Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2004
Publication Date: 6/15/2004
Citation: Schrader, S.M., Wise, R.R., Wacholtz, W.F., Ort, D.R., Sharkey, T.D. 2004. High leaf temperature limits photosynthesis in Pima cotton II. thylakoid membrane responses to moderate heat stress. Plant Cell and Environment. 27:725-735. Interpretive Summary: High temperatures often inhibit plant growth, with photosynthesis considered among the most sensitive plant functions to high temperature. Most temperate C3 plants exhibit a broad photosynthetic temperature optimum between 20° and 35°C with peak CO2 assimilation often occurring near 30'C. It is well understood that increasing leaf temperatures beyond this range reduces photosynthetic efficiency by stimulating photorespiration. We found that in moderately heat-tolerant crop Pima 6 cotton (Gossypium barbadense) high leaf temperatures within the range encountered under normal field conditions there is damage to the photosynthetic membrane that also contributes to the lower efficiency of photosynthesis. The outcome would be worse except that cyclic electron flow is induced that helps overcome the loss of efficiency. This work provides physiological markers for selecting more heat tolerant crops.
Technical Abstract: Photosynthesis is inhibited by high temperatures which plants are likely to experience under natural conditions. Both increased thylakoid membrane ionic conductance and rubisco deactivation have been suggested as the primary cause. We used the moderately heat-tolerant crop Pima 6 cotton (Gossypium barbadense) to examine heat stress-induced inhibition of photosynthesis. Previous field-work indicated that moderate heat stress (T = 35 to 45) is associated with very rapid leaf temperature changes. Therefore, we devised a system for rapid heating of intact, attached leaves to mimic natural field heat stress conditions and monitored rubisco activation, carbon-cycle metabolites, thylakoid leakiness and photosystem I activity. As a proxy for NADPH and stromal redox status we measured NADP-malate dehydrogenase (NADP-MDH) activation state. In dark-adapted cotton leaves, heating caused an increase in thylakoid permeability at temperatures as low as 36°C. The increased permeability did not cause a decline in ATP levels during steady-state or transient heating. Rapid heating caused a transient decline in ribulose 1,5-bisphosphate without a decrease in rubisco activation because of increased photorespiration. Sustained heating caused a decline in rubisco activation and also oxidized the stroma as judged by NADP-MDH activation and this is hypothesized to result from increased NADPH-dependent cyclic photophosphorylation, explaining the maintenance of ATP content in the face of increased thylakoid membrane ion leakiness.