After being interviewed by a newspaper reporter on high tunnels and explaining in great detail what a high tunnel is and how it is different from a greenhouse, you can guess my shock to read the headline “High Tunnels—A Poor Man's Greenhouse.” High tunnels do not offer the precision of conventional greenhouses for environmental control, but they do sufficiently modify the environment to enhance crop growth, yield, and quality and provide some frost protection, but their primary function is to elevate temperatures a few degrees each day over a period of several weeks. In addition to temperature control, there are benefits of wind and rain protection, soil warming, aid in control of insects, diseases, varmints, and birds. They are relatively inexpensive, about $1.30/sq. ft., excluding labor. This system is particularly appealing to new-entry growers with limited capital who utilize retail-marketing channels. High tunnels like plastic-covered greenhouses are generally quonset-shaped with a peak, constructed of metal bows that are attached to metal posts, which have been driven into the ground about 2 feet deep. They are covered with one layer of 6-mil greenhouse-grade polyethylene, and are ventilated by manually rolling up the sides each morning and rolling them down in early evening. There is no permanent heating system, although it is advisable to have a standby portable propane unit to protect against unexpected below-freezing temperatures. There are no electrical connections. The only external connection is a water supply for trickle irrigation.
High synchrony, rate, and germination of needle palm [Rhapidophyllum hystrix (Pursh) H.A. Wendle & Drude] seeds were achieved only after removing the sclerotesta and embryo cap, which imposed physical dormancy. After scarification, recently harvested seeds or seeds stored for 12 months at 5C and 100% relative humidity had 96% and 98% final germination (G), with 9 to 11 days required to achieve 50% of final germination (T50) at 30C. Germination temperature controlled G, T50, and days between 10% and 90% of final germination (T90 - T10) of scarified seeds, with respective values of 98%) 9 days, and 5 days at 30C, and 18%, 31 days, and 12 days at 15C. Seeds with 36% moisture at harvest had no reduction in G until moisture was <14%. Germination of seeds with 19% moisture declined from 80% if stored at 0C to 33% if stored at -l0C; no seeds germinated after storage at less than -l0C.
Greenhouse and field methods were developed to screen Allium spp. for resistance to Botrytis leaf blight (caused by Botrytis squamosa Walker). In the green-house, plants were sprayed with laboratory-grown inoculum and incubated in a temperature-controlled enclosure containing an atomizing mist system. For field inoculations, a portable misting system with windbreaks was erected, and the plants were sprayed with laboratory-grown inoculum. Greenhouse and field incubation conditions maintained leaf wetness without washing inoculum from the leaves. Botrytis leaf blight symptoms in greenhouse and field evaluations were identical to symptoms in commercial onion fields. A total of 86 selected USDA Allium collection accessions were evaluated using these methods. All A. fistulosum accessions and A. roytei were highly resistant to immune, as were most accessions of A. altaicum, A. galanthum, A. pskemense, and A. oschaninii. Nearly all of the A. vavilovii and A. cepa accessions were susceptible. However, one A. cepa accession (PI 273212 from Poland) developed only superficial lesions, which did not expand to coalesce and blight leaves. This work confirms previous reports of Botrytis leaf blight resistance in Allium spp., and suggests that strong resistance exists with A. cepa.
The effect of root zone temperature (temp.) on 18-month-old plants of `Gulf Coast' blueberries (predominantly Vaccinium corymbosum L.) grown in temperature-controlled water tanks during Summer 1993 were determined for plant growth and leaf nutrient status. Soil medium (1 sand: 1 peat, v/v) was maintained at or above –20 k Pa. Six singleplant replicates were placed in either a 24 or 31C tank. After 60 days, plants grown at 24C had more leaves, greater total leaf area, and higher leaf and stem fresh weight. Leaf moisture (P < 0.09) and stem dry wt (P < 0.07) were greater at the lower root temp. Root: shoot ratio and total root dry weight were not affected by root temp. Leaf S and Cu levels were higher and NO3 levels lower in plants grown at the 24C root temp. compared to those grown in the 31C root temp. Nitrogen, K, Na, Ca, Mg, P, Fe, Mn, and Zn (order of decreasing concentration) were not affected by root temp. The total N: NO3-N ratio was higher at the lower root zone temp.
Commercial production of Easter lily (Lilium longiflorum Thunb.) requires precise temperature control to ensure that the crop flowers in time for Easter sales. The objective of this project was to develop and validate a greenhouse decision-support system (DSS) for producing Easter lily to predetermined height and flower date specifications. Existing developmental models were integrated with a knowledge-based system in a DSS to provide temperature recommendations optimized for Easter lily scheduling and height control. Climate data are automatically recorded in real time by linking the DSS to a greenhouse climate control computer. Set point recommendations from the DSS can be manually set or automatically implemented in real time. Potential benefits of the project include improved crop quality and the transfer, validation, and integration of research-based models. The DSS was implemented at several research and commercial locations during the 1994 Easter lily season. DSS recommendations were compared with the strategies of experienced growers. The system design, implementation, production results, quality of recommendations, and potential are discussed.
Rooted cuttings of crepe myrtle (Lagerstroemia indica L. × L. fauriei `Muskogee') were transplanted into 3.8-L black polyethylene containers filled with a bark-based rooting substrate and exposed for 2 months during Summer 1995 to either of three container shielding treatments: containers shielded from insolation (container shielded inside a whitewashed 11.4-L black polyethylene container), containers exposed to insolation, or containers shielded for 1 month then exposed for 1 month. Mean highest temperature in the western quadrant of rooting substrate of exposed containers was 16°C higher than for those in shielded containers. Containers exposed for 2 months had reduced root and shoot growth and increased leaf N compared with the other two treatments. Crape myrtle plants were next transplanted into 27.0-L polybags, transferred into a temperature-controlled glasshouse, and fertigated to container capacity every 3 days with humic acid extract at concentrations of 0, 50, 150, or 300 μl·L–1 for 2 additional months. Effects of the container shielding treatments for all growth parameters remained evident until the end of the experiment. Shoot and root extension growth of plants previously in containers shielded for 2 months and containers exposed for 2 months, responded in a quadratic fashion to humic acid extract concentration levels.
Selected tomato (Lycopersicon esculentum Mill) genotypes were evaluated for their fruit-setting ability under high-temperature field conditions. A temperature-controlled greenhouse study was conducted to determine the percent fruit set from the total number of flowers and fruit produced per plant. Ratings for set obtained under high-temperature field conditions were significantly (P = 0.001) correlated with percent fruit set determined under similar greenhouse conditions. Most of the Asian Vegetable Research and Development Center (AVRDC) selections, Beaverlodge lines, `Nagcarlan', and `Red Cherry' could be considered heat-tolerant. Small-fruited, abundantly flowering genotypes were less affected by heat stress than larger-fruited cultivars. Prolonged periods of high temperature caused drastic reductions in pollen fertility in most genotypes, except `Red Cherry' and L. esculentum var. cerasiforme (PI 190256). Stigma browning and stigma exsertion commonly occurred on all lines, except AVRDC CL-5915-553 and PI 190256. Diallel analyses indicated that pollen fertility and fruit set under high field temperatures were primarily under additive gene control.
Abstract
Tulip bulbs (Tulipa spp.) were placed under ventilated low pressure storage (LPS) conditions for 14 days in either August or September. Compared to 760 mm Hg stored bulbs, LPS suppressed leaf growth and floral development. These effects were highly visible after storage in air at either 76 or 150 mm Hg and in the month of August. When tulip bulbs were forced, LPS treatments applied in August delayed flowering of most cultivars and flower size was occasionally reduced; in September treatments, LPS ventilation with additional O2 and CO2 accelerated flowering of 2 cultivars, but flower size was reduced. When stored under 76 mm Hg in air in August, most cultivars of hyacinth (Hyacinthus spp.) were subsequently delayed in flowering, but daffodils (Narcissus spp.) were not. Except for one cultivar of each species, LPS did not affect the percent of plants flowering, plant height or flower size. Penicillium growth on the bulb tunics was enhanced by humidifying the air under LPS conditions. It is concluded that LPS provides no advantages over the ventilated, temperature controlled units presently employed.
Chlorophyll fluorescence was measured under both laboratory and greenhouse conditions in an effort to develop a quick, reliable, and inexpensive laboratory procedure capable of predicting heat stress experienced by tomato (Lycopersicon esculentum Mill.) under greenhouse conditions. The laboratory tests consisted of measurements of the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) performed on leaf discs taken from whole tomato leaves and placed on a temperature controlled plate. Comparisons were made with greenhouse measurements of the same parameter conducted on intact leaves of whole plants exposed to different temperature treatments imposed by manipulation of the aerial environment of the greenhouse. Dark adaption periods ranging from 15 min to all day in the greenhouse and temperature exposure periods ranging from 5 min to 60 min in the laboratory were compared to find the best correlation between the two tests. Best agreement was obtained with 60 min treatment times in the laboratory and 60 min dark adaption periods in the greenhouse. Fv/Fm decreased quadratically with increasing leaf temperature in a similar fashion in both tests, suggesting that the laboratory approach can adequately predict plant response to greenhouse heat stress.
Soybeans (Glycine max [L.] Merr. cv. Williams) were grown to a standard developmental stage (6th trifoliate leaf c. 50% fully expanded) in a sunlit, temperature-controlled greenhouse (27±3°C) or in growth chambers (27±1°C) under microwave-powered (MP) E lamps (Fusion Systems, Rockville, MD) or a 50-50 mixture of high pressure sodium and metal halide (HID) lamps. Daily PAR in growth chambers was 44 mol m-2, provided either as a square-wave (HID; 875 μmol m-2s-1) or in steps (MP; peak irradiance c. 1650 μmol m-2s-1). Growth chamber experiments were conducted at 400 μl l-1 or 700 μl l-1 CO2“ambient” or “elevated”, respectively). Total dry matter was similar for all treatments at ambient CO2 but MP-grown plants were more like greenhouse plants with respect to Leaf Area Ratio, Specific Leaf Weight, and length of stem and petiole. Axillary growth, however, was much less under greenhouse conditions. Elevated CO2 resulted in a significant stimulation of plant growth under both HID and MP, but gains were greater under MP illumination. Enhanced growth of MP plants was marked by increased partitioning into roots. It is possible that morphological modifications in MP plants rendered them more efficient at conversion of PAR into dry matter.