS·m-1, respectively.
Holly L. Scoggins and Marc W. van Iersel
Several probes have been been recently developed that can be inserted directly into the growing medium of container-grown crops to get electrical conductivity (EC) or pH measurements. However, for many floriculture and greenhouse crops, EC interpretation ranges are based on substrate solution extraction methods such as the 1:2 v/v dilution, saturated media extract (SME), and more recently, the pour-through. We tested the sensitivity and accuracy of four in situ EC probes at a range of substrate moisture content and fertilizer concentrations. We also compared results from in situ probes with currently used methods of EC measurement. Concerning the effects of substrate volumetric water content (VWC) on the in situ probes, our results indicate little differences exist among probes when VWC exceeds 0.50, though drier substrates yielded differences depending on the measurement method. The SigmaProbe and W.E.T Probe measure the EC of the pore water specifically and show a decrease in EC with increasing water content, as the fertilizer ions in the pore water becomes more diluted as VWC increases. Results with the Hanna and FieldScout probes increased with increasing water content as the added water helps conduct the current of these meters. The EC measured with the various in situ probes differed slightly among the probes, but was highly and positively correlated with all three of the solution extraction methods over the range of fertilizer concentrations. It would be possible to convert substrate EC guidelines that have been established for any of the laboratory methods for use with the in situ probes, though our results indicate the substrate VMC must be above 0.35 for the interpretation to be valid.
Marc W. van Iersel and Sue Dove
Efficient water use in nurseries is increasingly important. In recent years, new soil moisture sensors (ECH2O probes) have become available, making it possible to monitor the moisture content of the growing medium in containers. One piece of information that is lacking for fully-automated irrigation systems is how much water actually needs to be present in the growing medium to prevent detrimental effects of drought on plants. We determined the effect of substrate moisture on photosynthesis and plant water relations of hydrangea and abelia. Growth rates of these species were measured during two subsequent drying cycles to determine how drought affects the growth rate of these species. Whole-plant photosynthesis, an indicator of growth rate, of both species remained stable as the volumetric moisture content of the substrate dropped from 25% to 15%, with pronounced decreases in photosynthesis at lower substrate moisture levels. Abelias and hydrangeas wilted when the substrate moisture level dropped to 6.3% and 8.3%, respectively. At wilting, abelias had lower leaf water potential (–3.7 MPa) than hydrangeas (–1.8 MPa). After the plants were watered at the end of the first drying cycle, the photosynthesis of the plants did not recover to pre-stress rates, indicating that the drought stress caused a long-term reduction in photosynthesis. Despite the more severe drought stress in the abelias (both a lower substrate water content and lower water potential at wilting), abelias recovered better from drought than hydrangeas. After the plants were watered at the end of the first drying cycle, the photosynthetic rate of abelias recovered to ≈70%, while the photosynthetic rate of the hydrangeas recovered to only 62% of the pre-stress rate.
Krishna S. Nemali and Marc W. van Iersel
Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO2 concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m3·m-3 for all species and did not differ between a θ of 0.15 and 0.22 m3·m-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m3·m-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m3·m-3. Based on these results, a θ of 0.22 m3·m-3 for salvia and impatiens and a slightly lower θ of 0.15 m3·m-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m3·m-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO2 transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.
Stephanie Burnett, Paul Thomas and Marc van Iersel
We previously found that incorporation of PEG-8000 into the growing medium delayed germination and resulted in shorter seedlings. However, in that study, we were unable to determine whether the reduced height was merely the effect of delayed germination or of reduced elongation after germination. To answer this question, we studied whether postgermination drenches with PEG-8000 can reduce seedling height. Annual salvia (Salvia splendens F. Sellow. ex Roem. & Shult. `Bonfire') and French marigold (Tagetes patula L. `Boy Orange') seedlings were treated with drenches of PEG-8000: 0, 15, 20, 30, 42, 50, 62, 72, or 83 g·L–1. At least 20% of seedlings treated with 62 to 83 g·L–1 of PEG-8000 were dead 14 d after treatment. Salvia and marigolds treated with the remaining PEG-8000 concentrations were up to 34% and 14% shorter than untreated seedlings, respectively. Leaf water (Ψw) and turgor potential (Ψp) also decreased for salvia which were grown with greater concentrations of PEG-8000, one probable cause of the observed reduction in elongation. Since the PEG-8000 in this study was applied after germination, it is clear that PEG-8000 does not reduce elongation merely by delaying germination, but also by reducing the elongation rate. Thus, postgermination drenches with PEG-8000 can be used to produce shorter seedlings.
Marc W. van Iersel and Lynne Seymour
Respiration is important in the overall carbon balance of plants, and can be separated into growth (Rg) and maintenance respiration (Rm). Estimation of Rg and Rm throughout plant development is difficult with traditional approaches. Here, we describe a new method to determine ontogenic changes in Rg and Rm. The CO2 exchange rate of groups of 28 `Cooler Peppermint' vinca plants [Catharanthus roseus (L.) G. Don.] was measured at 20 min intervals for 2 weeks. These data were used to calculate daily carbon gain (DCG, a measure of growth rate) and cumulative carbon gain (CCG, a measure of plant size). Growth and maintenance respiration were estimated based on the assumption that they are functions of DCG and CCG, respectively. Results suggested a linear relationship between DCG and Rg. Initially, Rm was three times larger than Rg, but they were similar at the end of the experiment. The decrease in the fraction of total available carbohydrates that was used for Rm resulted in an increase in carbon use efficiency from 0.51 to 0.67 mol·mol-1 during the 2-week period. The glucose requirement of the plants was determined from Rg, DCG, and the carbon fraction of the plant material and estimated to be 1.39 g·g-1, while the maintenance coefficient was estimated to be 0.031 g·g-1·d-1 at the end of the experiment. These results are similar to values reported previously for other species. This suggests that the use of semicontinuous CO2 exchange measurements for estimating Rg and Rm yields reasonable results.
Marc W. van Iersel and Jong-Goo Kang
To determine the effect of fertilizer concentration on plant growth and physiology, whole-plant C exchange rates of pansies (Viola ×wittrockiana Gams.) subirrigated with one of four fertilizer concentrations were measured over 30 days. Plants were watered with fertilizer solutions with an electrical conductivity (EC) of 0.15, 1.0, 2.0, or 3.0 dS·m-1 (N at 0, 135, 290, or 440 mg·L-1, respectively). Plants watered with a fertilizer solution with an EC of 2 dS·m-1 had the highest shoot dry weight (DW), shoot to root ratio, leaf area, leaf area ratio (LAR), and cumulative C gain at the end of the experiment compared to those watered with a solution with a higher or lower EC. Shoot tissue concentrations of N, P, K, S, Ca, Fe, Na, and Zn increased linearly with increasing fertilizer concentration. A close correlation between final DW of the plants and the measured cumulative C gain (CCG) (r2 = 0.98) indicated that the C exchange rates were good indicators of plant growth. There were quadratic relationships between fertilizer EC and gross photosynthesis, net photosynthesis, and dark respiration, starting at 13, 12, and 6 days after transplanting, respectively. Although plants fertilized with a fertilizer solution with an EC of 2 dS·m-1 had the highest C exchange rates, the final differences in shoot DW and CCG among ECs of 1.0, 2.0, and 3.0 dS·m-1 were small and it appears that pansies can be grown successfully with a wide range of fertilizer concentrations. Plants with a high LAR also had higher DW, suggesting that increased growth was caused largely by increased light interception. A detrimental effect of high fertilizer concentrations was that it resulted in a decrease in root DW and a large increase in shoot to root ratio.