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I.L. Goldman

I thank Paul Williams and Dan Lauffer for their inspiration, help, and guidance with the Wisconsin Fast Plant System, Geoffrey Schroeck for assistance with maintenance of the populations, the students of Horticulture 502 for their thoughtful

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Sunita K. Agarwal, David J. Schultz, and Dennis A. Schaff

Most cells have an active turnover of many of their nucleic acids (particularly some types of RNA) which through degradative processes result in the release of adenine, guanine and hypoxanthine. These free purines are converted to their corresponding nucleotides through salvage pathways. Adenine is converted to its nucleotide form AMP by Adenine phosphoribosyltransferase (APRT) which is one of the enzymes associated with the purine salvage pathway. Since all organisms have a de novo pathway for the formation of AMP, APRT is classified as a `salvage enzyme'. The APRT enzyme, in general, does not show a high degree of specificity for the exact structure of adenine and can also act on cytokinins and adenine derivatives like 2,6-diaminopurine, 2-fluoroadenine and 6-methylpurine. The APRT enzyme can utilize adenine analogues as substrate and convert them into their nucleotide forms which are toxic. Plants that lack APRT activity (APRT-plants) survive in the presense of these analogues. The amount of adenine analogue used for selecting APRT-plants is such that it kills all APRT+ (wild type) plants. APRT+ plants survive when grown in the presense of azaserine and alanosine that block de novo synthesis of AMP. APRT-plants transformed with the wild type cloned gene can be selected from a mixture of transformed and non-transformed plants by selecting in the presense of adenine, azaserine and alanosine. The presense of APRT activity can be confirmed by assaying for the APRT enzyme. APRT activity has been detected in many plant species. The presense of a positive forward and backward selection system can thus allow the use of APRT as a selectable marker in plant gene transfer systems.

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Erin Schroll, John G. Lambrinos, and David Sandrock

-tolerant succulents [particularly stonecrop ( Sedum ) species] have been the dominant plant selections for extensive green roof applications ( Getter and Rowe, 2006 ). However, expanding the plant palette available for green roofs is desirable. Green roof vegetation

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Karen H. S. Taylor, Dr. Greg Cobb, and Dr. Jayne Zajicek

Designing a landscape involves the selection of plants with certain characteristics such as height, color, hardiness zone, bloom time, etc. A Hypercard stack, which is a specific type of software application for Macintosh computers, was developed to aid landscapers in the location of plants with the desired characteristics. This Hypercard stack, called the “Plant Stack”, is based on the book, Identification Selection and Use of Southern Plants for Landscape Design, by Dr. Neil Odenwald and James Turner. The stack is also useful as an educational tool; for example, it can be used as a set of flash cards. Use of the software for selecting southern plants will be discussed as will use of the same software as an educational tool.

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Anson E. Thompson, Dennis T. Ray, Margaret Livingston, and David A. Dierig

Abstract

Single plant selections from a diverse guayule (Parthenium argentatum Gray) breeding population were evaluated for rubber and resin concentration, rubber and resin yield, and 12 growth characteristics. Thirty-seven superior selections (≈9% of the total of 421 single plant selections) exhibit combinations of satisfactory biomass production, high rubber yield, and strong regeneration and survival after harvesting by clipping the branches at a height of 0.10 m. Rubber concentration (%) was not highly correlated with the 12 plant characteristics measured. In contrast, rubber yield (g/plant) was highly correlated with plant dry weight and other plant characters related to biomass production. Twelve of the 37 superior selections had yields >125 g of rubber per plant, and the rubber concentration of seven of these 12 plants exceeded the mean (7.1%) of the 37 superior selections. Selection for high rubber concentration and yield with strong top regrowth after clipping appears to be feasible, and should expedite the development of a multiple harvesting system and rapid commercialization of guayule as a new domestic source of natural rubber.

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JoAnn Robbins and Thomas M. Sjulin

Abstract

Strawberry (Fragaria × ananassa Duch.) seedlings were planted in a greenhouse at 3- to 4-week intervals and simultaneously inoculated at ages 80, 101, 129, or 157 days with either of two naturally occurring virus sources each of which contained a mottle, mild yellow edge, and crinkle virus complex. Inoculation by aphids with either virus source reduced vigor, petiole length, leaflet width, stolons per plant, and vegetative dry weight of plants in the greenhouse. The tendency of virus inoculation to reduce vigor and petiole length was inversely proportional to increasing seedling age. In the field, inoculated seedlings were also less vigorous than control seedlings. Virus source effects and seedling age interactions with virus source were not significant. Selection for virus tolerance, based on greenhouse vigor, petiole length or leaflet width measurements, increased the frequency of seedlings subsequently classified as virus-tolerant in the field in both 80- and 101-day-old seedlings. Selection based on greenhouse vigor or petiole length increased the frequency in 129-day-old seedlings. No greenhouse selection method evaluated was effective in 157-day-old seedlings.

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I.L. Goldman

Wisconsin Fast Plants are rapid-cycling versions of various Brassica species amenable to a variety of genetic studies due to their short life cycle and ease of handling. I have recently developed a model system using Brassica rapa L. Fast Plants for teaching the cyclical selection process known as recurrent selection in the context of a course on plant breeding. The system allows for up to three cycles of recurrent selection during a 15-week semester and enables students to gain experience in planting, selection, pollination, and seed harvest during each cycle. Fourteen cycles of replicated, recurrent mass selection for high (H) and low (L) levels of anthocyanin pigment expression in hypocotyl tissue were practiced by students in Horticulture 502 during a period of four semesters. In addition to bi-directional selection; replicated unselected (D) control populations were maintained forcomparative purposes. Over 14 cycles, highly significant gains and losses in hypocotyl pigment production were realized for H and L populations, respectively. Plants in D populations showed no directional response to random selection and therefore did not exhibit genetic drift. Plants in H populations exhibited production of anthocyanin pigment in organs other than hypocotyls, suggesting selection goals could be modified to include pigmentation of specific organs or whole plants. Results from this selection program suggest significant gains from recurrent selection can be visualized through student-based selection activities in the classroom.

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Michelle M. Leinfelder and Ian A. Merwin

Apple replant disease (ARD) is a common problem typified by stunted growth and reduced yields in successive plantings of apple (Malus ×domestica Borkh.) in old orchard sites. ARD is attributed to biotic and abiotic factors; it is highly variable by sites, making it difficult to diagnose and overcome. In this experiment, we tested several methods of controlling ARD in a site previously planted to apple for >80 years. Our objective was to evaluate practical methods for ARD management. We compared three different experimental factors: four preplant soil treatments (PPSTs) (compost amendments, fumigation with Telone C-17, compost plus fumigation, and untreated soil); two replanting positions (in the old tree rows vs. old grass lanes); and five clonal rootstocks (`M.26', `M.7', `G.16', `CG.6210', and `G.30') during 4 years after replanting. The PPSTs had little effect on tree growth or yields during 4 years. Tree growth was affected by planting position, with trees planted in old grass lanes performing better than those in the old tree rows. Rootstocks were the most important factor in overcoming ARD; trees on `CG.6210' and `CG.30' grew better and yielded more than those on other rootstocks. Rootstock selection and row repositioning were more beneficial than soil fumigation or compost amendments in controlling ARD at this orchard.

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Kristine M. Lang, Ajay Nair, and Kenneth J. Moore

( Ambrózy et al., 2016 ; Day, 2010 ; Díaz-Pérez, 2013 , 2014 ; Kong et al., 2013 ), which makes it more difficult to predict plant performance under shade within high tunnel production systems. Research from the Southeast region using shade structures in

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D.T. Lindgren

68 WORKSHOP 9 (Abstr. 662–666) Ornamental Plant Breeding in the Midwest