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Developmental physiology, nitrogen preference, and estimated biofuel production of Opuntia ficus-indica (prickly pear cactus) in the United States
AdvisorCushman, John C
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Opuntia ficus-indica (Prickly pear cactus) is a large succulent cactus species that has a long history of agricultural production for carmine dye, fruits, and edible young cladodes. As a CAM plant O. ficus-indica uses crassulacean acid metabolism (CAM) which is characterized by assimilation of CO2 at night, which greatly increases water use efficiency. More recently, O. ficus-indica has been recognized as having high biomass and biogas (CH4) production values comparable to other large CAM species. Opuntia eliator, a closely related tropical epiphyte species, has been shown to predominantly use C3 photosynthesis in seedlings before switching to CAM photosynthesis. Similarly, young cladodes in Opuntia ficus-indica have been shown to predominantly fix CO2 during the day until they mature. Some plants can also switch to CAM photosynthesis when preferred nitrogen sources (nitrate or ammonium) become limiting and O. ficus-indica has been shown to have a slight increase in nitrogen uptake in ammonium vs.. nitrate. A field trial, and life cycle assessment has been conducted on biogas production from O. ficus-indica in Mexico, but biogas production values in the United States are unknown. Here, measured carbon isotope ratios, 24-hour gas exchange, and tissue acidity in O. ficus-indica seedlings and daughter cladodes to determine the occurrence of CAM vs. C3 photosynthesis. Results demonstrated that O. ficus-indica seedlings used predominantly CAM photosynthesis to assimilated CO2 even under well-watered conditions, while developing cladodes were shown to be sink tissues that switched directly from C3 photosynthesis to CAM photosynthesis in greenhouse conditions. Here, N preference was investigated by placing O. ficus-indica cladodes in sand culture and providing nutrient solutions with varying amounts of nitrate and ammonium for one month, and measuring differences in growth, biochemistry, and CAM and N-related gene expression. Statistical differences in O. ficus-indica growth, chlorophyll content, tissue acidity, soluble sugars, nitrate reductase activity, nitrate and ammonium content, glyoxylic acid content, N:C ratio and relative expression of genes involved N metabolism, and CAM activity were all detected when nitrate and ammonium were varied in sand culture. A life cycle inventory of biogas production from O. ficus-indica was built using results from an irrigation field trial that took place in Logandale, NV. Estimated biogas production from O. ficus-indica in the United States was 13,004.29-26,877.85 Nm3 ha-1 yr-1 produced from plants receiving 716 mm year−1 which is comparable to that of actual production values in Mexico.