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Gary J. Wilfret

Production of stock plants is essential for the asexual propagation of poinsettia, but variability exists among cultigens in the development of axillary shoots under high day/night temperatures of central Florida. Thirty-eight and 44 cultigens were grown during 1995 and 1996, respectively, and were evaluated for cutting production and subsequent growth of harvested cuttings. Plants were pruned twice prior to cutting harvest, with a projected cutting number of 21 in 1995 and 27 in 1996. Cuttings were graded into three groups based upon stem caliper and overall quality: #1, #2, and cull. Number of #1 cuttings/plant in 1995 ranged from 3.4 to 18.6, represented by `Cortez' and `Ball 838', respectively; a majority of the cultigens produced between 14 and 16 cuttings in the top grade. Number of usable (#1 and #2) cuttings ranged from 4.9 to 30.0, represented by `Cortez' and `Jolly Red', respectively, with a mean of 20.2. Stem caliper of cuttings measured 7 cm from terminal apex ranged from 0.55 cm of `Mikkel 520' to 0.91 cm of `Ball 838'. Cuttings of `Cortez' and `Red Splendor' had poor lateral development. During 1996, number of #1 cuttings ranged from 9.8 (`Picacho') to 22.2 (`Freedom'), with a mean of 16.6. Number of usable cuttings ranged from 14.2 to 31.9, represented by `Cortez' and `Spotlight Dark Red', with a mean of 25.3. Stem caliper ranged from 0.55 cm (`Ball 865') to 0.79 cm (`Supjibi'). Cuttings taken from plants of the `Cortez' series produced few, if any, laterals, while `Marblestar' and `Jolly Red' had up to 50% aborted axillary buds.

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David S. de Villiers and Robert W. Langhans

Protein is an important and essential dietary component. Common bean, a major source of vegetable protein in the Americas, was chosen for study in controlled environments with a view to its potential for use in space colonies. Eighteen 0.58-m2 stands of the cranberry type of bean, `Etna', were grown in the greenhouse at plant densities of 7, 15, and 28 plants/m2 in a recirculating ebb-and-flow system. Duration of photoperiod and thermoperiod was 16 h. Day/night temperatures settings were 25/20 °C. Daily light integral was matched across greenhouse sections by means of supplemental lighting; it averaged 17 mol/m2 per day. Crop cycle was 70 days from seed to harvest. At harvest, plants were dismembered so that dry weights of leaf, branch, stem, pod, and bean yields could be separately measured by node of origin. Internode lengths were recorded, and all loose trash recovered. The relationship between yield and plant density followed the form expected. Yield of edible biomass at 7 plants/m2 (284 g/m2) was 88% of that at 28 plants/m2 (324 g/m2), a significant difference. At 15 plants/m2 it was 97%. The trend suggests that further gains (but only very small) in yield can be expected with increased density in this cultivar. Productivity and quantum yield at 28 plants/m2 were 4.69 g/m2 per day and 0.27 g/mol, respectively. The coefficient of variation for plants grown at 28 plants/m2 was three times that of plants grown at 7 plants/m2 (0.88 vs. 0.26). Yield component analysis, harvest index, and plant morphology at the different planting densities are discussed.

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Yu Sung, Daniel J. Cantliffe, and Russell T. Nagata

Lettuce (Lactuca sativa L.) seeds can fail to germinate at temperatures above 24 °C. The degree of thermotolerance is thought to be at least partly related to the environment under which the seed developed. In order to study the effects of temperature during seed development on subsequent germination, various lettuce genotypes were screened for their ability to germinate at temperatures ranging from 20 to 38 °C. Seeds of the selected genotypes `Dark Green Boston' and `Valmaine' (thermosensitive), `Floricos 83', `Everglades', and PI 251245 (thermotolerant) were produced at 20/10, 25/15, 30/20, and 35/25 °C day/night temperature regimes in plant growth chambers. Seeds were germinated on a thermogradient bar from 24 to 36 °C under 12 h light/dark cycles. As germination temperature increased, the number of seeds that failed to germinate increased. Above 27 °C, seeds matured at 20/10 or 25/15 °C exhibited a lower percent germination than seeds that matured at 30/20 or 35/25 °C. Seeds of `Dark Green Boston' and `Everglades' that matured at 30/20 °C exhibited improved thermotolerance over those that matured at lower temperatures. Seeds of `Valmaine' produced at 20/10 °C exhibited 40% germination at 30 °C, but seeds that matured at higher temperatures exhibited over 95% germination. Germination of `Valmaine' at temperatures above 30 °C was not affected by seed maturation temperature. The upper temperature limit for germination of lettuce seed could thus be modified by manipulating the temperature during seed production. The potential thermotolerance of seed thereby increased, wherein thermosensitive genotypes became thermotolerant and thermotolerant genotypes (e.g., PI251245) germinated fully at 36 °C. This information is useful for improving lettuce seed germination during periods of high soil temperature, and can be used to study the biology of thermotolerance in lettuce.

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Michael P. Harvey and Mark H. Brand

Studies initiated in Spring 1998 analyzed the influence of division size, shade, and temperature on the growth rate of the ornamental grass Hakonechloa macra `Aureola' in nursery-container production. To determine the optimum division size for production, container-grown stock plants were used to make early spring divisions of four sizes (1-2, 4-6, 8-10, and 12-15 buds). Divisions were established in 325-ml pots for 1 month before being transplanted to 3.7-L nursery containers. Plants were grown outdoors and received topdressed 17-6-10 slow-release fertilizer (containing micronutrients) and drip irrigation from June through September. Average leaf area, shoot number and bud count increased linearly as division size increased, but average height remained the same between each treatment. Plants of all division sizes exhibited healthy growth, with 50% of the plants in the 4-6 buds/division treatment growing to marketable size compared to 45%, 35% and 15% in the 8-10, 12-15, and 1-2 buds/division treatments, respectively. Four shade densities (0%, 30%, 50%, and 70%) were tested to determine which promoted optimum growth. As shading increased, average shoot number per plant decreased, average height and shoot length increased and bud count remained the same. To determine the optimum growing temperature for Hakonechloa, divisions were grown in 325-ml pots under four different day/night temperatures (15/10, 21/16, 27/22, and 33/28 °C) for 12 weeks in growth chambers. Plants were fertigated daily with a 5-25-5 liquid fertilizer. Average bud count, leaf area, plant height, plant width, shoot length, and shoot number increased as temperature increased to 27/22 °C, then decreased significantly beyond this temperature optimum.

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Matthew G. Blanchard and Erik S. Runkle

The production value of potted orchids has increased by 155% in the past decade, and they are now the second-most valuable potted flowering plant in the United States. Scheduling orchids to flower on specific dates requires knowledge of the environmental parameters that regulate flower induction. However, the flowering requirements of the vast majority of orchid species and hybrids have not been well described. Odontioda is a cool-growing, epiphytic genus originating from the Andes Mountains of South America, and several hybrids are commercially grown for their bright-colored flowers and compact habit. We quantified the promotion of inflorescence initiation and time from visible inflorescence (VI) to anthesis at constant and fluctuating day/night temperatures. Odontioda George McMahon `Fortuna' and Lovely Penguin `Emperor' were grown at constant temperatures of 14, 17, 20, 23, 26, or 29 °C, and day/night (12 h/12 h) temperatures of 20/14, 23/17, 26/14, 26/20, 29/23, or 29/17 °C. Plants were grown in glass greenhouses under a 12-h photoperiod, and shading was provided so that the maximum instantaneous irradiance was ≤300 μmol·m-2·s-1. After 6 weeks at the various temperature setpoints, heat stress symptoms were observed on plants grown at 26, 29, 26/14, 26/20, 29/23, and 29/17 °C. After 14 weeks, ≥60% of both hybrids had VI when grown at 14, 17, 20, or 20/14 °C. Data for time from VI to anthesis were converted to a rate and a thermal-time model relating temperature with inflorescence development was developed. This information could be used by commercial orchid growers to schedule flowering Odontioda orchids for specific market dates.

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Cholani K. Weebadde and James F. Hancock

While it is of great significance for strawberry breeders to know the genetics of day-neutrality (DN), evidence for inheritance of the trait is still contradictory. A linkage mapping approach is being used to determine how many QTL regulate DN and the proportion of the variability explained by each. A preliminary genetic linkage map was constructed for 125 individuals of the day neutra× short day (SD) cross `Tribute' × `Honeoye' using single dose restriction fragments (SDRFs) of amplified fragment length polymorphic (AFLP) markers. Over 500 SDRFs from 55 AFLP primer combinations were used to build the map using the software tool Join Map 3.0 at a LOD score of 3.0. Single marker analysis using WinQTL cartographer software previously determined 27 SDRF markers to co-segregate with DN for 57 individuals of the mapping population phenotyped in the field for the years 2002 and 2003, indicating putative QTL for DN. These markers were included in the linkage analysis and seven of them mapped to five different linkage groups that may indicate the quantitative nature of the trait. For determining QTL and percentage of phenotype governed by each QTL, however, accurate phenotypic evaluation is critical. Therefore, controlled environment (growth chamber) studies were used to obtain flowering response data under SD and long day (LD) conditions with two day/night temperatures. This study was conducted for the entire mapping population (over 400 individuals) so that QTL detected can be confirmed by fine mapping the QTL regions. We will also test how robust the QTL detected are, by analyzing the same segregating population at six different field locations in the United States (California, Maryland, Michigan, Minnesota, New York, and Oregon) for their flowering response under SD and LD conditions.

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Nadine Ledesma and Nobuo Sugiyama

The effects of high-temperature stress on pollen viability and in vitro and in vivo germinability were studied in two facultative, short-day strawberries (Fragaria ×ananassa Duch.), `Nyoho' and `Toyonoka.' Plants were exposed to two day/night temperature regimes of either 23 °C/18 °C (control) or 30 °C/25 °C (high temperature) from when the first inflorescence became visible until anthesis. Pollen viability in `Nyoho' was only slightly affected at 30 °C/25 °C when compared with pollen from plants grown at 23 °C/18 °C. In `Toyonoka', however, pollen viability was significantly lower at 30 °C/25 °C than at 23 °C/18 °C. The in vitro germination percentages were significantly lower in pollen from plants grown at 30 °C/25 °C and germinated at 30 °C than from plants grown at 23 °C/18 °C and germinated at 23 °C in both cultivars. But the percentages were much lower in `Toyonoka' than in `Nyoho', particularly at the 30 °C germination temperature. Pollen from plants grown at 23 °C/18 °C also extended longer pollen tubes than pollen grown at 30 °C/25 °C in both cultivars, but `Nyoho' had longer pollen tubes than `Toyonoka' at 30 °C/25 °C. Fluorescence microscopy revealed that most of the `Nyoho' pollen germinated on the stamen, elongated through the style and reached the ovule regardless of temperature treatment. In `Toyonoka', pollen germination and elongation were greatly inhibited at 30 °C/25 °C, resulting in unfertilized ovules. These results suggest that certain strawberry cultivars produce heat-tolerant pollen, which in turn could result in higher fruit set.

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Matthew G. Blanchard and Erik S. Runkle

Volatile energy costs and lower profit margins have motivated many greenhouse growers in temperate climates to improve the energy efficiency of crop production. We performed experiments with dahlia (Dahlia ×hybrida Cav. ‘Figaro Mix’), French marigold (Tagetes patula L. ‘Janie Flame’), and zinnia (Zinnia elegans Jacq. ‘Magellan Pink’) to quantify the effects of constant and fluctuating temperatures on growth and flowering during the finish stage. Plants were grown in glass-glazed greenhouses with a day/night (16 h/8 h) temperature of 20/14, 18/18, 16/22 (means of 18 °C), 24/18, 22/22, or 20/26 °C (means of 22 °C) with a 16-h photoperiod and under a photosynthetic daily light integral of 11 to 19 mol·m−2·d−1. Flowering times of dahlia, French marigold, and zinnia (Year 2 only) were similar among treatments with the same mean daily air temperature (MDT). All species grown at 20/14 °C were 10% to 41% taller than those grown at 16/22 °C. Crop timing data and computer software that estimates energy consumption for heating (Virtual Grower) were then used to estimate energy consumption for greenhouse heating on a per-crop basis. Energy costs to produce these crops in Charlotte, NC, Grand Rapids, MI, and Minneapolis, MN, for a finish date of 15 Apr. or 15 May and grown at the same MDT were estimated to be 3% to 42% lower at a +6 °C day/night temperature difference (DIF) compared with a 0 °C DIF and 2% to 90% higher at a −6 °C DIF versus a 0 °C DIF. This information could be used by greenhouse growers to reduce energy inputs for heating on a per-crop basis.

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M.M. Peet

The environmental and physiological causes of cracking or splitting of soft fruits and citrus as they ripen are not well understood. This paper explores factors contributing to radial cracking in tomatoes, gives suggestions for prevention of cracking, and suggests directions for future research. Fruit cracking occurs when there is a rapid net influx of water and solutes into the fruit at the same time that ripening or other factors reduce the strength and elasticity of the tomato skin. In the field, high soil moisture tensions suddenly lowered by irrigation or rains are the most frequent cause of fruit cracking. Low soil moisture tensions reduce the tensile strength of the skin and increase root pressure. In addition, during rain or overhead irrigation, water penetrates into the fruit through minute cracks or through the corky tissue around the stem scar. Increases in fruit temperature raise gas and hydrostatic pressures of the pulp on the skin, resulting in immediate cracking in ripe fruit or delayed cracking in green fruit. The delayed cracking occurs later in the ripening process when minute cracks expand to become visible. High light intensity may have a role in increasing cracking apart from its association with high temperatures. Under high light conditions, fruit soluble solids and fruit growth rates are higher. Both of these factors are sometimes associated with increased cracking. Anatomical characteristics of crack-susceptible cultivars are: 1) large fruit size, 2) low skin tensile strength and/or low skin extensibility at the turning to the pink stage of ripeness, 3) thin skin, 4) thin pericarp, 5) shallow cutin penetration, 6) few fruits per plant, and 7) fruit not shaded by foliage. Following cultural practices that result in uniform and relatively slow fruit growth offers some protection against fruit cracking. These practices include maintenance of constant soil moisture and good Ca nutrition, along with keeping irrigation on the low side. Cultural practices that reduce diurnal fruit temperature changes also may reduce cracking. In the field, these practices include maintaining vegetative cover. Greenhouse growers should maintain minimal day/night temperature differences and increase temperatures gradually from nighttime to daytime levels. For both field and greenhouse tomato growers, harvesting before the pink stage of ripeness and selection of crack-resistant cultivars probably offers the best protection against cracking. Areas for future research include developing environmental models to predict cracking and exploring the use of Ca and gibberellic acid (GA) sprays to prevent cracking.

Open access

Elisa Solis-Toapanta, Paul Fisher, and Celina Gómez

To identify practices that may simplify the use of small-scale hydroponic systems for indoor gardening, we compared two nutrient solution management treatments for basil (Ocimum basilicum) production. Experiments were conducted for 8 weeks to evaluate the effect of biweekly replacement of the nutrient solution (W) vs. biweekly fertilizer addition without nutrient solution replacement (W/O) on growth and nutrient uptake of basil ‘Genovese Compact’ grown in either a greenhouse or an indoor environment. Greenhouse day/night temperature was 29/24 ± 4 °C, relative humidity (RH) was 65 ± 4%, and daily light integral (DLI) was 26.1 mol·m‒2·d‒1. The indoor environment had a constant ambient temperature of 21 °C, RH of 65%, and DLI of 9 mol·m‒2·d‒1 provided by broadband white lamps. Four plants were grown in 7.6-L replicate hydroponic systems, with 178 mg·L‒1 N from a complete nutrient solution in two experimental runs. Shoot fresh and dry mass, leaf number, and leaf area showed an increasing quadratic trend over time when plants were grown in the greenhouse. In contrast, growth over time was linear for plants grown indoors. Within each environment, solution management treatment did not affect growth, indicating that the simpler W/O strategy was adequate under these conditions. Plants grown in the greenhouse required more frequent refill water applications compared with indoors, which resulted in three to four times more refill water applied. Because indoor-grown plants had a decreased growth rate, nutrient uptake rate, and volume of water applied compared with plants grown in the greenhouse, electrical conductivity (EC) for the W/O treatment increased over time. Final nutrient solution concentration was highest for indoor-grown plants under the W/O treatment, and final tissue nutrient concentration was higher for plants grown indoors compared with the greenhouse. Final nutrient uptake (dry mass × nutrient concentration) was higher for plants grown in the greenhouse rather than indoors. Considering that EC increased in the solution of indoor-grown plants under W/O, an appropriate strategy using this treatment would require reducing fertilizer input indoors. To refine simple and robust fertilizer management strategies for indoor gardeners, further research is needed to test variables such as different plant species, cultivars, and water qualities.