Photosynthetic depression in leaves of frost-hardened ivy is not caused by feedback inhibition via assimilate accumulation

The aim of this study was to examine whether an impairment of photosynthetic activity following frost hardening is associated with an accumulation of soluble saccharides usually occurring during the development of frost tolerance. Rates of photosynthetic CO2 uptake and O-2 evolution as well as contents of glucose, fructose and sucrose were determined in leaves of ivy plants (Hedera helix L.) with varying source/sink relations prior to and after hardening for 4 weeks at 5/0 degrees C day/night temperatures. Effects of feeding inorganic phosphate (P-i) on photosynthesis and of dehardening after transfer to 20/15 degrees C were also recorded. Photosynthesis was reduced by about 30% after hardening, accompanied by a strong increase in sugar contents (sucrose 2 x, glucose and fructose 10 x). Photosynthesis and sugars tended to normalise after 4 days at 20/15 degrees C. Photosynthetic depression in frost-hardened leaves was similar at ambient and saturating CO2, irrespective of source/sink relations. Feeding the leaves with P-i hardly restored their photosynthetic activity. These findings do not support the hypothesis that photosynthetic impairment during frost hardening is a result of feedback inhibition via assimilate accumulation.