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United States Department of Agriculture

Agricultural Research Service

Title: Sensitivity to Photosynthesis in the C4 Plant, Maize, to Heat Stress.

item Crafts-Brandner, Steven
item Salvucci, Michael

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 1, 2002
Publication Date: August 1, 2002

Interpretive Summary: Net photosynthesis (Pn) is inhibited by moderate heat stress in many plant species. This inhibition is caused, in large part, by the temperature-induced decrease in the activation state of Rubisco, the enzyme that converts CO2 into plant biomass. Heat stress does not directly inhibit Rubisco, but it inhibits the enzyme, Rubisco activase, that regulates the activation state of Rubisco. In addition to heat stress, high levels of atmospheric CO2 also inhibit the activation state of Rubisco in plants with C3 photosynthetic metabolism. Plants with C4 photosynthetic metabolism, such as maize, are known to be less sensitive to heat stress than C3 plants, and they also have an internal CO2 concentrating system that elevates the CO2 level in chloroplasts to very high levels. Here we provide the first report of high temperature effects on Rubisco activation and photosynthesis in a C4 plant. Moderate heat stress leads to decreased Rubisco activation, but Pn did not decrease until very high temperature due to the CO2 concentrating system of maize. High levels of atmospheric CO2 did not affect Rubisco activation. Under moderate heat stress Pn would significantly increase if Rubisco activation remained high. Energy supply was not limiting to Pn under moderate heat stress; the primary limitation was the sensitivity of Rubisco activase to heat stress. Heat tolerance of C4 species could be substantially improved by improving the thermal tolerance of Rubisco activase. These results will be of interest to research scientists in the public and private sectors.

Technical Abstract: Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize, a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38 C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30 C, indicating increased thylakoid energization even at temperatures that did not inhibit net photosynthesis. Compared to CO2 assimilation, the maximum quantum yield of photosystem II (Fv/Fm) was relatively insensitive to leaf temperatures up to 45 C. The activation state of PEP carboxylase decreased marginally at leaf temperatures above 40 C and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45 C. The activation state of Rubisco decreased at temperatures exceeding 32.5 C with nearly complete inactivation at 45 C. Levels of 3-PGA and RuBP decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30 C.

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