Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The effect of chilling temperatures on porphyrin biosynthesis was studied in maize (Zea mays, L.). The results were consistent with the hypothesis that there are at least 3 temperature sensitive sites that compromise the accumulation of chlorophyll. Week old 28{dollar}\sp\circ{dollar}C light grown seedlings synthesized negligible amounts of chlorophyll when exposed to 12{dollar}\sp\circ{dollar}C for a subsequent 6 d. The effects were reversible in that plants returned to 28{dollar}\sp\circ{dollar}C had the chlorophyll content restored to control levels. The results were not consistent with a low temperature mediated decrease of carotenoid content, depletion of chlorophyll per chloroplast, accelerated senescence, or a dessication mediated depletion of chlorophyll.;A methodology for the spectroscopic estimation of porphyrin intermediates was developed and refined. One temperature sensitive site was revealed by measurement of porphyrin intermediates in etiolated seedlings. When illuminated at different temperatures negligible accumulation of intermediates occurred at temperatures of 22{dollar}\sp\circ{dollar}C and less. A second site was demonstrated by incubation of etiolated tissue with {dollar}\delta{dollar}-aminolevulinic acid in darkness at various temperatures from 12{dollar}\sp\circ{dollar}C to 28{dollar}\sp\circ{dollar}C. At temperatures in the range of 14{dollar}\sp\circ{dollar}C to 18{dollar}\sp\circ{dollar}C the accumulation of precursors was less than in 28{dollar}\sp\circ{dollar}C controls. The optimal temperature for accumulation of intermediates was 22{dollar}\sp\circ{dollar}C.;The third site has been shown to lie in the synthesis of {dollar}\delta{dollar}-aminolevulinic acid. In vivo studies showed negligible synthesis of {dollar}\delta{dollar}-aminolevulinic acid below temperatures of 18{dollar}\sp\circ{dollar}C. Pre-illumination of leaf tissue at 28{dollar}\sp\circ{dollar}C results in lowering this threshold temperature. A comparison of {dollar}\delta{dollar}-aminolevulinic acid versus porphyrin accumulation indicated that {dollar}\delta{dollar}-aminolevulinic acid accumulation responds to temperature differently than porphyrin synthesis indicating that chill sensitivity is mediated at different sites in the porphyrin pathway.;The underlying biochemical bases of the chilling effects demonstrated in this study have not been established. The possibility of a chill enhanced degradation of the intermediates and the effects on synthesis of the enzymes were considered.



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