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

Abstract

The influence of exogenously applied sucrose on cold hardening of raspberry (Rubus idaeus L.) in vitro was examined. Raspberry plants (cv. Preussen) were cultured on Murashige‐Skoog (MS) media with different levels (1, 3, 5 and 7%) of sucrose and subjected to low‐temperature acclimation (3/−3°C day/night temperature, 8‐h photoperiod) for 14 days. Cold hardiness (LT50 in controlled freezing), shoot moisture content, osmolality and the amounts of sucrose, glucose and fructose were determined. Exogenously applied sucrose was taken up by plants, but the uptake corresponded to less than 10% of total sugar reserves in the culture. Cold hardiness was primarily affected by acclimation treatment, but sucrose increased cold hardiness of nonacclimated plants and significantly enhanced the effect of acclimation treatment, 5% sucrose in the culture medium being optimal for cold hardening. LT50 values ranged between −4.1 and −7.1°C for nonacclimated, and between −14.2 and −20.7°C for cold‐acclimated shoots. Shoot moisture content was inversely related to medium sucrose level and declined only slightly during cold acclimation. After cold acclimation, plant osmolality predicted hardiness better than shoot moisture content. Plant osmolality and sugar content were increased by increasing the medium sucrose level and, to a greater extent, by cold acclimation. Sucrose, glucose and fructose accumulated during hardening. Sucrose was the predominant sugar, and the rate of sucrose accumulation during cold acclimation was independent of the medium sucrose level or the initial plant sucrose content. A close correlation between cold hardiness and total sugars, sucrose, glucose and fructose was established. These results suggest that sugars have more than a purely osmotic effect in protecting acclimated raspberry plants from cold.