Activities of the Georgia Envirotron facility include interdisciplinary researches in Environmental Science, Food Safety, Crop and Soil Sciences, Entomology, Plant Pathology, Urban Agriculture etc. Research summaries of leading scientists, graduate, undergraduate and Young Scholars Internship students are presented below (selected).
- Research at the Envirotron
- Research at the Rainout Shelters
- Collaborative Research Projects
- Young Scholars Program
- Other Projects
- Refereed Publications
Testing irrigation and fertilization rates for young peach plants in controlled environment
Bruno Casamali, Marc W. van Iersel, and Dario J. Chavez (Department of Horticulture, University of Georgia)
Accurate irrigation and fertilization management for agricultural crops has become a subject of interest, due largely to widespread problems with drought and fertilizer runoff. Irrigation and fertilization guidelines are needed for peach growers in the southeastern U.S. to avoid plant stresses and improve production. Although studies in controlled environment cannot be easily translated to field situations, they can provide a better understanding the trees’ physiological responses to different irrigation and fertilization conditions. Greenhouse experiments were conducted: one to test different irrigation rates and another to test different fertilization rates, with both experiments testing two scions (‘Flavorich’ and ‘Julyprince’) and two rootstocks (‘MP-29’ and ‘Guardian’). For the irrigation experiment, flower bud break percentage was affected by the scion and rootstock treatments. Plants receiving the highest irrigation level (volumetric water content (VWC) of 45%) were ~35% taller than the treatments with a VWC of 15 and 35%. Plants growing with a VWC of 45% or those grafted onto ‘Guardian’ displayed the greatest trunk cross-sectional area increase (TCSAI). In general, plants receiving the lowest irrigation level (VWC of 15%) had the lowest stem water potential in comparison to those receiving more water. For the fertilizer experiment, ‘Flavorich’ scion reached peak bloom faster and had more abundant bloom than ‘Julyprince’. Photosynthetic activity was affected by the interactions between the fertilizer vs. scion and fertilizer vs. rootstock treatments. Plants receiving greater amounts of fertilizer (18, 13.5, and 9 g of N per plant) had ~2x the TCSAI than the plants receiving the lower amounts of fertilizer (4.5 and 2.3 g of N per plant). Higher doses of irrigation and fertilization tend to increase plant height and TCSAI. However, the photosynthetic activity appeared to be less dependent of the irrigation and fertilization rates, similar to the bud break progression, which was found to be more dependent on the scion and rootstock cultivars.
Phenotyping for Gummy Stem Blight Resistance in Watermelon
Winnie Gimode and Cecilia McGregor, University of Georgia
Institute of Plant Breeding, Genetics and Genomics, University of Georgia.
Gummy stem blight (GSB) is a major fungal disease of field-grown watermelons in southeastern US, caused by Stagonosporopsis pathogen. This pathogen thrives in warm and humid conditions that is conducive for germination of the spores. Infection is optimum at temperatures between 21°C-26°C, and high moisture levels (above 90% relative humidity), accompanied by leaf wetness. The impact of GSB on watermelon production can be severe, resulting in huge yield losses.
Resistance to fungicides poses a major challenge in management of GSB. In addition, the repeated use of fungicides may have a negative impact on the environment, particularly if residues persist in the soil. Using cultivars resistant to GSB would be a cheaper, more environmentally sustainable option for disease management. Currently, no commercial watermelon cultivars are resistant to GSB.
This study therefore aims to identify quantitative trait loci (QTL) associated with gummy stem blight resistance in watermelon and further introgress the loci into watermelon cultivars. In order to achieve this, we are utilizing the growth chambers for phenotyping purposes, since the temperature, light and humidity settings can be controlled. Phenotyping for GSB involves screening watermelon seedlings by inoculating them with the fungus (Stagonosporopsis citrulli) through spraying. Due to the high humidity and consistent temperature levels required for development of this disease, the growth chambers have been ideal for these purposes, and we are getting relatively consistent results!
Photosynthetic Thermotolerance of Peanut Seedlings Exposed to Different Temperatures at Planting and Early Season
Pilon*, W.S. Monfort, C. Weaver, T.L. Grey, V. Tishchenko, Department of Crop and Soil Sciences, University of Georgia, Tifton, GA 31793.
Sub-optimal temperatures at planting and early-season is known to decrease peanut growth. However, the effect of sub-optimal temperature conditions on the photosynthetic thermotolerance of peanut seedling has not been fully investigated.
The objective of this study was to assess the photosynthetic thermotolerance of peanut seedlings grown under different temperature regimes. Georgia-06G seeds were maintained in four storage conditions (cold room, greenhouse, office, and wagon) for 72 days to promote different seed vigor. They were then transferred to a cold room until planting. Seeds from each storage condition were planted in pots under two temperature regimes, 18/24 ᵒC and 21/29 ᵒC during the dark/light period. Emergence was recorded daily from 5 to 18 days after planting. At 18 DAP, OJIP fluorescence was measured in the uppermost, fully-expanded, mainstem leaf. Leaf discs were also collected from the same leaves for pigment concentrations. Plants were harvested and separated into leaves and stems and oven dried at 60 ᵒC for dry matter.
Higher emergence was observed in pots grown under 21/29 ᵒC. Overall quantum efficiencies (φPo, φEo, and φRo) and performance indices (Fv/F0, IABS and PItotal) were higher in plants grown under 21/29 ᵒC compared to those grown under 18/24 ᵒC. OJIP fluorescence-derived structural indicators or pigment concentrations were unaffected by the temperature regimes. Dry matter of leaves and stems was higher for the plants grown under 21/29 ᵒC compared to 18/24 ᵒC. Overall, plants grown at 21/29 ᵒC were more efficient at absorbing light, and trapping and conserving energy during the thylakoid reactions, which likely contributed to the enhanced growth and development of peanut seedlings. Seed storage impacted emergence speed, but did not influence overall peanut seedling growth. Further studies will be performed to support and validate these results.
Eschericha coli O157:H7 pre-harvest contamination of ready-to-eat produce.
Dr. Marilyn Erickson. Center for Food Safety, University of Georgia.
Over the past six years, growth chambers at the Envirotron have been in use by Dr. Marilyn Erickson from the Center for Food Safety to explore a number of issues associated with pre-harvest contamination of ready-to-eat produce. As a source of contamination of enteric pathogens (Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes), animal manure-based soil amendments are of concern for their potential to introduce and contaminate fields and crops grown in those fields. Aerobic composting of animal manures is one process that may be used to reduce pathogen levels; however, insufficient heat generated at the surface of compost heaps may arise leading to pathogen survival. Although turning of compost piles is advocated to facilitate exposure of all material to heat generated from microbial breakdown of compost materials, Dr. Erickson’s lab has also been investigating nonthermal parameters that affect pathogen inactivation at surface sites of composting piles by holding compost mixtures in small trays or containers within the growth chambers under defined temperature, humidity, and light conditions. Results from several of these studies are highlighted below:
- Using “fresh” chicken manure, both Salmonella and Listeria monocytogenes survived for longer periods at 25°C in mixtures formulated with wheat straw to a carbon:nitrogen ratio of 40:1 compared to formulations of 30:1 and 20:1.
- Holding or aging of chicken manure for 3 weeks prior to mixing with carbon amendments had a detrimental effect on pathogen inactivation. No decreases in initial Salmonella levels (~5 log CFU/g) were noted in compost systems formulated with aged chicken manure and stored for 4 weeks at 25°C whereas the pathogen was not detected in compost mixtures prepared with “fresh” chicken manure and stored for 1 week at 25°C.
- Inactivation of Salmonella was also significantly impacted by the carbon amendment used in compost formulations with decreased survival occurring in wheat straw systems compared to pine needle systems.
Another focus of research that has required our use of controlled and contained conditions in growth chambers is to ascertain whether lettuce-infesting insects could influence the fate of either surface or internalized populations of E. coli O157:H7. Brief exposure (~18 h) of lettuce leaves to insects (5 cabbage loopers, 10 thrips, or 10 aphids) prior to inoculation of plants with E. coli O157:H7 resulted in significantly reduced internalized populations of the pathogens within these leaves after approximately 2 weeks, as compared with leaves not exposed to insects. These results suggest that internalization of E. coli O157:H7 may be minimized by plant defenses that are induced in response to intrusive insect activities.
A third area of research employing leafy green plants cultivated in growth chambers has been to examine the potential for internalization of enteric pathogens through roots or leaf stomata. E. coli O157:H7 in contaminated water was applied by application to the soil or by spraying the leaves of the plants. Uptake of pathogen into roots occurred when soil contained 7 log CFU/g. Greater internalization of E. coli O157:H7 occurred in lettuce and parsley roots surrounded by saturated soil compared to moist soil. Uptake into roots occurred later for parsley than lettuce or spinach under saturated conditions. Exposure of the leaf surface to 7 log CFU/ml spray led to internalization of the pathogen into leaves of mature lettuce and spinach plants. When sprayed at this concentration, no differences in the degree of internalization occurred for virulent and non-virulent surrogate strains of E. coli O157:H7. Internalized populations of E. coli O157:H7 were greater in spinach compared to lettuce one week after the spray event implying differences in the level of defenses activated by these leafy greens.
Currently, growth chambers are being used to explore the short-term survival of E. coliO157:H7 sprayed on five different cultivars of mature cabbage plants. Pathogen survival on these cultivars will be compared to the plant’s expression of several plant defense protein genes and to the plant’s constitutive levels of total phenols and antioxidant capacity. In addition, chemical treatments that have previously been shown to alter phytochemical levels in leafy greens will be applied to one cultivar one week prior to pathogen exposure. Subsequently, treated and untreated plants will be applied to determine if pathogen survival has been impacted.
Interspecific Arachis Breeding Project.
Noelle Barkley, Roy Pittman, Angie Lewis. USDA
The growth chambers are being utilized to grow four wild Arachis species which are being used as female parents for interspecific crosses. The males Arachis species are being grown in our greenhouse facility. The use of the growth chambers allow us to synchronize pollinations by controlling light and temperature so that the male flowers can be collected and immediately used to pollinate the females. In the greenhouse, male flowers are collected in the morning and then used in the evening to pollinate the female plants. Each day flowers from the male plants are harvested and transported to the growth chamber to apply pollen to the stigma. This process is repeated each day flowers are available to help increase the success of obtaining a true hybrid since Arachis species are vigorous self pollinating plants. The resulting hybrid F1’s will be used to create synthetic allotetraploids by treatment with colchine which doubles the chromosomes producing a tetraploid from a diploid. (The majority of Arachis species are diploid whereas cultivated peanuts are tetraploid). The resulting synthetic allotetraploids will be crossed with cultivated peanuts to create mapping populations and introgress disease resistance traits into cultivated peanuts.
Morphological Characterizations of Subterranean Clover for Determination of Genetic Redundancy.
Brad Morris. Plant Genetics Resources Conservation Unit.
Genetic redundancy is of prime concern for curation of crop species. The USDA, ARS, PGRCU Trifolium subterranean collection is suspect of having genetic redundant accessions. Our goal was to determine whether or not genetic redundant accessions do in fact exist within the U.S. subterranean clover collection. Subclover seed were planted in potting soil within each of five4″ plastic pots. A sub-sample of 90 subclover accessions were tested. Soon after seed germination, each pot utilized in the test were moved to growth chambers at the Georgia Envirotron. Plants were grown in a 16 hour photoperiod regime with a 27°C / 17 °C day/night temperature setting. Successful morphological characterizations were recorded for leaf marking, flower color, and stipule color.
Combined Effects of Elevated Carbon Dioxide Levels and Temperature on the Biology of the Mealybug Phenacoccus madeirensis Green (Homoptera: Pseudococcidae)
Juang-Horng Chong1, Marc W. Van Iersel2, and Ron D. Oetting2
1 Department of Horticulture, Sunchon National University, South Korea, 2 Department of Horticulture
The combined effects of elevated CO2 levels (400 and 700 µL/L) and temperatures (20, 25 and 30 0C) on the development, survival and reproduction of two generations of the mealybug Phenacoccus madeirensis were investigated. Mealybugs were reared on chrysanthemums grown in growth chambers set at a specific CO2 level and temperature. The duration to egg hatching and to adulthood of the mealybugs was recorded by examining the mealybug cohorts daily. Hatching rates of eggs and survival rate to adulthood were determined by recording the number of individuals that successfully molted into the next developmental stage. The proportion of females in the population was determined by fractioning the number of females over the total number of adults at the end of the experiment. Adult females were isolated in leaf cages and their eggs were collected daily to determine fecundity. The nutritional status (carbon concentration, nitrogen concentration, and the relative water content of leaves) of chrysanthemum were also studied to interpret the performance of mealybugs at elevated CO2 level and temperature. The development of mealybug is temperature-dependent. Duration of development did not differ among different CO2 level treatments and generations. A female completed its development in about 20 days at 30 0C, 28 days at 25 0C, and 47 days at 20 0C. Males have longer duration of development than females. Survival rates, proportion of females, fecundity, duration of reproduction, and the parameters of host plant nutritional status did not differ significantly among temperature and CO2 level treatments and between generations.
Fate of Eschericha coli O157:H7 in Manure Compost Applied to Soil to Grow Vegetables in an Envirotron Growth Chamber.
Mahbub Islam1, Jennie Morgan1, Michael P. Doyle1 and Xuiping Jiang2.
1Center for Food Safety, 2University of Clemson.
Animal waste in the form of raw manure or composted manure is routinely applied to the land as a crop fertilizer and/or soil amendment. A potential risk arising from the disposal of animal waste of fecal origin is the spread of enteric pathogens. Many outbreaks or cases of E. coliO157:H7 infection have been associated with water or food directly or indirectly contaminated with animal manure. Cross-contamination of produce from manure or improperly composted manure used on the farm can be a source of pathogen contamination during preharvest. Although competition with soil microorganisms and adverse environmental conditions can reduce pathogens, there is little information regarding the ability of E. coliO157:H7 to survive in manure-amended soils. In this study, our objective was to determine the fate of E. coli O157:H7 in soil and on vegetables in a controlled and contained plant growth chamber environment.
A five-strain mixture of green fluorescent protein (GFP)-expressing i O157:H7 was prepared and inoculated at 107 CFU/g into the compost. The inoculated compost was mixed with Tifton clay soil at a ratio of 1: 100. Twenty horticultural pots for each of baby carrot and green onion plants were filled with inoculated and fertilized soil (ca.5000 g). Three healthy transplants of each plant were planted into each pot 100 mm apart from each other, and then irrigated with city tap water. The pots were placed in the Envirotron with control of light, temperature, and CO2 levels. Special air filters was installed to prevent pathogens from spreading to the environment. Plants were irrigated every other day, and fertilized with soluble fertilizer (Sam’s Choice Deep Feeding All purpose Food) every two weeks. Soil samples from around the plant (Soil), plant leaves and stem samples (Plant), and soil samples just under the roots (S/p) in triplicate were analyzed for E. coli O157:H7 at approximately weekly intervals for the first four weeks, and every 2 weeks for the rest of plant growth cycle (up to 3 months). Soil moisture content and pH were also determined. Over a period of 64 days in onion, the population of GFP-expressing E. coli O157:H7 in soil and soil under roots samples was steadily reduced by 3 log , whereas in plant samples was reduced by 2. With carrot, it took 84 days to achieve a reduction of 2.3 log in soil. Seventy days were needed to get a reduction of 1.7 log in carrot plant.
Image analysis for non-destructive and non-invasive quantification of root growth and soil water content in rhizotrons.
Rolf O. Kuchenbuch1 and Keith T. Ingram2.
1Center for Agricultural Landscape and Land Use Research, Muncheberg, Germany, 2Department of Crop and Soil Sciences.
Studies aiming at quantification of roots growing in soil are often constrained by the lack of suitable methods for continuous, nondestructive measurements. A system is presented in which maize (Zea mays L.) seedlings were grown in acrylic containers – rhizotrons – in a soil layer 6-mm thick. These thin-layer soil rhizotrons facilitate homogeneous soil preparation and nondestructive observation of root growth. Rhizotrons with plants were placed in an Envirotron CG72 growth chamber, on a rack slanted to a 45o angle to promote growth of roots along the transparent acrylic sheet. At 2- to 3-day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and root diameters were measured using RMS software. Images taken during the course of the experiment were also analyzed with QUACOS software that measures average pixel color values. Color readings obtained were converted to soil water content using images of reference soils of known soil water contents.
To verify that roots observed at the surface of the rhizotrons were representative of the total root system in the rhizotrons, they were compared with destructive samples of roots that were carefully washed from soil and analyzed for total root length and root diameter. A significant positive relation was found between visible and washed out roots. However, the influence of soil water content and soil bulk density was reflected on seminal roots rather than first order laterals that are responsible for more than 80% of the total root length.
Changes in soil water content during plant growth could be quantified in the range of 0.04 to .26 cm3 cm–3 if image areas of 500 x 500 pixel were analyzed and averaged. With spatial resolution of 12 x 12 pixel, however, soil water contents could only be discriminated below 0.09 cm3 cm-3 due to the spatial variation of color readings.
Results show that this thin-layer soil rhizotron system allows researchers to observe and quantify simultaneously the time courses of seedling root development and soil water content without disturbance to the soil or roots.