Effects of Freezing Stress on Photosynthetic Reactions in Cold Acclimated and Unhardened Plant Leaves

Abstract

Leaves of cold acclimated and unhardened plants of Triticum aestivum, Viola wittrockiana, Tetragonia tetragonioides, and Hedera helix were subjected to controlled frost treatments. After thawing, rates of photosynthetic CO₂ uptake and respiratory CO₂ release by the leaves were measured at room temperature. Simultaneously, physical parameters related to photosynthetic activities of the thylakoid membranes, such as chlorophyll fluorescence, light scattering and membrane potential-dependent absorbance changes were monitored.

According to measured rates of photosynthesis, cold acclimation led to significant frost tolerance of Tritiam, Viola, and Hedera. The Temperature inflicting 50% inhibition , T₅₀, was decreased by acclimation from about -6°C to -15°C in Triticum and Viola; T₅₀ in hardened leaves of Hedera was about -23°C. In Tetragonia acclimation lowered the T₅₀ by maximally 2°C from about -5.5 to -7.5°C.

Except for Tetragonia, respiration was distinctly less sensitive to a freeze-thaw cycle than photosynthetic CO₂ uptake; T₅₀ of respiraion frequently was 1–3 degrees lower than T₅₀ of photosynthesis.

In leaves of Triticum, Viola, and unhardened Tetragonia, the physical signals indicating light — driven processes in the thylakoids were affected at significantly lower freezing temperatures than CO₂ assimilation.

From these results , a decrease in photosynthetic CO₂ fixation presently appears as the earliest detectable sign of freezing injury in plant leaves.