<p>Endophytes are a unique group of microorganisms that spend at least part of their life cycle within plant tissues. These microbes are increasingly being recognized for their potential to improve the health and productivity of their host plants. Recent studies indicate that endophytes could also influence human health by altering the composition of chemical compounds within plants, thereby affecting their nutritional quality and flavor. In addition, the presence of endophytes in edible plant tissues could directly affect human health by introducing microbes that can stimulate the immune system or act as opportunistic pathogens in people with compromised immune systems. However, despite their potential importance for plant and human health, these plant-associated microbes have been understudied due to a lack of visible symptoms associated with their presence and difficulty in isolating them from plant tissues. In the present studies, we hypothesized that endophytes play an important role in carrot, one of the most important vegetable crops in the world.</p> <p> Carrot is well known as an important source of vitamins, antioxidants and other nutrients in the human diet, and carrot taproots are often consumed raw. Carrot crops are slow to establish and are subject to assault by a wide range of pests that negatively affect the health and productivity of this crop, as well as the storage potential of its taproots. Consequently, the aim of these studies was to examine endophyte dynamics in carrot. Studies were carried out in field, greenhouse and laboratory trials using a diverse set of carrot genotypes with broad genetic backgrounds and physiological characteristics. Endophyte communities were studied using traditional culture-based techniques, along with low and high throughput sequencing technologies.</p> <p>Results of these studies demonstrate that carrot seeds and taproots are colonized by an abundant and diverse set of endophytic microbial taxa. Many of these endophytes could solubilize phosphorous, fix atmospheric nitrogen, produce siderophores and auxin and suppress infection by a key carrot pathogen, <i>Alternaria dauci</i>, demonstrating their potential importance for maintaining carrot health and productivity. Some of the endophytes identified in these trials were vertically transmitted to progeny inside carrot seeds, indicating that they could be part of a core microbiome that evolved alongside carrot plants, and are likely to be critical in early seedling establishment. We also determined that carrot endophytes could be acquired via horizontal transmission from soil with greater soil health in an organic relative to a conventionally managed system resulting in greater populations of endophytes with antagonistic activity against <i>A. dauci</i>. Finally, endophyte communities varied among the genotypes evaluated in this study, with some being more responsive to the presence of greater populations of beneficial endophytes in their environments. This indicates that it could someday be possible to begin selecting for these beneficial plant microbial relationships in breeding programs.</p> <p> Based on the results of these studies, we conclude that endophytes do indeed play an important role in carrot. Additional research aimed at determining how these microbes functionally interact with carrot plants and identifying practical approaches to manipulate these communities to enhance the productivity and quality of carrot taproots, are recommended. A new isolation technique identified in these trials will aid in these efforts.</p>

Part I Four-week old tomato (Lycopersicon esculentum Mill.) seedlings were exposed to different constant temperatures of 15, 25, 35, or 45 c. To determine the effect of temperature on Abscisic Acid (ABA) young and old leaves were harvested at 0, 6, 12, 24, or 48 hand free, hydrolyzable and total ABA were measured using gas liquid chromatographic methods. Temperature had a significant effect on free , hydrolyzable and total ABA in both young and old leaves. Time had a significant effect as a cubic function on all ABA measurements in old leaves but in young leaves a significant effect was observed only on the hydrolyzable ABA. Interactions between temperature and time were observed in both young and old leaves. Young leaves had significantly higher amounts of free and hydrolyzable ABA than did old leaves . The total ABA levels were not significantly different in young and old leaves . Suboptimal temperatures increased ABA levels in the plants, in a manner similar to that of other kinds of stresses. Part II Five- week old tomato plants (Lycopersicon esculentum Mill.) were exposed to day-night temperatures of 10-5, 15-10, 25 - 15, 35-25, or 45-35 C. The day length was 16 h with a light intensity of approximately 400 ~ E m- 2s-1 . Plant tops were sampled at 12, 24 , 48, 68, or 72 h. Free, hydrolyzable and total absciscis acid (ABA) quantities were measured using standard gas chromatographic techniques. Stressful temperatures significantly increased both free and hydrolyzable ABA levels . The highest ABA levels were observed in plants grown at 10-5 C. Although time significantly affected ABA levels, its role needs more precise definition . Due to the similar involvements of ABA in temperature-induced and other stresses, ABA may be hypothesized to be a common mediator for different stresses. Part III Warm season crops: bean (Phaseolus vulgaris) CV . Burpee Stringless Green Pod; corn (Zea mays) CV. Golden ,Jubilee; Cantaloupe (Cucumis melo) cv. Hales Best; eggplant (Solunum melongena) CV. Ichiban; and okra (Hibiscus esculentus) CV . Dwarf Green and cool season crops: beet (Beta vulgari s) CV. Early wonder; lettuce (Lactuca sativa) •CV. Great Lakes; cabbage (Brassica oleracea) cv. Savoy; radish (Raphanus sativus) CV. Scarlet Globe; and pea (Pisum sativum) cv. Little Marvel were exposed to 10, 25 , or 40 C. After 24 h free and hydrolyzable ABA and DPA were measured in the plant tops using standard gas chromatographic methods. Warm season crops exhibited elevated levels of PABA, HABA and DPA under 10 C, compared to those at 25 or 40 c. Cool season crops showed similar PABA , HABA and DPA contents under all temperatures, except pea which had higher PABA and HABA levels at 40 C and beet which showed lower levels of HABA at 25 C as compared to 10 and 40 C. DPA existed at much higher concentrations than ABA in all plants of the two groups. It is concluded that while 1 0 C is stressful to warm season crops and thus the ABA and DPA increases, it is not so favorable to cool season crops. A temperature of 40 C would be stressful to both groups, but as long as water supply to the plant is not limiting the plants would not actively produce ABA . Higher levels of DPA under stressful conditions would warrant consideration and taking into account the role of metabolism in regulation of total ABA in the plant.

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Water management for the irrigation of containerized production remains a task reserved to a highly qualified personnel. Without proper instruments to determine the actual water needs, nursery crops can face growth losses of 20 to 30%. / Reviewing the different methods that are used in field production, tensiometer have shown the greatest potential of adaptation to potted plants. / Tests on the utilization of tensiometer were realized on two peat-vermiculite substrates, using two types of porous cup (1 bat--standard flow and 1 bar--high flow). The instruments were tested while permanently fixed in the growing medium, and temporarily fixed for three hours in the growing medium. / The results show that the instruments give excellent measurements for normal condition of growth, requiring saturation level between 50 and 100%, and that for both mode of operation. / Preferably, the choice of the instrument should be based on the hydraulic characteristics of the substrate. Little difference has been found using both instrument in the substrate 1, but in substrate 2 however, the high flow porous cups have given better response. The high flow cups have higher porosity and bigger pores that suit better substrate 2, which is made of coarser particles.

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Thesis (doctoral)--Universität, Giessen, 2006. / Includes bibliographical references (p. 206-219).