RURALS: Review of Undergraduate Research in Agricultural and Life Sciences

Stomatal Frequency, Distribution, and Needle Hydrophobicity in Cloud Forest Spruce and Fir, Southern Appalachian Mountains

Jennifer E. Reed et al. — Thu, 15 Nov 2012 11:06:04 PST

Red spruce (Picea rubens Sarg.) and Fraser fir, [Abies fraseri (Pursh) Poir] are two species of conifer trees that dominate the mountaintop spruce-fir forests of the Southern Appalachian Mountains, USA, a community that is considered a boreal remnant of the last glacial maximum. Their persistence has been attributed, at least partially, to the frequent cloud immersion that matches the distribution pattern of these remnant boreal species. Although fundamental to photosynthetic carbon uptake and growth, little is known in general about stomatal morphology or behavior in cloud-immersed, conifer forest species, although immersion has been hypothesized as facilitating carbon dioxide uptake while reducing evaporative water loss, for an enhanced water use efficiency. It is also known that the absence of any water film formation on the leaf surface (hydrophobicity) is critical for photosynthetic gas exchange. Yet, studies have also alluded to the possible importance of water vapor in cloud-immersed species. In the present study, stomatal distribution patterns and surface water repulsion from needles at high and low elevation sites were measured and compared between the two species to evaluate the potential importance of these traits for growth in a cloud-dominated habitat. It was hypothesized that hydrophobicity and stomatal densities would increase with increasing altitude in both species in response to a greater occurrence and duration of cloud immersion. This hypothesis was not supported by data, which showed that these leaf surface features were statistically similar at both altitudes sampled. However, the data did show statistically significant differences in these characteristics among the two species at the lower elevation site, raising questions about possible functional differences.

Influence of Pasture Management Regimes on the Abundance of Four Target Beetle Groups

Christina Doehling et al. — Mon, 29 Oct 2012 15:46:02 PDT

Limited information is available on the impact of pasture management regimes on the local arthropod community. The objectives of this study were to identify the insect orders and families associated with managed smooth bromegrass pastures and determine the effect of three management strategies on the abundance and occurrence of 4 key beetle groups. Pitfall samples were collected weekly from June 2 until September 2during the summer of 2010. Sixty-one families of insects, representing nine orders, were collected in the traps. A predatory ground beetle in the genus Cyclotrachelus made up 71.5% of all target beetles collected. No significant differences were detected in beetle numbers among the management regimes. Results from this research suggest that any of the three management strategies evaluated could be implemented without negative impacts on the beneficial beetle groups studied

Misting Effects of the Microbial Quality of Retail Leaf Lettuce

Rachel Rossman et al. — Mon, 02 Apr 2012 17:58:44 PDT

Limited research has been done on the microbial quality of produce that has been misted at retail food settings. Therefore, our objective was to determine the microbial quality of leaf lettuce displayed at retail in misted or non-misted displays. Three grocery stores that mist and three grocery stores that do not mist lettuce were selected to obtain leaf lettuce. Leaf lettuce was analyzed for aerobic plate count (APC), yeast, mold, and Enterobacteriaceae. APC ranged from 6.23 to 6.36 Log10/g for misted leaf lettuce compared to 5.20 Log10/g for non-misted leaf lettuce. Yeast counts ranged from 4.34 to 4.91Log10g for misted leaf lettuce compared to 4.04 Log10/g for non-misted leaf lettuce. Mold counts ranged from 2.88 to 4.46 Log10/g for misted leaf lettuce compared to 4.08 Log10/g for non-misted leaf lettuce. Enterobacteriaceae ranged from 6.23 to 6.53 Log10/g in misted leaf lettuce compared to 4.08 Log10/g for non-misted leaf lettuce. From this study, we have demonstrated that there is a higher amount of microorganisms on leaf lettuce under misted conditions; determination of specific pathogenic microorganisms on the misted and non-misted leaf lettuce needs to be examined.

Favorable Team Scores Under the Team-Based Learning Paradigm: A Statistical Artifact?

Trevor Hefley et al. — Fri, 09 Mar 2012 12:14:32 PST

Team-based learning (TBL) is a highly structured method of active small group learning and consists of identical tests taken individually and in groups. We reviewed literature to determine the scoring methods used for individual and group testing. Twenty-eight percent of the studies used multiple choice formats for the individual test and formats that allow for immediate feedback and corrective scoring for the group test. We calculated the expected value of both individual and group tests and found that by default groups will score higher than individuals, even if there is no actual difference in performance. We urge TBL practitioners to use statistically equivalent scoring for individual and group testing.

Performance of Mapping-Grade GPS Receivers in Southeastern Forest Conditions

Michael D. Ransom et al. — Mon, 07 Jun 2010 06:00:52 PDT

This study represents an assessment of two recently-available GPS receiver configurations used in mature southern pine and hardwood forests in the Piedmont of Georgia. Six control points were visited ten times, with WAAS (Wide Area Augmentation System) enabled, and 50 position fixes were recorded during each visit to each control point. We found that horizontal position accuracy was on the order of 2 m when WAAS was enabled in those receivers and post-collection differential correction was not employed. This was significant in that the accuracy of the receivers evaluated was greater than recent studies suggested even without the use of differential correction, revising our notion of how well GPS receivers perform in real time in forested conditions. In general, there was no significant difference in horizontal position accuracy between the two receiver configurations when the error of an average position (from a set of position fixes) was analyzed. However, when the error was assessed for each position fix, and then averaged, there was a significant difference with one of the receiver configurations when used in the pine and hardwood stands, and there were significant differences between the two receiver configurations when used in the hardwood stand. In addition, in general there was no correlation between horizontal position error and PDOP (Positional Dilution of Precision), signal-to-noise ratio, relative humidity, air temperature, and atmospheric pressure values at the time of data collection. However, one of the two receiver configurations seemed sensitive to air temperature. These results illustrate the real-time horizontal position accuracy that can be obtained with current technology in similar forest conditions throughout the Piedmont of the United States during leaf-off (winter) conditions.

Storage and breakdown of starch aid P. parviflorus in leaf re-greening after nitrogen deficiency

Kevin A. Korus et al. — Sat, 15 May 2010 02:56:30 PDT

Plectranthus parviflorus, common Swedish ivy does not lose leaves when it is deprived of nitrogen. Instead this plant retains its yellow leaves and upon reintroduction of nitrogen will re-green and start to grow. In two experiments, rooted cuttings of common Swedish ivy were grown with (150 ppm N) and without nitrogen for 3 weeks. After some plants were sampled the others were either switched or kept at 0 or 150 ppm N and allowed to grow for another 3 weeks. After another sampling, plants were again switched or kept at 0 or 150 ppm N for a final 3 weeks. At each harvest, leaves were tested for starch, sampled for microscopy and then dried and weighed for soluble carbohydrate extraction. Data collected indicates that yellow leaves store and breakdown starch into soluble carbohydrates (specifically reducing sugars) in order to keep leaves from senescing. When nitrogen is re-supplied to these plants, leaves re-green and the plant continues to grow. We propose that common Swedish ivy’s ability to store and breakdown starch aids in the process of leaf re-greening.

Monitoring Early Season Mosquito and Bird Populations: Implications for West Nile Virus in Lancaster County, Nebraska

Jason Thiele et al. — Mon, 12 Oct 2009 08:44:21 PDT

West Nile virus (WNV) surveillance efforts in Nebraska focus on the late summer and early fall months. We studied mosquito and bird populations in Lancaster County, Nebraska to evaluate the potential for WNV transmission in the late spring and early summer, prior to occurrences of human WNV infections. Culex tarsalis is the most important vector of WNV in Nebraska and was the focal species for the study. Mosquitoes were trapped at six locations representing three habitat types from May 14 to July 11, 2007. The C. tarsalis population, as estimated by the number of individuals caught in the traps each night, peaked early in the study period and then declined. Surveys of avian communities at the sites showed that competent WNV reservoir species were present throughout the study period. We also tested Culex mosquitoes for the presence of WNV. One of 95 pools of Culex mosquitoes tested for WNV using the Rapid Analyte Measurement Platform (RAMP^®) method returned a positive reading. Modeling the distribution of the RAMP test results indicated that additional pools might have contained mosquitoes with WNV. The high C. tarsalis population observed in the late spring and the WNV-positive pools suggest that studying early season mosquito populations can provide valuable information for evaluating the risk of West Nile virus to humans later in the year.

Analysis of Green Roof Growing Media Porosity

Kelsey Latshaw et al. — Sat, 18 Jul 2009 09:27:17 PDT

When selecting a growing media for a green roof, a rooftop environment should be distinguished from traditional plant settings. Therefore, it becomes critical that the media’s pore space allows for adequate storage and drainage of water. In this experiment, rooflite™, a certified green roof growing media, was tested at depths of 7.62 cm (3 in) and 10.16 cm (4 in), for drainage and water-holding capacity. The main objective of this analysis is to determine if these two depths will meet the manufacturer’s recorded values. The depths will also be compared in terms of porosity. When analyzing the results, it was determined that the media could hold and drain enough water on a green roof with a 10.16 cm. depth. In detail, the results indicated that an average of 117.3 mL of water was stored and 35.3 mL of water was drained from a media depth of 7.62 cm (3 in). An average of 153.7 mL of water was held and 46 mL was drained from a media depth of 10.16 cm (4 in). It was concluded that the media depth of 10.16 cm (4 in) had increased water storage and drainage capabilities.

Incidence of Salmonella on Reptiles in the Pet Trade

Casey R. Cain et al. — Fri, 20 Feb 2009 20:27:48 PST

Reptiles are very common carriers of Salmonella and can be infected without showing any symptoms. Humans can contract reptile-associated Salmonellosis (RAS) by handling a Salmonella-infected reptile or any object contaminated by a Salmonella-infected reptile then failing to properly wash their hands, or by allowing reptiles to contaminate areas of food preparation.

This study tested reptiles for Salmonella in four Nebraska pet shops in March of 2007 to estimate the prevalence of Salmonella in captive reptiles in Nebraska. Of 80 pet reptiles tested, 53.8% (43) yielded Salmonella, which is much higher than the 6.6% (12 of 182) prevalence of Salmonella in wild reptiles in Nebraska from previously-gathered data.

Two generalized linear models and two mixed models suggested that pet shops may account for the variation in prevalence of Salmonella, while taxonomic groups do not. Presence of Salmonella in Nebraska reptiles appears to be mainly dependent upon location—geographic proximity to other reptiles with Salmonella in the wild, or the presence of Salmonella in other pet reptiles within a pet shop.

Determination of Biofilms on Plastic Cutting Boards

Kristen Neth et al. — Fri, 10 Oct 2008 04:12:32 PDT

Biofilm formation on food processing surfaces has been well documented. Plastic cutting boards used in the foodservice/commercial food preparation kitchen setting would be ideal surfaces for a biofilm to form. The purpose of this experiment was to determine if biofilms were capable of forming on cutting boards that were used in a foodservice/commercial food preparation kitchen. Three 25cm2 surface areas were randomly selected on 15 plastic cutting boards (n = 45) from a food preparation kitchen to be swabbed before and after proper cleaning and sanitizing. Swabs were diluted in buffered peptone water, serial dilutions made on Plate Count Agar and incubated for 48 hours at 37°C. The average aerobic plate count for the plastic cutting boards before sanitation was 1.31 x 10 E4 + 2.00 x 10 E4 CFU/25 cm2. After sanitation of the plastic cutting boards, the average aerobic plate count was 1.35 x 10 E3 + 1.91 x 10 E3 CFU/25 cm2. The results indicate that plastic cutting boards, even after cleaning and sanitizing, can support biofilms when they are heavily scarred and discolored.

Sensory Evaluation of Ice Cream made with Prebiotic Ingredients

Adeline K. Lum et al. — Tue, 07 Oct 2008 22:36:18 PDT

Fructooligosaccharides (FOS) and inulin are considered prebiotic ingredients and are FDA approved for use in food for human consumption. A prebiotic is a food ingredient that benefits the host by selectively stimulating the growth and activity of the beneficial bacteria in the colon. In this project, consumer acceptability of ice cream made with 10% of the sugar substituted with either inulin or FOS was tested. Physical tests were also conducted. The texture, water activity and L and b color values did not differ significantly for the treatments (Inulin and FOS) versus the control sample. The a value for color (red/green) differed (p=0.02) for the treatment samples when compared to the control. For the sensory evaluation, 71 participants ranked the ice cream samples made with inulin and FOS equally as well liked (6.56 and 6.80, respectively) on a 17 cm hedonic scale (0 = like and 17 = dislike). However, the participants liked the control ice cream better than the ice cream with a prebiotic ingredient (P<0.001). Inulin and FOS are potential ingredients for use in ice cream.

Results of a Twenty-Two Year Old Progeny Test of a Pinus sylvestris Plantation in Eastern Nebraska

Adam D. Rupe et al. — Fri, 22 Aug 2008 11:29:20 PDT

The goal of this study was to examine the performance and adaptability of a 22-year-old Pinus sylvestris L. (Scots pine) full-sibling progeny plantation representing genetic material from seven European countries (Spain, France, Russia, Ex-Yugoslavia, Italy, Greece, and Germany), to the Great Plains environment. The study site was located at the University of Nebraska Agricultural Research and Development Center near Mead, NE. A total of 92 individuals originating from 23 crosses were studied for the following growth parameters: specific leaf area (SLA, cm²g^-1), needle length (cm), canopy width (m), and tree height (m). Analysis of variance and orthogonal contrasts were used to compare crosses. Principal component analysis (PCA) was used to understand the factors controlling seed source classification. Taller trees, wider canopies and long needles characterized crosses that scored high in the PCA, while crosses that scored low had high SLA. Results indicated that crosses with maternal genetic materials originating from northern latitudes (i.e. Russia and Germany) performed better than the remaining crosses. Additionally, crossing northern seed sources with southern sources (i.e. Italy and Greece) improved growth. PCA analysis showed that height was the most valuable indicator variable, which is important because tree height is one of the easiest and fastest measurements that can be obtained in the field, allowing it to be used by producers and non-scientists in the future as an inexpensive and simple method of testing.

Weed Response to Broadcast Flaming

Andre C. Domingues et al. — Wed, 13 Aug 2008 07:19:36 PDT

A field experiment was conducted during the summer of 2007 at the Haskell Agricultural Laboratory near Concord, NE, to determine the response of four weeds species to broadcast flaming, including two grasses [barnyardgrass (Echinochloa crus-galli) and green foxtail (Setaria viridis)] and two broadleaves [velvetleaf (Abutilon theophrasti) and morningglory (Ipomoea hederacea)]. The V3 barnyardgrass (5 cm tall), V4 green foxtail (6 cm), V4 morningglory (11 cm) and V5 velvetleaf (8 cm) were flamed using propane rates of 0, 12, 31, 48, 69 and 87 kg/ha. The responses of the barnyardgrass, green foxtail, velvetleaf, and morningglory to propane flaming were described by logistic models. The percentage of weed control in terms of visual injury rating and dry matter (DM) reduction increased with increasing propane rates. In general, grasses were more tolerant to flaming than the broadleaf species when based on visual ratings. About 90% control of barnyardgrass was achieved with 55 kg/ha compared to 26 kg/ha for velvetleaf and morningglory. In contrast to visual ratings, the dose response curves based on DM showed that comparatively higher propane dose was needed to obtain 90% DM reduction in morningglory than the other species. For example, propane rate of 41 kg/ha was required to get 90% DM reduction in morningglory 14 days after treatment compared to 36, 20 and 24 kg/ha for barnyardgrass, green foxtail and velvetleaf, respectively. Broadcast flaming has good potential for the control of both grass and broadleaf weed species.

Corn (Zea mays) and Soybean (Glycine max) Tolerance to Broadcast Flaming

Teixeira Z. Heverton et al. — Wed, 18 Jun 2008 05:57:34 PDT

Propane flaming can be used as one of the alternatives to chemical weed control. A field experiment was conducted during summer 2007 at the Haskell Agricultural Laboratory near Concord, NE to determine the responses of corn (Zea mays) and soybean (Glycine max) to broadcast flaming utilizing different rates of propane. The 5-leaf corn (V5, plant height 25 cm) and V3 soybean (plant height 8 cm) were flamed using propane rates of 0, 12, 31, 48, 69 and 87 kg/ha. The responses of the corn and soybean plants were described by logistic models. In general, soybean was more susceptible to propane flaming than corn; 20% injury was achieved with 21 kg/ha in soybean compared with 46 kg/ha in corn. Dose response curves of propane for soybean were similar among evaluation dates. For example, the propane dose (also known as effective dose, ED), which caused injury levels of 5% (ED5), 10% (ED10) and 20% (ED20) at 3 hours after treatment were 11, 13, and 17 kg/ha, respectively, and these rates did not change significantly over time. In contrast, dose response curves were different over time for corn. For example, the ED20 values for 3 hours, 7, and 14 days after treatment were 14, 22, and 46 kg/ha, respectively, suggesting that the corn crop was able to recover after flaming. This is likely because the growing point at time of flaming was below the ground level and thus remained unaffected. Broadcast flaming for weed control has more potential for use in corn than soybean. Further research is needed to determine if these relationships are valid at other growth stages.

Inhibition of Mold Growth by Sourdough Bread Cultures

Pei Ven Kam et al. — Wed, 11 Jun 2008 06:58:47 PDT

Sourdough bread cultures are mixtures of wild yeasts and Lactobacillus bacteria living in flour and water, where they form an interesting symbiosis that makes the culture quite stable. The presence of lactic acid bacteria (LAB) in sourdough bread cultures increases the shelf life of the sourdough bread and other sweet baked goods made with these cultures, due to the inhibitory effect of organic acids on spoilage molds. In addition, it has been found that when sourdough LAB are cultivated they produce antifungal substances, such as organic acids (in particular, lactic acid and acetic acid), carbon dioxide, ethanol, and hydrogen peroxide, and other, as yet unidentified inhibitory substances, that prevent mold growth. The inhibitory effect of four American sourdough cultures were tested for antifungal activity against the common spoilage molds, Aspergillus flavus, Aspergillus niger, Penicillium expansum, Penicullium roqueforti, and Cladosporium cladosporioides. In these experiments, actively growing sourdough cultures were inoculated into modified deMan Rogosa and Sharpe (mMRS) broth and incubated. After incubation, the cultures were centrifuged and filtered through 0.2µm membrane filters, the supernatants were collected and mixed with Potato Dextrose Agar (PDA). This mixture was used as culture medium for the growth of the spoilage molds. The growth rates of molds growing in the presence of the sourdough culture supernatants were compared with controls, where the molds were cultivated only in PDA. The results showed that Aspergillus flavus and Cladosporium cladosporioides were inhibited the most by the sourdough cultures. Therefore, the use of sourdough cultures shows promise for preserving food products from spoilage, as they could be a source of natural antimicrobial and antimycotic agents for use in the food industry.

Effects of Consumer Perceptions on the Marketing of Second Generation Genetically Modified Products

Anneke Gustafson et al. — Wed, 11 Jun 2008 06:54:13 PDT

The paper examines how the market/consumer acceptance of a second generation, consumer oriented, genetically modified product (new GMP) affects a company’s marketing strategy and its decision to invest in the research and development (R&D) of the new product. Two relationships between the products available in the market and their effect on the monopolist’s marketing strategy are considered, namely, vertical and horizontal product differentiation. Analytical results show that when the new GMP in a vertically differentiated market is viewed as the low quality product it has to be priced lower than the conventional product to capture a positive market share while when it is viewed as the high quality product, it may be able to capture the entire market. Results also show that the innovating firm may have to price the new GMP lower in the horizontally differentiated than in the vertically differentiated market in order to enhance its market share.

Dry Bean Intake of Women Ages 19-45

Julie Eihusen et al. — Wed, 11 Jun 2008 06:51:40 PDT

Dry beans are considered an excellent source of folate. A mail-out survey was conducted to measure the dry bean intake of women between the ages of 19 and 45 years and to estimate the amount of folate from beans in the diet. One hundred and seventy-six women reported on the frequency with which they consume foods made with dry beans and the types of dry beans they have previously consumed. The average number of servings of food made with beans per month was 18.5, and the most commonly eaten food made with dry beans was refried beans, averaging 2 times per month. It was estimated that women, 19-45, consume approximately 21.67 mcg of folate from foods made with beans each day, making up a small portion of the recommended daily intake of 400 mcg per day. Most respondents reported having eaten kidney beans, and approximately two-thirds of the respondents said they had consumed pinto beans, lima beans, and small red beans. Taste and nutrition were the top reasons participants included dry beans in their diet, although taste was also cited by 22% of the participants as the reason they chose not to consume dry beans.

An Investigation of the Genetic Variation between Blissus occiduus Barber and Blissus leucopterus leucopterus (Say)

Lanae M. Pierson et al. — Wed, 11 Jun 2008 06:44:49 PDT

In Nebraska, there are two chinch bug species that are of major economic importance: the common chinch bug, Blissus leucopterus leucopterus (Say) and the western chinch bug, B. occiduus Barber. The lack of key morphological characters to accurately differentiate between these two species in the immature stage and their extensive overlap of plant hosts and geographic distribution underscore the need to identify molecular markers to distinguish between these two chinch bugs. The objective of this research was to investigate the genetic diversity between B. l. leucopterus and B. occiduus using Amplified Fragment Length Polymorphism (AFLP). Five primer combinations were selected from 20 primer combinations to be used for testing 15 samples of each chinch bug species. The five primer combinations included a total of 151 AFLP markers. Of these, 148 AFLP markers (or 98.01%) were polymorphic between populations. Within B. occiduus, 133 AFLP markers (or 88.08%) were polymorphic and within B. l. leucopterus, 132 AFLP markers (or 87.42%) were polymorphic. Approximately 63% of the variation in the data set could be attributed to genetic variation within the populations according to the AMOVA analysis. Conversely, approximately 37% of the genetic variation occurred between populations. Several distinct molecular markers were identified that can be employed to distinguish between the two species when morphological characteristics show minimal, if any differences, during the immature stages. This research provides a genetic marker that can be used to differentiate between these two economically important chinch bug species. This new diagnostics tool will allow species-specific management options to be employed. In addition, this baseline data can advance future research on chinch bug genetics, including comparisons of additional species.

Consumption & Attitudes about Whole Grain Foods of UNL Students Who Dine in a Campus Cafeteria

Kimberly J. Bisanz et al. — Wed, 11 Jun 2008 06:33:56 PDT

Strong, research-based evidence indicates the consumption of whole grains lowers the risk of chronic diseases, including type 2 diabetes mellitus, cardiovascular disease, and certain cancers, along with aiding weight regulation. Consequently, the 2005 Dietary Guidelines for Americans set forth by the Department of Health and Human Services (HHS) and the Department of Agriculture (USDA) placed an increased emphasis on whole grains in the American diet. It is recommended that at least one-half of the grains in a daily diet, or approximately 3 one-ounce servings, should be whole grain. The hypothesis of this research was that students eating in campus dining centers do not meet this recommendation, making intervention necessary. Subjects (n=205) were self-selected diners at Selleck Dining Center on the University of Nebraska – Lincoln campus. Participants completed a one-page survey addressing attitude toward, consumption level of, knowledge of, and preferences for whole grain products. Results indicated a total of 86% of students reported eating whole grains, however, only 36% of those students reported average consumption levels at or above the recommended 3 ounces per day. Knowledge questions indicated that those participants reporting 3 or more servings of whole grain per day did not have a clear understanding of how to identify whole grains thus indicating their reported intakes were likely overestimated. Campus dining centers have the opportunity to be pivotal in increasing whole grain consumption among their users. Suggested appropriate intervention by dining center staff includes education on whole grain benefits, placement of identifiable markers for whole grain products, and offering a wider selection of whole grain products.

Development of a System for Directed Evolution of Arabidopsis Formate Dehydrogenase to Utilize NADP as a Cofactor

Brittany L. Prather et al. — Mon, 04 Dec 2006 15:37:38 PST

Formate dehydrogenase (FDH) is a NAD-dependent enzyme found in methylotrophic bacteria, yeast and plants. This enzyme catalyzes the reversible oxidation of formate to carbon dioxide. The goal of this research was to determine the feasibility of using a directed evolution approach to generate an altered Arabidopsis FDH with a high affinity for NADP as a cofactor. A PCR procedure that induced approximately 1.5 mutations in the wild-type Arabidopsis FDH sequence per thousand base pairs was developed and the amplified products were transformed into E. coli cells. Approximately 1300 cell lines were assayed in 96-well microplates for activity with NADP⁺ and 100 putative mutants were selected for further study. One particular mutant line, pFDH-18, possessed reproducible NADP⁺-FDH activity. Sequence analysis showed that a single T in the wild-type DNA sequence had been changed to a G. The result of this mutation was that an isoleucine (Ile) residue at position 188 in the wild-type enzyme was converted to a methionine. This particular Ile residue is conserved in the known FDH sequences from higher plants and is located in the region of the enzyme that contains the binding domain for the NAD cofactor.