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  • Author or Editor: Jonathan M. Frantz x
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Two warm-season bedding plant species, zinnia (Zinnia elegans) and vinca (Catharanthus roseus), were used to determine if phosphorus (P) supply should be adjusted with light supply, and if deficiency and/or oversupply symptoms were apparent at different P rates when growth rates were altered by light levels. An additional goal was to determine the influence of P and light on overall P uptake efficiencies and water use efficiencies. Plants were grown in a greenhouse with or without shade over portions of the bench and supplied 0.1, 0.2, 0.5, 1, 2, or 4 mm P along with complete nutrient solution as needed with no leaching fraction. Optimum plant growth and flower development rate occurred at a P supply of 0.5 mm regardless of the light supply. Plant growth was greatly reduced by P supply below 0.5 mm regardless of shade conditions. Tissue P concentration was not influenced by light, but overall P content (mg P per plant) was higher when plants were grown without shading as a result of larger plants in higher light environments. The appearance or severity of deficiency symptoms also was not influenced by light. Water use efficiency was maximized when growth was not limited by P supply (at or above 0.5 mm). One hundred percent recovery of applied P was obtained at the 0.5 mm P supply in vinca, whereas recovery was less at the same P supply in zinnia. These results indicate no benefit for plant growth and flowering to P supply above 0.5 mm and illustrate how P content is demand-driven. However, there was no induction or delay of nutrient stress symptoms as a result of different plant growth rates in the different light differences environments.

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A major source of power consumption in controlled-environment crop production is plant-growth lighting. Methods developed to minimize this source of power consumption will reduce the negative environmental impact of crop production through more-efficient management of non-renewable resources. One such method uses “intracanopy lighting,” in which the plants are allowed to grow through multiple levels of low-intensity lamps to irradiate the understory that normally is shaded when traditional overhead lighting is used. Early results with cowpea (Vigna unguiculata L. Walp `IT87D-941-1') indicate a significant reduction in net power consumption within a given growth area or volume while enhancing the harvest index (HI = percent edible biomass). Incorporation of mylar reflectors and manipulation of lamp geometries for more-efficient use of available photosynthetically active radiation, while maintaining low power consumption are the focus of present experiments. Photosynthetic rates by leaves of different ages and positions within the canopy are measured as a way of determining lighting efficiency. The productivity parameters HI, edible yield rate (EYR = gDW × m–2 × day–1), yield efficiency rate (YER = gDW edible × m–2 × day–1 [gDW non-edible]-1), energy conversion efficiency (ECE = EYR × [kW·h]–1), and energy partition efficiency (EPE = YER × [kW·h]–1) express the costs of edible biomass production in terms of the spatial, temporal, energetic, and non-edible biomass penalties. [Research supported in part by NASA grant NAGW-2329.]

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Fuel prices have fluctuated wildly in the last several years, and faced with unpredictable or rising fuel costs, growers often lower temperature set points to decrease fuel use. However, plant growth and development are influenced by lower temperatures and may actually cause increases in fuel use as a result of longer production times. Alternative strategies to efficient crop production are needed. Fertility, light, and CO2 are other environmental factors that can be manipulated within a greenhouse but how all three interact together on growth and development are surprisingly not well known. Petunia ×hybrida Vilm. were grown in controlled environments in a 2 × 2 × 2 factorial study investigating how light, fertility, and CO2 influence growth and development, including shoot partitioning, nutrient uptake, and carbohydrate concentration. Generally, light enhanced flowering, both mass and fraction of total biomass, whereas increased fertility was detrimental to the proportion of biomass allocated to flowers. The influence of CO2 was complex with high CO2 suppressing flowering and enhancing leaf growth, but only midway through the 7-week experiment. Carbohydrate concentration remained high in elevated CO2, even when light and fertility were not limiting. This suggests a sink limitation, so even in high light and fertility, crop response to enhanced CO2 was low. Although CO2 had no size effect late in growth, CO2 suppressed nutrient concentrations. Together, these data suggest strategies that growers may have in controlling their crop growth and development and indicate that enhanced growth (leaf and steam mass) may be at the detriment of development (flowering mass and allocation).

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In protected environments, temperature is often regulated to produce ornamental crops for specific market dates. Temperature primarily controls plant developmental rate and thus production time, but it can also interact with light quantity to affect crop quality attributes such as flower number, branching, and biomass accumulation. We quantified how mean daily temperature (MDT) between 14 and 26 °C influenced quality characteristics of 15 common bedding plant crops. American marigold (Tagetes erecta), cup flower (Nierembergia caerulea), diascia (Diascia barberae), flowering tobacco (Nicotiana alata), geranium (Pelargonium ×hortorum), globe amaranth (Gomphrena globosa), heliotrope (Heliotropium arborescens), nemesia (Nemesia foetans), New Guinea impatiens (Impatiens hawkeri), osteospermum (Osteospermum ecklonis), pot marigold (Calendula officinalis), snapdragon (Antirrhinum majus), stock (Matthiola incana), and torenia (Torenia fournieri) were grown under two mean daily light integrals (9.0 and 18.0 mol·m−2·d−1) in five environmentally controlled greenhouse compartments with a 16-h photoperiod. As MDT increased from 14 to 26 °C, flower or inflorescence number decreased for nearly all crops. In six crops, flower or inflorescence size decreased as MDT increased, whereas in five crops, there was an initial increase in flower size with an increase in MDT and then a subsequent decrease at MDT greater than 20 °C. In 10 of the crops, shoot weight at flowering decreased linearly or quadratically with an increase in MDT. Branch number was inversely related with MDT in eight crops and was positively correlated with an increase in flower number. We conclude that in a majority of the crops studied, plant quality decreased as the MDT increased, which can at least partially be attributed to earlier flowering at the higher MDTs. Therefore, there is often a tradeoff between faster crop timing and higher plant quality, especially for plants with a low estimated base temperature (Tmin) for development.

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Growers tend to over fertilize their plants as a way to minimize the likelihood of encountering nutrient deficiencies that would reduce the quality of their plants. Much of the nutrition literature focuses on the nutritional extremes namely of toxicity and deficiency. Once plants get to this stage, little can be done to correct the problem. Characteristics of plant performance in super-optimal conditions, yet below toxic levels, is less well known, and needs to be developed to help growers identify problems in their production practices before they impact sales. New Guinea Impatiens were grown over a wide range of N, K, and B levels, from 15% to 400% full strength Hoagland's solution for each nutrient after establishing transplanted rooted cuttings in a peat: perlite soilless media. Plants were grown for four weeks during treatment, during which time the flowers were pinched. After only 2 weeks of treatment, plants with 200% and 400% N were significantly shorter than control plants and plants with 15% N. Reflectance measurements and photographs were made twice a week. At the end of the four weeks, plant tissue was analyzed for form of N, root development and structure, and leaf area. Tissue samples were also analyzed with SEM and energy dispersive X-ray analysis to determine changes in nutrient location and tissue structure. This data provides insight into the nutrition economy of plants in general, tests the use of reflectance spectrometry as a method of detecting super-optimal fertilizer concentrations, and will help growers optimize their fertilization requirements to reduce production costs yet maintain high plant quality.

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There are several commercial materials available that have remarkable hydrating properties and many claim them to be ideal for use in horticulture and deliver water to the roots better than other soilless media. These are often referred to as “hydrogels.” There is general agreement in the literature that the physical characteristics of hydrogels are altered in the presence of divalent cations such as Ca and Mg. Tap water can reduce the water holding capacity by 70% or more. Unfortunately, the literature agrees on little else in terms of the performance of hydrogels. Some of the confusion is caused in part by comparing one type of hydrogel to another but treating all as equal. There has been no mathematical performance evaluation of hydrogel and what affect the environment may play in that performance to predict potential irrigation savings or shelf life extension. In a series of greenhouse and laboratory studies, we have evaluated the physical characteristics of several types of hydrogels and characterized bedding plant performance throughout a typical growth cycle. We measured leaf expansion, water content of the media, root growth, flowering, and fresh and dry masses. We have found little to no differences in the rate of leaf expansion when using hydrogels, but enhanced root growth early in production with the hydrogels. Our results indicated that plant growth was enhanced early in production, but any advantage they may have was lost by the end of production. Plants grown in hydrogels needed irrigation less frequently than those without hydrogel, but the effect was diminished over time. Since the use of the material can add about 15% to the cost of potting media, this data is designed to assist growers in hydrogel use and to determine any benefits of the added costs.

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“Float-bed” (FB) is a simple hydroponic system used by the tobacco industry for transplant production. “Ebb-and-flood” (EF) is a modified FB system with periodic draining of the bed to limit water availability and control plant growth. Field-bed cabbage (Brassica oleracea L. gp. Capitata) transplant production was compared with FB, EF, and overhead-irrigated plug-tray greenhouse systems. Plants were produced in May and June and transplanted in a field near Blacksburg, Va., in June and July of 1994 and 1995, respectively. Beds for FB and EF production consisted of galvanized metal troughs (3.3 × 0.8 × 0.3 m) lined with a double layer of 0.075-mm-thick black plastic film. In 1994, both EF and FB seedlings were not hardened before transplanting, were severely stressed after transplanting, and had higher seedling mortality compared with plants from other systems. Plug-tray transplants showed the greatest increase in leaf area following transplanting and matured earlier than seedlings produced in other systems. In 1995, EF- and FB-grown cabbage plants were hardened by withholding water before transplanting, and seedlings had greater fresh mass and leaf area than plug-tray or field-bed seedlings 3.5 weeks after transplanting. Less succulent cabbage transplants were grown in EF and FB systems containing 66 mg·L-1 N (40% by nitrate) and 83 mg·L-1 K. Compared with the FB system, the EF system allowed control of water availability, which slowed plant growth, and increased oxygen concentration in the root zone. Both EF and FB systems are suitable for cabbage transplant production.

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The cause of sudden substrate pH decline by geranium is unknown. Low Fe and low P have been shown to cause many plant species to acidify the substrate. Research was done to determine if low Fe or P stresses caused four geranium (Pelargonium ×hortorum Bailey) cultivars to acidify nutrient solution. Two cultivars were susceptible and two resistant to substrate acidification based on a grower survey. Rooted geranium cuttings were transferred to 4-L containers containing modified Hoagland's solution with N supplied as 15% NH4 and 85% NO3. The plants were grown in a greenhouse for 44 days. Treatments consisted of a complete nutrient solution and two similar solutions devoid of either Fe or P. Solutions pH was set at 5.8, changed weekly, and tested 3 and 6 days after each change. Because all cultivars showed similar responses, results were combined. Twenty days after transplanting (DAT), plants in all treatments, including control, caused solution pH to fall below 5. At 37 DAT, the solution pH levels for control, minus Fe, and minus P treatments were 4.1, 3.7, and 3.6, respectively. Results indicated that geranium is an acidifying plant when N is supplied as 15% NH4 and 85% NO3. Additionally, low Fe and low P stresses increase the acidification rate. Total dry weights of minus-P plants were about half that of minus-Fe plants. This indicated that plants under P stress had a higher specific rate of acidification than plants under Fe stress.

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The cause of sudden substrate pH decline by geranium (Pelargonium ×hortorum Bailey) is unknown. Published reports indicate that this response can be influenced in other plants by temperature and light extremes. The first of five experiments compared plants with all flowers removed to plants that were allowed to flower. Experiment 2 compared plants grown at four light levels (105, 210, 450 and 1020 μmol·m–2·s–1). Experiment 3 compared plants grown at four temperatures (14/10, 18/14, 22/18 and 26/22 °C day/night). Experiment 4 was a repeat of Experiment 1 and Experiment 5 was a factorial combining the three highest light levels and the three highest temperature levels. Plants allowed to form flowers had a final substrate pH of 5.7 compared to 6.3 for plants where flowers were removed. With increasing increments of temperature, substrate pH declined from 6.8 to 4.6 and with increasing light intensity from 6.1 to 4.8. There was no effect of flower removal in Experiment 4. Light and temperature had no consistent effects in Experiment 5 throughout 46 days after planting, with most pH values remaining in the acceptable range of 5.6–6.1. By 60 days, temperature treatments began to segregate, with pH being highest in the low-temperature treatments and lowest, down to 5.5, in the highest-temperature treatments. High temperature stimulated geranium acidification in both experiments, with the effect more severe in the first experiment. The flowering and high light effects were not duplicated in the second trial. This indicates that an additional factor is involved in expression of the light, temperature, and flowering control of acidification.

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Currently, formulation of inorganic fertilizers is based on cation amounts such as NH4, K, Mg, Ca, Fe, MN Cu, and Zn, whereas anion species and amounts are viewed, with few exceptions, as necessary fillers. The delivery of cations in the nutrient solution is associated with an anion such as Cl, SO4, NO3, PO4 or CO3. These anions at higher concentrations can result in different growth responses by altering the rhizosphere pH, soluble salts, and influencing the uptake of both cations and anions. The impact of these anions has not been extensively studied in the formulation of inorganic fertilizers. Several experiments assessed the effect of SO4 and Cl on root and shoot growth and development of bedding plants represented by petunia, impatiens, and vinca. In all treatments, plant height, shoot and root dry weight, and flower number decreased with an increase in Cl concentration. Root morphology was marked by fewer total roots and shorter primary and secondary roots when grown with Cl anions compared to the plants grown with SO4 anions. This indicates that anions have a larger role in determining optimum fertilizer formulation than previously believed. This information provides an additional tool in formulating fertilizers for greenhouse bedding plant production.

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