PLANT PHYSIOLOGY , Vol 102, Issue 1 295-302, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Optimal Thermal Environments for Plant Metabolic Processes (Cucumis sativus L.) (Light-Harvesting Chlorophyll a/b Pigment-Protein Complex of Photosystem II and Seedling Establishment in Cucumber)
J. J. Burke and M. J. Oliver
United States Department of Agriculture Cropping Systems Research Laboratory, Route 3, Box 215, Lubbock, Texas 79401
Analysis of the temperatures providing maximal photosystem II fluorescence
reappearance following illumination and thermal kinetic windows (TKWs),
obtained from the temperature characteristics of enzyme apparent Km values,
have been proposed as indicators of the bounds of thermal stress in plants.
In this study, we have evaluated the temperature optimum for the
accumulation of the chlorophyll a/b light-harvesting complex of photosystem
II (LHCP II), its mRNA, and the mRNA of the small subunit of
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in cucumber
(Cucumis sativus L. cv Ashley) as a broader measure of metabolism than that
provided by either the fluorescence reappearance or TKWs. The TKW for
cucumber is between 23.5 and 39[deg]C, with the minimum apparent Km
occurring at 32.5[deg]C. The photosystem II variable fluorescence
reappearance following illumination was maximal between 30 and 35[deg]C.
Maximum synthesis of the LHCP II occurred at 30[deg] C. The light-induced
accumulation of the LHCP II and the small subunit of Rubisco mRNAs showed
similar temperature characteristics. Suboptimal temperatures delayed
germination, altered cotyledonary soluble sugar content, and broadened the
temperature range for chlorophyll accumulation. These results demonstrate
an effect of seed reserve mobilization on the range of temperatures for
chlorophyll accumulation, and suggest that metabolic temperature
characteristics may be broadened by increasing available substrates for
enzyme utilization. This study provides new information about the
relationship between TKWs and cellular responses to temperature. In
addition, the results suggest that the temperature range outside of which
plants experience temperature stress is narrower than traditionally
supposed.