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 photosyn thetic activity following frost hardening is associated with an accumu lation of soluble saccharides usually occurring during the development of frost tolerance. Rates of photosynthetic CO2 uptake and O-2 evolut ion as well as contents of glucose, fructose and sucrose were determin ed 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 da y/night temperatures. Effects of feeding inorganic phosphate (P-i) on photosynthesis and of dehardening after transfer to 20/15 degrees C we re also recorded. Photosynthesis was reduced by about 30% after harden ing, accompanied by a strong increase in sugar contents (sucrose 2 x, glucose and fructose 10 x). Photosynthesis and sugars tended to normal ise after 4 days at 20/15 degrees C. Photosynthetic depression in fros t-hardened leaves was similar at ambient and saturating CO2, irrespect ive of source/sink relations. Feeding the leaves with P-i hardly resto red their photosynthetic activity. These findings do not support the h ypothesis that photosynthetic impairment during frost hardening is a r esult of feedback inhibition via assimilate accumulation.