COLD HARDENING OF SPRING AND WINTER-WHEAT AND RAPE RESULTS IN DIFFERENTIAL-EFFECTS ON GROWTH, CARBON METABOLISM, AND CARBOHYDRATE CONTENT

The effect of long-term (months) exposure to low temperature (5 degree s C) on growth, photosynthesis, and carbon metabolism was studied in s pring and winter cultivars of wheat (Triticum aestivum) and rape (Bras sica napus). Cold-grown winter rape and winter wheat maintained higher net assimilation rates and higher in situ CO2 exchange rates than the respective cold-grown spring cultivars. In particular, the relative g rowth rate of spring rape declined over time at low temperature, and t his was associated with a 92% loss in in situ CO2 exchange rates. Asso ciated with the high photosynthetic rates of cold-grown winter cultiva rs was a P-fold increase per unit of protein in both stromal and cytos olic fructose-1,6-bisphosphatase activity and a 1.5- to 2-fold increas e in sucrose-phosphate synthase activity. Neither spring cultivar incr eased enzyme activity on a per unit of protein basis. We suggest that the recovery of photosynthetic capacity at low temperature and the reg ulation of enzymatic activity represent acclimation in winter cultivar s. This allows these overwintering herbaceous annuals to maximize the production of sugars with possible cryoprotective function and to accu mulate sufficient carbohydrate storage reserves to support basal metab olism and regrowth in the spring.