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|Title:||Inhibition of Photosynthesis in Whole Tomato Plants Following Chilling in the Light: The Role of Enzymes of the Photosynthetic Carbon Reduction Cycle|
|Author(s):||Sassenrath, Gretchen Frances|
|Doctoral Committee Chair(s):||Ort, Donald R.|
|Department / Program:||Biology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Certain plants which are accustomed to growing under temperate conditions show a distinct inhibition of function when subjected to brief periods at temperatures that are well below the optimal growing point but above freezing. Photosynthesis is often the first physiological process that displays a loss of activity following a low temperature exposure, and damage is exacerbated when low temperatures are sustained in the presence of high light intensities. Under normal growing conditions, the component processes of whole plant photosynthesis appear to be balanced in such a way that it is unlikely that any single biochemical reaction is ever a sole limiting factor to photosynthesis or to growth. However, after a plant has been subjected to a stress, a lesion may develop in one step, creating a dominant limiting factor. It is the aim of this study to discover the nature of inhibition of photosynthesis and thence localize the essential biochemical processes that may be specifically weakened during chilling in the light.
The maximal rate of photosynthesis is inhibited more than 60% in intact tomato leaves following a 6 hour exposure to 4 C under saturating illumination (1000 uE/m$\sp2$-s). A comparable reduction is observed in the quantum yield. Following light chill stress, the major limitation to photosynthesis is due to a direct inhibition at the level of the chloroplast.
Direct measurement of enzyme activities, coupled with measurements of in vivo metabolite pool sizes, were used to determine if enzymatic activity limits CO$\sb2$ assimilation following a light chill treatment. After light chilling, the level of fructose-1,6-bisphosphate increases more than two fold, while that of ribulose 1,5-bisphosphate declines by half. This indicates that a step between the utilization of FBP and the regeneration of RuBP is inhibited. Measurements of enzymatic activities indicate that FBPase is substantially inhibited following a light chill treatment. However, the FBPase seems to be fully functional, since maximal activity can be regained by activating the enzyme in vitro with dithiothreitol. The loss of activity of FBPase following a light chilling may depend on the thioredoxin:ferredoxin system, which is responsible for the light-mediated reductive activation of the FBPase.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
|Date Available in IDEALS:||2015-05-14|