title: Rapid heating of intact leaves reveals initial effects of stromal oxidation on photosynthesis.
abstract: Heat stress in leaves under natural conditions is characterized by rapid fluctuations in temperature. These fluctuations can be on the order of 10 degrees C in 7 s. By using a specially modified gas-exchange chamber, these conditions were mimicked in the laboratory to analyse the biochemical response to heat spikes. The decline in ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity during prolonged heat stress is generally associated with an increase in ribulose 1,5-bisphosphate (RuBP) levels. However, rapid heating caused an initial decline in RuBP which was subsequently followed by a small decline in Rubisco carbamylation. The ratio of RuBP to Rubisco sites declined from a saturating concentration to a sub-saturating concentration, providing a possible mechanism for the decarbamylation of Rubisco. If RuBP is saturating (>1.8 RuBP Rubisco site(-1)), it acts as a cap on the catalytic site and keeps Rubisco activated. Measurements of triose-phosphate levels and NADP-malate dehydrogenase activation (a stromal redox proxy) indicated that the regeneration of RuBP by the Calvin cycle was limited by the availability of redox power.
authors.affiliation: Department of Botany, University of Wisconsin-Madison, Madison, WI 53706, USA. tsharkey@wisc.edu
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Schrader
authors.forename: Kyle R
authors.initials: KR
authors.lastname: Kleinbeck
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Sharkey
mesh_terms: D006358:Hot Temperature; D050538:Malate Dehydrogenase (NADP+) / Q000378:metabolism; D010084:Oxidation-Reduction; D010788:Photosynthesis; D018515:Plant Leaves / Q000201:enzymology / Q000378:metabolism / Q000502:physiology*; D012274:Ribulosephosphates / Q000378:metabolism
chemical_list: D012274:Ribulosephosphates; C001933:ribulose-1,5 diphosphate; D050538:Malate Dehydrogenase (NADP+)