PLANT PHYSIOLOGY , Vol 111, Issue 1 169-178, Copyright © 1996 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
The Effect of Leaf Temperature and Photorespiratory Conditions on Export of Sugars during Steady-State Photosynthesis in Salvia splendens
J. Jiao and B. Grodzinski
Department of Horticultural Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Export and photosynthesis in leaves of Salvia splendens were measured
concurrently during steady-state 14CO2 labeling conditions. Under ambient
CO2 and O2 conditions, photosynthesis and export rates were similar at 15
and 25[deg]C, but both declined as leaf temperature was raised from 25 to
40[deg]C. Suppressing photorespiration between 15 and 40[deg]C by
manipulating CO2 and O2 levels resulted in higher rates of leaf
photosynthesis, total sugar synthesis, and export. There was a linear
relationship between the rate of photosynthesis and the rate of export
between 15 and 35[deg]C. At these temperatures, 60 to 80% of the carbon
fixed was readily exported with sucrose, raffinose, and stachyose, which
together constituted over 90% of phloem mobile assimilates. Above 35[deg]C,
however, export during photosynthesis was inhibited both in
photorespiratory conditions, which inhibited photosynthesis, and in
nonphotorespiratory conditions, which did not inhibit photosynthesis.
Sucrose and raffinose but not stachyose accumulated in the leaf at
40[deg]C. When leaves were preincubated at 40[deg]C and then cooled to
35[deg]C, export recovered more slowly than photosynthesis. These data are
consistent with the view that impairment of export processes, rather than
photosynthetic processes associated with light trapping, carbon reduction,
and sucrose synthesis, accounted for the marked reduction in export between
35 and 40[deg]C. Taken together, the data indicated that temperature
changes between 15 and 40[deg]C had two effects on photosynthesis and
concurrent export. At all temperatures, suppressing photorespiration
increased both photosynthesis and export, but above 35[deg]C, export
processes were more directly inhibited independent of changes in
photorespiration and photosynthesis.
