Cold hardening of raspberry plants in vitro is enhanced by increasing sucrose in the culture medium

The influence of exogenously applied sucrose on cold hardening of raspberry (Rubus idaeas L.) in vitro was examined. Raspberry plants (cv, Preussen) w ere cultured on Murashige-Skoog (MS) media with different levels (1, 3, 5 a nd 7%) of sucrose and subjected to low-temperature acclimation (3/-3 degree s C day/night temperature, 8-h photoperiod) for 14 days. Cold hardiness (LT 50 in controlled freezing), shoot moisture content, osmolality and the amou nts of sucrose, glucose and fructose were determined. Exogenously applied s ucrose was taken up by plants, but the uptake corresponded to less than 10% of total sugar reserves in the culture. Cold hardiness was primarily affec ted by acclimation treatment, but sucrose increased cold hardiness of nonac climated plants and significantly enhanced the effect of acclimation treatm ent, 5% sucrose in the culture medium being optimal for cold hardening. LT5 0 values ranged between -4.1 and -7.1 degrees C for nonacclimated, and betw een -14.2 and -20.7 degrees C for cold-acclimated shoots, Shoot moisture co ntent was inversely related to medium sucrose level and declined only sligh tly during cold acclimation. After cold acclimation, plant osmolality predi cted hardiness better than shoot moisture content. Plant osmolality and sug ar content were increased by increasing the medium sucrose level and, to a greater extent, by cold acclimation. Sucrose, glucose and fructose accumula ted during hardening. Sucrose was the predominant sugar, and the rate of su crose accumulation during cold acclimation was independent of the medium su crose level or the initial plant sucrose content. A dose correlation betwee n cold hardiness and total sugars, sucrose, glucose and fructose was establ ished. These results suggest that sugars have more than a purely osmotic ef fect in protecting acclimated raspberry plants from cold.