Effect of heat stress on the inhibition and recovery of the ribulose-1,5-bisphosphate carboxylase/oxygenase activation state |
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Authors: | S J Crafts-Brandner R D Law |
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Institution: | (1) USDA-ARS Western Cotton Research Laboratory, 4135 E. Broadway Road, Phoenix, AZ 85040-8830, USA, US |
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Abstract: | Experiments were conducted to determine the relative contributions of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco;
EC 4.1.1.39) activation state vis-à-vis Rubisco activase and metabolite levels to the inhibition of cotton (Gossypium hirsutum L.) photosynthesis by heat stress. Exposure of leaf tissue in the light to temperatures of 40 or 45 °C decreased the activation
state of Rubisco to levels that were 65 or 10%, respectively, of the 28 °C control. Ribulose-1,5-bisphosphate (RuBP) levels
increased in heat-stressed leaves, whereas the 3-phosphoglyceric acid pool was depleted. Heat stress did not affect Rubisco
per se, as full activity could be restored by incubation with CO2 and Mg2+. Inhibition and recovery of Rubisco activation state and carbon dioxide exchange rate (CER) were closely related under moderate
heat stress (up to 42.5 °C). Moderate heat stress had negligible effect on Fv/Fm, the maximal quantum yield of photosystem
II. In contrast, severe heat stress (45 °C) caused significant and irreversible damage to Rubisco activation, CER, and Fv/Fm.
The rate of Rubisco activation after alleviating moderate heat stress was comparable to that of controls, indicating rapid
reversibility of the process. However, moderate heat stress decreased both the rate and final extent of CER activation during
dark-to-light transition. Treatment of cotton leaves with methyl viologen or an oxygen-enriched atmosphere reduced the effect
of heat stress on Rubisco inactivation. Both treatments also reduced tissue RuBP levels, indicating that the amount of RuBP
present during heat stress may influence the degree of Rubisco inactivation. Under both photorespiratory and non-photorespiratory
conditions, the inhibition of the CER during heat stress could be completely reversed by increasing the internal partial pressure
of CO2 (Ci). However, the inhibition of the CER by nigericin, a K+ ionophore, was not reversible when the Ci was increased at ambient or high temperature. Our results indicate that inhibition
of photosynthesis by moderate heat stress is not caused by inhibition of the capacity for RuBP regeneration. We conclude that
heat stress inhibits Rubisco activation via a rapid and direct effect on Rubisco activase, possibly by perturbing Rubisco
activase subunit interactions with each other or with Rubisco.
Received: 25 February 2000 / Accepted: 13 May 2000 |
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Keywords: | :Gossypium (heat stress Rubisco) Heat stress Photosynthesis Ribulose-1 5-bisphosphate carboxylase/oxygenase Rubisco activase |
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