PHYSIOLOGICAL-EFFECTS OF WINTER RAPE (BRASSICA-NAPUS VAR. OLEIFERA) PREHARDENING TO FROST - I - FROST-RESISTANCE AND PHOTOSYNTHESIS DURING COLD-ACCLIMATION

The objective of the research was to define the changes in photosynthe tic activity induced by prehardening and to determine their involvemen t in frost tolerance of winter rape. Prehardening of winter rape, cons isting of keeping the plants at + 12 degrees C during the light period s from sprouting until the beginning of the Ist stage of cold hardenin g, contributed to increasing its effectiveness. After 42 days of harde ning at + 2 degrees C the resistance of the prehardened plants equalle d that attained by winter rape in the most favourable seasons of veget ation in the field. Prehardening stimulated the efficiency of photosyn thesis at chill temperatures (+2-5 degrees C). Differences in photosyn thetic efficiency, like those in frost resistance, increase with succe ssive weeks of hardening. They also concern the leaves already develop ed at the hardening temperature. A prehardened photosynthetic apparatu s is less susceptible to the progress of photoinactivation taking plac e when the seedlings are kept at + 2 degrees C. It also demonstrated g reater activity even during the first hour of hardening or in the newl y expanding leaves, and also at higher temperatures, most probably bec ause of the more efficient progress of the dark processes. The describ ed changes in the functioning of the photosynthetic apparatus induced by prehardening were thus qualitatively very similar to those observed during long-term growth at +5 degrees C, already described in the lit erature.